PUBLICATION NO. FHWA-NHI- FHWA-NHI- Innovative Intersections and Interchanges Participant Notebook

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Innovative Intersections and Interchanges Participant Notebook

INTRODUCTION Thank you for choosing to participate in this training workshop on innovative intersections and interchanges offered through the National Highway Institute (NHI). In the United States, over the last several years an average of one-quarter of traffic fatalities and roughly half of all traffic injuries occur at, or near, intersections. In addition to being a substantial safety challenge, intersections may also become very congested when traffic volumes are high, resulting in user delay and frustration that ultimately may compound into larger regional mobility problems.

Motorists, pedestrians, and bicyclists face greater mobility challenges and safety risks at intersections as traffic volumes grow and congestion worsens. Agencies need safer, more balanced designs that keep people moving. Innovative intersection designs offer many safety and operational benefits, and are being built more often because they can deliver more for less. The Federal Highway Administration (FHWA) launched the Every Day Counts (EDC) state-based initiative to identify and rapidly deploy proven but underutilized innovations aimed at reducing project delivery time, enhancing roadway safety, reducing congestion, and improving environmental sustainability. In 2012, Intersection & Interchange Geometrics (IIG) was selected as a featured innovative technology in EDC Round-2. IIG consists of a family of innovative intersection designs that improve intersection safety while also reducing delay, and at lower cost and with fewer impacts than comparable traditional solutions.

In continuing effort to advance the deployment of innovative intersection designs, FHWA is pleased to offer this training workshop to assist transportation professionals in better understanding these intersections and the potential benefits they can provide when correctly implemented.

For further information or to offer comments on this training material, please contact:

Mark Doctor - FHWA Resource Center: [email protected] Jeff Shaw – FHWA Office of Safety: [email protected]

TARGET AUDIENCE The target audience for this training includes state and local transportation agency personnel, and/or consultants having responsibilities for developing and designing elements pertaining to intersections and interchanges.

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GOALS AND OUTCOMES The overall course goal is to make design engineers aware of new intersection and interchange designs that improve traffic operations and safety at a much lower cost than most other viable alternatives.

Specifically, participants should be able to perform the following after attending this course:

• Describe the principal features of the innovative geometric designs presented including key design and operational features • List the advantages and disadvantages of their use • Assess what innovative designs would be applicable at a given location given the conditions and constraints • Identify resources to acquire additional information on these designs and their implementations

COURSE CONTENT

Module 1: Introduction – This session includes a brief overview of common intersection problems that agencies are trying to solve and introduce the concepts of intersection conflict points to understand safety issues and phasing issues at signalized intersections.

Module 2: Restricted Crossing U-Turn (RCUT) – Also known as J-turns, superstreets, and reduced conflict intersections, this module describes the features, safety advantages and potential applications of the RCUT.

Module 3: Median U-Turns (MUT) and Other Indirect Left Turns – Focuses on the features of the MUT as well as other indirect left turn intersections with similar features.

Module 4: Displaced Left Turn Intersection (DLT) – Commonly referred to as the Continuous Flow Intersection (CFI), this module will concentrate on the DLT features and benefits.

Module 5: Continuous Green-T Intersections – A short discussion on this design for three-leg intersections and their applicability.

Module 6: Diverging Diamond Interchange (DDI) – A comprehensive discussion of the features and benefits of the DDI will be explored.

Module 7: Assessment and Evaluation Process – Describes the advantages of utilizing a formal intersection / interchange evaluation process to determine which geometric solution might work best at any given location.

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Module 1: Welcome and Introduction

Meet Your Instructor(s):

Mark Doctor, PE Jeffrey Shaw, PE FHWA Resource Center FHWA Office of Safety [email protected] [email protected] 404-562-3732 708-283-3524

Participant Introductions:

. Your name

. Your Agency/Organization

. What would you like to learn from this workshop?

Logistics:

. Please silence cell phone devices

. Please ask questions

. Schedule

o Breaks

o Lunch . Restrooms/Emergency exits

. Sign-in sheet and registration form Source: Google Images

MODULE 1 LEARNING OUTCOMES

 Define “intersection conflict points”

 Describe the “Safe Systems” concept as potentially applied to intersections

 List the common characteristics of “innovative” or “alternative” intersections and interchanges

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WHY INNOVATIVE INTERSECTIONS & INTERCHANGES?

Which of these intersection problems/challenges do you face?

• Increasing Congestion

• Too Many Crashes

• Mobility for all modes (Bicycles, Pedestrians, Transit)

• Not Enough Funding

• Time Consuming Projects

• Inability for more right-of-way

• Impacts of projects (Environmental, social, economic)

INTERSECTION SAFETY FACTS

Intersections are usually bottlenecks along high-volume roadways and are often a safety concern.

With Intersections: BIGGER is not always BETTER

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Important Safety Facts: Statistics show that about half of all severe crashes occur at an intersection.

Angle crashes account for over

40% of fatal

crashes at

intersections

Left-turn crashes account for over 20% of fatal crashes at

intersections

Ped & Bike crashes account for 25% of fatal crashes at signalized intersections

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INTERSECTION CONFLICT POINTS

Intersections are inherently locations for users crossing paths (i.e. conflict points). If users make incorrect judgements or disobey traffic control devices, those conflicts could result in a collision. Assuming single lane approaches, a typical 4-leg intersection has 32 conflict points among vehicles.

Breaking down the 32 conflict points:

. 16 crossing conflict points (most critical)

. 16 merging/diverging conflict points

Source: FHWA’s Signalized Informational Guide – Exhibit 4-3(b)

Every vehicle conflict point is affected either directly or indirectly by left-turn movements

There are four vehicle-pedestrian conflicts per crossing.

Conflicts involving right turn on red and “permissive” left-turns can be particularly problematic.

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SAFE SYSTEMS APPROACH

“Safe System is the management and design of the road system such that impact energy on the human body is firstly avoided or secondly managed at tolerable levels by manipulating speed, mass and crash angles to reduce crash injury severity.”

Source: Austroads Report AP-R560-18 Towards Safe System Infrastructure: A Compendium of Current Knowledge

Basic Principles:

• There is shared responsibility for safety among the Human, the Vehicle, and the Infrastructure that works together to minimize harm.

• Recognition and acceptance that crashes will occur due to human error, but seek to reduce systemic opportunities for errors to occur.

• When crashes do occur, manage their energy so that the outcomes are not severe.

Firstly – try to avoid the crash!!!

 Simplify Driver Decisions and Clarify with Positive Guidance

. Human Factors Guidelines

. Reduce the potential for a collision

. Intersection Conflict Points

 Manage Speed – higher speeds equate to longer distances to react and brake Modest reductions in travel speed approaching intersections can produce quite large reductions in the risk of deaths and serious injuries. When a conflict happens at slower speeds, a driver travels a shorter distance in the time required to react, increasing the opportunity to either avoid a collision or reduce the speed upon impact. Once the brakes are applied the stopping distance increases with the square of the initial speed.

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Even if a vehicle can’t stop completely before impact, a modest reduction in approach speed makes for a much greater difference in the speed on impact.

View an exceptional public service announcement developed in Australia: https://www.youtube.com/watch?v=ZfdbeecfiJo

The human body’s tolerance to the physical force of being in a crash is at the center of a Safe System approach to considering safety. Applying a Safe System approach to intersections requires an understanding of the relationship between speeds and angle of impact (the inescapable laws of physics) to determine how much force (kinetic energy) the human body experiences in the event of a crash. Reducing the overall kinetic energy of the impact can increase the odds for it to be survivable.

The figure below is from Australian research conducted on the effects of speed and angle of impact to the amount of transferable kinetic energy to a human involved in a vehicular collision. Note how a reduction in angle of impact from 90 degrees to about 40 degrees reduces the transferable kinetic energy by about the same as would reducing the travel speed from 90 km/h (56 mph) to 60 km/h (37 mph).

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INTERSECTION TRAFFIC OPERATIONS

Intersections are usually bottlenecks along high-volume roadways and adding more lanes often does not solve the issue.

Alternative intersections and interchanges that are signalized, improve operations by improving the signal operations.

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Basic two-phase signal operation allows for the north-south traffic to go in one phase and then east-west traffic to go in the other phase.

. All the left turns are made as “permissive-only”

o This operation is common for intersections with modest left- turning volumes

Source: MnDOT Traffic Signal Timing and Coordination Manual

Adding “protected” left-turn phases is common as volumes increase

Three Basic Left-turn Signalization Options:

1. Protected-only – Left turn on a green arrow only

2. Permissive-only - No green arrows 3. Protective-permissive – Signals with a green arrow; usually preceding, and then permissive lefts as gaps in traffic allow.

Which of the three options is the safest?

Which option typically offers the best operations?

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Adding more phases essentially “steals” time away from the major through movement and can increase overall intersection delays.

Fewer phases is generally better! A common feature to many innovative designs with signals is to reduce the number of traffic signal phases by strategically handling left turn movements differently.

Innovative intersections offer the potential to improve safety and reduce delay, often at a lower cost and with fewer impacts than traditional alternatives.

MODULE 1 LEARNING OUTCOMES

 Define “intersection conflict points”

 Describe the “Safe Systems” concept as potentially applied to intersections

 List the common characteristics of “innovative” or “alternative” intersections and interchanges

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Module 1: Learning Check

What are intersection conflict points?

A. Crashes that occur when the signal malfunctions

B. Crashes that occur when the driver makes an error

C. Combinations of the various through and turning movements that can be classified into three different types: crossing, merging, and diverging

What characteristics are common to innovative intersections?

A. They are designed to confuse and frustrate drivers

B. Pedestrians are prohibited from using them because they are too dangerous to walk through

C. They are designs that improve the way traffic makes certain movements to eliminate, relocate or modify conflict points and/or strategically improve the signalization phasing

What are the potential advantages of innovative intersections?

A. Fewer crashes

B. Less congestion

C. Lower cost

D. Better for pedestrians, bicyclists and transit users

E. Faster to implement

F. Fewer social, environmental and economic impacts

G. All of the above

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MODULE 2: RESTRICTED CROSSING U-TURN (RCUT) Learning Outcomes

 Describe the features of the RCUT intersection and its safety advantages

 Describe the potential applications for the RCUT The RCUT is an at-grade intersection with directional median channelization such that minor road traffic must turn right and make a U-turn back to cross or make the left-turn maneuver. The major road typically allows all maneuvers (some options limit left-turns).

The RCUT is also known as a J-turn, superstreet, or reduced conflict intersection and is usually implemented along multilane highways with a divided median. To understand the benefit, consider the degree of difficulty in being the minor road vehicle driver attempting to cross or turn left onto the divided highway. Finding a gap across multiple directions and lanes, where vehicles are moving at high speeds can be intimidating and time consuming. As an alternative, the RCUT design asks a driver to make a series of less complex decisions, taking advantage of more frequent gaps, and making movements that are associated with less severe conflict types. Source: Wisconsin DOT

Multilane divided highways with open medians commonly experience problems with far-

side right angle

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RCUT Intersection Movements

RCUT General Applicability

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Unsignalized RCUTS:

These are also known as J-turns. A stop-controlled RCUT intersection is sometimes used as a safety treatment at an isolated intersection on a four-lane divided arterial in a rural area. There are known safety benefits for this type of RCUT intersection. In some cases, a stop controlled RCUT intersection is later converted to a signalized RCUT intersection as traffic volumes increase.

Source: FHWA RCUT Informational Guide – Exhibit 1-2

A merge- or yield-controlled RCUT intersection may be appropriate on lower volume rural high-speed divided four-lane corridors. This type of RCUT intersection may require longer distances to the U-turn crossovers to allow for the weaving movement.

Source: FHWA RCUT Informational Guide – Exhibit 1-3

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Signalized RCUTS:

A signalized RCUT intersection, or superstreet, allows favorable progression along the corridor. RCUT intersection signals typically require only two phases, which can offer great efficiency by minimizing lost time in the cycle and allow progression in both directions under a broader range of speeds and signal spacing.

Source: FHWA RCUT Informational Guide – Exhibit 1-4

What happens when the minor street volumes for through movements and left-turns gets higher?

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Feasible Volumes

At minor street demands below 5,000 vpd, consider unsignalized RCUTs

For minor street demands of more than 25,000 vpd, consider other alternative intersections (such as a MUT or DLT) that would generally serve the minor street more efficiently

OPERATIONAL CONSIDERATIONS

RCUTs may operate with shorter cycle lengths than comparable conventional intersections because each signal will typically have only two phases. Shorter cycles reduce delay for most vehicles and for pedestrians.

Source: FHWA RCUT Informational Guide – Exhibit 3-13

Signals in a RCUT intersection operate with a coordinated two-phase mode.

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BI-DIRECTIONAL PROGRESSION

There are multiple opportunities available when implementing bi-directional progression with RCUT corridors.

Some of the key characteristics:

. Enables each direction on a two-way arterial to operate independently

. No movement crosses both directions of the major street . Both directions can be progressed at any speed and signal spacing

. Sometimes known as “perfect progression” or “100% progression”

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While the RCUT can be implemented as an isolated intersection treatment, it is ideal as a corridor treatment.

As a corridor treatment, not only would the safety benefits be compounded, but certain operational benefits, such as more efficient signal timing and corridor progression, can also be realized.

SAFETY BENEFITS OF RCUT INTERSECTIONS

Positive safety benefits are anticipated from implementing RCUT intersections at appropriate locations. This is greatly attributable to the reduced vehicle-vehicle conflict points compared to a conventional intersection. Very importantly, crossing conflicts are reduced from 16 to 4.

Besides reducing conflict points (i.e. potential crash locations), the RCUT has been demonstrated to be safer in terms of crash rates and crash severity based on the studies conducted to date. Studies show the RCUT offers significant safety advantages over conventional arterials. Good evidence indicates that a right turn followed by a U-turn is much safer than a direct left turn out of a side street.

Field evaluation studies by FHWA of RCUTs in Maryland showed consistent crash reduction on the order of 28-44%.

Studies in North Carolina indicate that RCUTs decrease fatal and severe injury crashes by more than half.

The Missouri Department of Transportation performed field evaluation studies and analyses on five different sites that converted traditional intersections into J-turns and found a 35% total crash reduction and 54% reduction in fatal and injury crashes with ZERO fatal accidents following the J-turn implementation. Also, a public survey found most road users felt there was no adverse effect on trip times.

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MULTIMODAL CONSIDERATIONS

For the main street crosswalks in a RCUT intersection, pedestrian crosswalks are slightly different than the pedestrian crosswalks in a conventional intersection.

Main Differences:

. Wide geometric footprint may increase the total crossing distance

. Cycle lengths are shorter which can benefit pedestrians

“Z” Crossing Option:

Currently, the most commons means for serving pedestrians at a RCUT intersection is a “Z” crossing treatment.

A “Z” crossing allows all six desired pedestrian movements at an intersection. The two minor street crossings (A to B, C to D) are made similarly to a conventional intersection. Three of the movements (A to C, B to D, and A to D) require pedestrians to take a longer, unconventional route. The sixth movement (B to C) requires pedestrians to take a shorter, unconventional route.

Source: FHWA RCUT Informational Guide – Exhibit 3-1 Crossing the minor road (A to B or C to D) is relatively simple and may offer shorter crossings than traditional intersections. Crossing the major road will generally be made in two stages and require an extra crossing of the minor street.

Pedestrian –Vehicle Conflict Points:

The conventional intersection has 24 pedestrian-vehicle conflict points.

The RCUT intersection reduces the number to 8 pedestrian-vehicle conflict points.

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Offset Approach Option:

At offset approach intersections, pedestrian crosswalks can be implemented between the minor road and perpendicular to the travel lanes. An example of a crossing between the minor roads is illustrated below:

Source: FHWA RCUT Informational Guide – Exhibit 3-5

“T” Style Crossing:

At “T” style or 3-leg RCUT intersections, it is also possible to implement a pedestrian crosswalk perpendicular to the travel lanes. An example is illustrated below:

Source: FHWA RCUT Informational Guide – Exhibit 3-6

Mid-block Option:

Supplemental “mid-block” crosswalks could be added near the U-turn locations. Because each direction of travel on the arterial can operate independently (i.e., similar to individual one-way streets), negligible vehicle delay to major-street vehicles results when installing additional traffic signals, as the signals can be timed

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to progress major-street vehicles. This feature allows mid-block pedestrian signals to be installed with minimal impact on vehicular travel time.

Source: FHWA RCUT Informational Guide – Exhibit 3-6

Source: FHWA RCUT Informational Guide – Exhibit 3-6

Bicycle – Minor Street Through:

Bicycles on the major roadway travel though a RCUT intersection the same way they travel through a conventional intersection.

Minor street left-turning or through bicycles do not have a direct route at a RCUT intersection if they are travelling in vehicular lanes. At the same time, newer and reconstructed streets in many communities typically integrate Complete Streets policies that include bicycle facilities.

RCUT intersections can be designed to reduce or eliminate out-of-direction travel by bicyclists. Both the challenges and benefits RCUT intersections offer bicyclists must be carefully evaluated to guide project planning and design decisions.

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Benefits: Major street through and right-turning bicyclists at a RCUT intersection encounter relatively more green time percentages for their movements, resulting in lower delay and, potentially, fewer stops for red lights. RCUT intersections are generally constructed on higher-volume roadways, so physically separating bicycle lanes from general purpose lanes using buffered bike lanes, cycle tracks or similar treatments may be appropriate. Major street bicyclists turning left can ride in the left-turn lane or stop at the crosswalk and use the “Z” crossing like a pedestrian.

Challenges: A higher volume of major street right-turning vehicles occur at RCUT intersections, compared to conventional intersections, resulting in more exposure between bicycle through and vehicle right-turn movements. An increasingly common practice at conventional or alternative intersections is to shift the right turn lane to the right of the bicycle lane.

Bicyclists will appreciate the high green time percentage for the major-street through movement at Restricted Crossing U-turn Intersections. Bicyclists desiring to make a left turn or through movement from the side street will face a choice of using the pathway through the median designed for pedestrians or using the U-turn crossovers in a manner similar to drivers of motor vehicles. Bicycle crossing options at a minor road is illustrated below:

Source: FHWA RCUT Informational Guide – Exhibit 3-12 Bicycle Cut-Through Option:

Consider a bicycle cut-through option when planning alternatives.

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Bicyclists who desire to make a left turn or through movement from the minor street will be required to choose between using the “Z” crossing like a pedestrian, using the U- turn crossovers like a motorist, or passing through/across the channelizing island.

The “Z” crossing is the best choice for bicyclists if the pathway through the intersection is designed for shared-use and wide enough to be comfortable for bicyclists. Otherwise, bicyclists may have to dismount and walk their bicycles across. If a direct bicycle crossing is not available, the choice of crossing with pedestrians or motorists will likely depend on the distance to the U-turn crossover and the type of bicyclist.

DESIGN CONSIDERATIONS

When considering the design of an RCUT intersection, intersection skew angles can be favorable or unfavorable for turning movements depending on the orientation.

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What is the optimal distance between the U-turn and the main intersection?

. 425 feet . 880 feet

. 1320 feet

. It depends

RCUT INTERSECTION GEOMETRY

Below are some potential factors that may influence the location of a U-turn:

. Signalized or unsignalized?

. Right turn acceleration lane?

. How many lanes to weave across?

. Primarily passenger cars or substantial truck traffic?

. Low mainline volumes (large gaps) or high volume (fewer gaps)?

. Mainline operating speeds (45 MPH or 70+ MPH)? Influences deceleration lane lengths

. Horizontal alignment & sight distances

. Practical constraints (bridges, right-of-way availability, adjacent intersection, etc.)

The determination on the U-turn location should be found be engineering judgement. All of the factors above must be considered when determining the location of a U-turn At a signalized RCUT intersection, the median opening is typically 400-1000 feet downstream of the minor road.

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Distance between Intersection and Median:

There are a few issues that are associated with the distance between the intersection and the median opening:

. Vehicles from the minor road must perform weaving maneuvers to access the median U-turn.

. Longer distances are preferred for weaving, but as the distance between the intersection and median opening increases, the delay for the minor road left-turn and through vehicles increases.

. Longer distances also decrease the probability of spillback.

. Distance will depend on the cross-section (number of lanes), speed limit of the roadway, and traffic volume.

The image below illustrates the spacing considerations:

Source: FHWA RCUT Informational Guide – Exhibit 7-22

Right-turn Acceleration Lane: GOOD or BAD???

Another design consideration is whether to provide a right-turn acceleration lane from the minor street to the major street at a rural unsignalized RCUT intersection. This decision requires considering the factors and trade-offs. If volumes are high on the main road there will be fewer long gaps available and therefore having an acceleration lane may reduce delay for side-street traffic.

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Also, the right-turn acceleration lane may reduce risk of rear-end crashes from slower vehicles turning out from minor road. However, with a right-turn acceleration lane the location of the U-turn will typically need to be farther away from the main intersection. It is a “trade-off” and the answer is “It Depends”.

The figure below illustrates no right-turn acceleration lane:

Accommodating Truck Movements:

Bulb-outs at U-turn locations help facilitate the turning needs of larger vehicles on the roadway. A bulb-out is also known as a loon. The loon is useful to accommodate the turning radius of large vehicles at the median opening, particularly where the median width is relatively narrow. The image below illustrates a U-turn that has implemented a loon in its design:

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A loon may not be needed if the median is wide enough. The type of vehicle also plays a major part in determining if a loon is needed. The image below illustrates examples of the design vehicles and median widths:

Crossover Access Management:

. Driveways should be at least 100 feet away from crossovers.

. Crossovers should be for single-direction U-turns only The top image illustrates an example of a loon. The bottom image illustrates an example of a U-turn acceleration lane.

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Signing for RCUT Intersections:

. Provide sufficient guide signing for drivers on the minor road to understand the U- turn involved for making the left-turning or through movement. The use of diagrammatic style signing may be helpful.

. The minor street signing needs to clearly indicate the prohibition of left-turn and through movements from the minor road.

. If more than one-lane is turning right from the minor street, lane placement signing is important to position drivers using the median U-turn into the left most lane on the minor road. The figure below shows typical signing along the minor street:

The figure below shows typical signing along the major street indicating the U-turn location:

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Learning Outcomes

 Describe the features of the RCUT intersection and its safety advantages

 Describe the potential applications for the RCUT

TRUE or FALSE The RCUT is a very effective intersection safety measure for rural high-speed divided highways

The RCUT is very effective for an intersection of two high-volume roads each with ADTs > 30,000

The RCUT is very effective as a corridor treatment as well as at an isolated intersection location

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Module 3: Median U-Turns (and Similar Indirect Left)

Learning Outcomes

 Describe the features of the MUT intersection and its safety advantages

 Describe the potential applications for the MUT

 Describe the features of similar indirect left-turn intersections

WHAT IS THE MEDIAN U-TURN & WHERE IS IT APPLICABLE?

A median U-turn (MUT), also known as the Michigan Left, is an at-grade intersection with indirect left turns using a U-turn movement in a wide median and/or loon. The MUT eliminates direct left turns on both intersecting streets, reducing the number of signal phases and conflict points at the main intersection.

Source: FHWA MUT Informational Guide Drivers on the major street (or the street with the median) that want to turn left are directed through the main intersection to a U-turn movement at a downstream directional crossover (usually signalized), and proceed back to the main intersection to then turn right onto the minor street. The image below illustrates a left turn from the main street:

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Drivers on the minor street that wish to turn left at the major street are directed to turn right, make a U-turn movement at the crossover, and then proceed through the main intersection. The image below illustrates a left turn from the minor street:

MUT Intersection Movements

MUT Applications

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OPERATIONAL CONSIDERATIONS

Basic Signal Phasing:

. For the first phase, the through vehicles on the mainline are given the green, while all other movements are stopped.

. For the second phase, the through vehicles on the mainline are stopped while the side street and median U-turns are given the green.

The MUT does not utilize a left-turn phase, which results in fewer clearance intervals in the intersection cycle and may allow operations with a shorter overall cycle length than a comparable multi-phase cycle as shown in the comparison figure below of a

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90-second two-phase cycle vs. a 150-second multi-phase cycle for an intersection of the same volumes.

Source: FHWA MUT Informational Guide – Exhibit 5-3 Performance: Based on comparative traffic operations and simulation studies, MUT intersections had the following operational advantages compared to corridors with conventional intersections:

. Increased in total through put from 20% to 40%

. Vehicles stopping in the network were 20% to 40% lower

. Reduced travel times by 17%

SAFETY BENEFITS OF A MEDIAN U-TURN

By restricting direct left turns at the main crossing intersection, MUT intersections reduce vehicular intersection conflict points from 32 to 16, including the conflict points introduced at the median U-turn crossovers. The crossing conflicts are reduced from 16 to 4 and merging conflicts reduced from 8 to 6.

Source: FHWA MUT Informational Guide – Exhibit 4-3 34 Innovative Intersections and Interchanges Participant Notebook

Safety Performance Studies:

In general, MUT intersections show safety performance improvement compared to conventional intersections for most crash types and injury severities.

. 30 % Reduction in Intersection Related Injury Crashes

MULTIMODAL CONSIDERATIONS

A conventional intersection has 24 pedestrian-to-vehicle conflict points, but a MUT intersection reduces those conflicts from 24 down to 16. At a MUT, the left turns are removed from the main intersection (and shifted to the U-turns), thus removing pedestrian exposure to left-turning vehicles. Although the number of pedestrian conflict points at a MUT is reduced, since left-turn demand movements are consolidated into right-turn movements, the total number of vehicles crossing the crosswalk is the same. Consideration of treatments such as a Leading Pedestrian Interval or right turn on red (RTOR) prohibitions may mitigate the conflicts. Pedestrian Crossings:

Crosswalks at a median U-turn intersection are at the same places they would be at a conventional intersection. The major street crossing could be made in one or two stages with the median allowing for a refuge for a 2-stage crossing if pedestrian signals and push-buttons are installed in the median as well.

One Stage Crossing: A one-stage crossing, i.e., crossing both directions of the major street during one signal phase, may be possible if the distance is not long. At many median U-turn intersections in Michigan that have wide medians, two-stage crossings of the major street are provided.

Two-Stage Crossing: In a two-stage crossing, a pedestrian crosses one direction of the major street during one signal phase and the other direction during a second signal phase, thus incurring delay between the phases. Because there are only two signal phases and the cycle lengths are generally shorter, this delay to a pedestrian due to the two-stage crossing may be relatively small.

Source: FHWA MUT Informational Guide

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Pedestrians with vision impairments should find crossing a MUT intersection to be comparable to a conventional intersection. The cues pedestrians with vision impairments rely on to cross intersections, such as the sound of traffic parallel to their crossing, will be similar. All pedestrians should benefit from the simpler two- phase signal timing and the lower number of conflicting traffic streams than at a conventional intersection. The image below illustrates a median refuge for pedestrians crossing the main street:

Pedestrian Walk Phases:

The two-phase signal at a MUT benefits pedestrians by creating more pedestrian phases per hour along with less “don’t walk” time between “walk” times (i.e., less wait time between walk signals).

Source: FHWA MUT Informational Guide – Exhibit 3-3

A MUT intersection having a wide center median or refuge area may cause the total pedestrian crossing distance to be longer compared to a conventional intersection. However, the crossing distance may be mitigated at some installations by removing the left-turn lanes or on streets with narrower medians.

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Bicyclists:

Traditionally, MUT intersections were built along suburban arterials or other high- volume streets where bicycle accommodations were less likely to be included. However, the current trend in many communities and State DOTs is to integrate Complete Streets policies that include bicycle accommodations on all types of streets. There are at least two older MUT corridors in Michigan with marked bicycle lanes added after the MUT intersections were established.

Bicyclists have three options for navigating a MUT intersection when it comes to making a left turn. The navigation options are as followed:

1. Bicyclists making a two-stage left turn

2. Bicyclists following pedestrian crossing rules

3. Bicyclists following vehicle rules

Source: FHWA MUT Informational Guide – Exhibit 3-7

Of these options, the two-stage left turn is likely preferred. Light-Rail Transit and Bust Rapid Transit Vehicles:

The MUT intersection may provide a unique opportunity for light-rail transit (LRT) and bus rapid transit (BRT) vehicles to use the wide median required for the U-turn movements. In a conventional highway corridor, the median width is typically insufficient to accommodate LRT or BRT operations, and median openings and/or two-way left-turn lanes (TWLTLs) are incompatible with the requirements to provide a semi-exclusive running way for transit vehicles. However, a MUT corridor typically has sufficient median width to accommodate LRT and BRT vehicles, and limits the number and direction of crossings across the transitway.

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Source: FHWA MUT Informational Guide – Exhibit 3-9 DESIGN CONSIDERATIONS

MUT Crossover Spacing:

. The median opening is typically located 400 to 1000 feet downstream of the minor road

. Vehicles from the minor road must perform weaving maneuvers to access the median U-turn.

. Longer distances are preferred for weaving, but as the distance between the intersection and median opening increases, the delay for the minor road left-turn and through vehicles increases.

. Longer distances also decrease the probability of spillback.

. Distance will depend on the cross-section (number of lanes), speed limit of the roadway, and traffic volume.

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Source: FHWA MUT Informational Guide – Exhibit 7-15 and 7-16

MUT Median Widths:

The AASHTO Green Book provides guidance on minimum median widths for various design vehicles when designing for U-turns. The figure below provides the minimum median width required for seven different types of design vehicles turning from the inside lane of a four-lane divided street to either:

1. The inside lane in the opposing direction

2. The outside lane in the opposing direction

3. The outside shoulder in the opposing direction

Source: FHWA MUT Informational Guide – Exhibit 7-12 39 Innovative Intersections and Interchanges Participant Notebook

SIGNING, MARKING, SIGNALS & LIGHTING CONSIDERATIONS

Signing:

Signing for MUT intersections follows the same practice and MUTCD guidelines as for conventional intersections. However, unfamiliar drivers may be confused by the direct left turn prohibitions at the main intersection. MUT intersection signing should try to guide a motorist to make decisions at appropriate locations to complete the redirected left-turn movements. Motorists on the minor street, particularly on approaches with two or more lanes, must be informed they need to turn right at the main crossing intersection to make a left-turn movement. The image below is an example of a MUT intersection signing plan:

Source: FHWA MUT Informational Guide – Exhibit 8-6

Pavement Marking:

Pavement markings for the main intersection of a MUT intersection generally follow the same principles for those at conventional intersections, including markings for pedestrian and bicycle accommodations. The requirements within the MUTCD for edge lines, lane lines, pavement arrows, and words on the pavement are the same as with conventional intersection.

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The image below illustrates pavement markings at a directional crossover with dual lanes:

Source: FHWA MUT Informational Guide – Exhibit 8-11

SIMILAR INDIRECT LEFT-TURN INTERSECTIONS

Thru-Turn Intersections:

The Utah Department of Transportation utilizes the MUT concept in a design referred to as a ThrU Turn Intersection (TTI). The TTI is used to reduce the congestion, queueing and delay at high volume intersections with constraints for widening.

The TTI redirects some or all the left turn movements away from the intersection and utilizes signalized U-turns approximately 500-600 feet away from the main intersection. By moving the left turns away from the

intersection, more green time is dedicated to thru movements which will reduce the congestion at the intersection.

The adjacent image is an

illustration of the TTI in Draper, Utah.

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Although the term “ThrU-Turn” intersection has spread to be used in other locations such as Boise, ID, the concept also has different names. The term “Express Left” has been used in Tucson, AZ but the concept is the same. These are basically MUT intersections that utilize a larger bulb-out or “loon” in substitute for the lack of a median for the U-turn turnaround bays.

Bowtie Intersection:

Another way to facilitate redirected left-turn movements is by using roundabouts on the minor street to accommodate the U-turn movements for the arterial and cross- street left turns.

Arterial left turn movements would turn right at the cross- street and use the roundabout to "double back" thru the main intersection. Left turns at the main intersection are prohibited, eliminating the left turn bays and reducing right-of-way requirements. The main intersection operates under a simple two-phase signal control.

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Quadrant Roadway Intersection (QRI):

The QRI reroutes all four left-turn movements at a four-legged intersection onto a road that connects the two intersecting roads. The connection road can be in any quadrant, but factors such as the traffic volumes and availability of right-of-way set preference. At the main intersection, all left turns are prohibited and the signal allows a simple two- phase operation. Both junctions of the connector road are typically signalized also.

The figure below shows the movements at a QR intersection:

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JUGHANDLE INTERSECTIONS

There are three different designs of the “New Jersey” Jughandle intersection:

1. Forwards Ramp Style

2. Reverse Ramp Style

3. T-Intersection & U-turn Ramp Style Forward Ramp Style:

All turns are made from the rightmost lanes.

Reverse Ramp Style:

Left-turning traffic uses the rightmost lane downstream of the intersection into a loop ramp

Right-turning traffic turns at the intersection in a traditional manner (not onto the loop ramp).

Source: MUTCD, 2009

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T-Intersection & U-Turn Ramp:

Source: MUTCD, 2009

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Learning Outcomes

 Describe the features of the MUT intersection and its safety advantages

 Describe the potential applications for the MUT

 Describe the features of similar indirect left-turn intersections

Learning Check:

TRUE or FALSE

The MUT is a very effective intersection safety measure for rural low volume intersections.

The MUT is only possible on divided roads with medians > 60 feet.

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MODULE 4: DISPLACED LEFT TURN (DLT) INTERSECTION LEARNING OUTCOMES:

• Describe the features of the DLT intersection and its safety and operational advantages

• Describe the applicable conditions for considering the DLT

WHAT IS THE DLT & WHERE IS IT APPLICABLE?

The displaced left turn (DLT) intersection is also known as a continuous flow intersection (CFI) and a crossover displaced left-turn intersection. The concept involves relocating one or more left-turn movements on an approach to the other side of the opposing traffic flow. This allows left-turn movements to proceed simultaneously with the through movements and eliminates the left-turn phase for this approach. The number of traffic signal phases and conflict points (locations where user paths cross) are reduced at a DLT intersection, which can result in improvements in traffic operations and safety performance.

In some cases, the displaced left turns are on the minor street instead of the major street. The image below illustrates a four-legged DLT with displaced lefts on the major street:

Source: FHWA DLT Informational Guide

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Potential Configurations:

For the DLT intersection, there are several different varieties on how they can be configured:

. Three-legged DLT intersection with major street displaced left . Three-legged DLT intersection with minor street displaced left

. Four-legged DLT intersection with major street displaced lefts

. Four-legged DLT intersection with four displaced lefts

SAFETY BENEFITS OF DLT INTERSECTIONS

The DLT intersection offers no reduction in conflict points for a three-leg intersection, it results in a 6- to 12-percent decrease in conflict points for a four-leg intersection. The table below shoes the relationship between intersection legs, crossovers and conflict points:

A possible safety disadvantage is the unfamiliarity of drivers with the design. There are several counterintuitive design features of the DLT Intersection. These features could be expected to result in driver confusion in several ways including the following:

1. Drivers are familiar with making a left-turn maneuver at the main intersection. In the case of a DLT Intersection, the indirect left-turn occurs several hundred feet ahead of the main intersection. Even with adequate signage, this will require drivers to anticipate the left-turn in advance.

2. The counterintuitive design features of a DLT Intersection and the absence of turn movements at the main intersection can also lead to wrong-way movements. Wrong-way movements can be reduced by providing adequate signage and pavement markings.

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MULTIMODAL CONSIDERATIONS

Pedestrian Crossings:

The position of left-turn lanes between opposing through lanes and right-turn lanes presents pedestrians with an unfamiliar crossing scenario, and the DLT intersection’s wide geometric footprint can make it challenging to accommodate pedestrians as part of the traffic signal timing. To mitigate these issues, the design should include pedestrian islands (e.g., medians) to provide refuge, and the short cycle lengths associated with DLT intersection operations can help make pedestrian movements more comparable to crossing times at conventional intersections. Pedestrian crossing distances at DLT intersections are similar with those at large conventional intersections with channelized right turns. The design focus is to create crosswalks allowing pedestrians to move from the channelization to the outside of the intersection.

The image to the left illustrates pedestrian crossings with refuge islands:

Source: FHWA DLT Informational Guide – Exhibit 3-4

Bicycle Options:

Bicyclists traveling through on the cross road will conflict with right- turning vehicles trying to merge with traffic along the cross street. To address the bicyclist exposure at this location, consider a perpendicular bicycle crossing of the right-turn lane, as illustrated in the adjacent figure.

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Bicycle Left-Turns:

Another consideration for bicyclists making left-turns would be to install a bicycle box in the far-side pedestrian island for bicyclists to complete a left- turn through the main intersection.

Source: FHWA DLT Informational Guide – Exhibit 3-12

Potential Bus Stop Locations:

Transportation professionals can consider two strategies to place transit stops and need to work collaboratively with transit agencies to finalize transit stop locations. The first strategy would be at a mid-block location.

Source: FHWA DLT Informational Guide – Exhibit 3-14

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The second strategy would be at the main intersection.

Source: FHWA DLT Informational Guide – Exhibit 3-15

Source: FHWA DLT Informational Guide – Exhibit 3-16

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OPERATIONAL CONSIDERATIONS

Typical Signal Coordination: The multiple signalized intersections contained within a DLT intersection are usually coordinated so certain movements at separate intersections essentially operate during the same phase.

Source: FHWA DLT Informational Guide – Exhibit 5-6 In Step 1, the left-turn crossover upstream of the main intersection may give green to crossover vehicles at the same time the minor street movements occur at the main intersection.

By the time the crossover vehicles reach the main intersection, Step 2 (the next phase) will have begun, allowing the left turns to discharge at the main intersection.

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DESIGN CONSIDERATIONS

Crossover Intersection Spacing:

The distance between the main intersection and the crossovers generally ranges from 300 to 500 feet, but there are trade-offs to be considered:

• Short spacing may result in queue spillback and reduce the ability to clear queues through a single signal cycle

• Long spacing may be more difficult to coordinate signal operations Spacing greatly affects signal timing strategies and coordination between movements.

In general, a DLT approach with higher left-turn demand should have longer spacing.

Source: FHWA DLT Informational Guide – Exhibit 7-18

DLT Crossover Design:

The objective is to provide a smooth alignment for the through traffic.

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Align the left turns at the stop bar with the receiving lanes (for the displaced left turn pockets) to reflect desirable vehicle path alignment to minimize path overlap.

Source: FHWA DLT Informational Guide – Exhibit 7-3

It is also important to accommodate semi-trucks through the crossover and the left-turn lanes.

Left Turn Maneuvers

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Signalized Right-Turn Lane

DLTs Without Right-Turn Bypasses:

SIGNING, MARKING, SIGNALS & LIGHTING CONSIDERATIONS

• It is critical to provide sufficient signing, particularly for drivers on the minor road. It may not be intuitive for left-turning or through drivers on the minor road.

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• More guide signing is required than conventional intersection. Ex. Extra confirmation sign on each approach to confirm U-turn

• Of course, there needs to be signing to indicate the prohibition of left- turn and through movements from the minor road.

• Lane placement is also critical for minor road drivers; those utilizing the median U-turn should position themselves in the left most lane on the minor road.

An example of advanced signing for DLT is illustrated in the image below.

Source: FHWA DLT Informational Guide – Exhibit 8-6

Drivers have adjusted quickly to all three recent Restricted Crossing U-turn Intersection installations in North Carolina. Very few wrong way movements through crossovers have been observed at rural intersections and very few red light runners were observed at crossovers on U.S. Rt.17, and overall traffic seems to flow smoothly. One concern expressed prior to implementation for all three areas was that these were all areas with high concentrations of tourists and retirees who might be surprised or slower to adapt to new traffic patterns. Those concerns have generally subsided.

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Angular Arrow Signal Display:

An angular arrow signal display for the left-turn traffic at the crossover intersection could help guide motorists through the crossover intersection.

Pavement Markings:

For the general pavement marking concepts in the MUTCD, using the “KEEP CLEAR” pavement markings should be considered

Source: FHWA DLT Informational Guide – Exhibit 8-8

LEARNING OUTCOMES:

• Describe the features of the DLT intersection and its safety and operational advantages

• Describe the applicable conditions for considering the DLT

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Knowledge Check

TRUE or FALSE The DLT is a very effective intersection safety measure for rural low volume intersections

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MODULE 5: CONTINUOUS GREEN-T INTERSECTIONS Continuous Green-T

The continuous green T (CGT) intersection is an alternative 3-legged (or T intersection). CGT intersections are characterized by a channelized left-turn movement from the minor street approach onto the mainline (major street), along with a continuous mainline through movement that occurs at the same time.

The continuous-moving through lanes are not controlled by a traffic signal phase, while the other intersection movements are controlled by a three-phase signal. The through lanes on the mainline that have continuous flow typically contain a green through arrow signal indicator to inform drivers that they do not have to stop. The continuous through lanes are separated from the left-turn and merge lanes with delineators, curbed islands, pavement markings, or barrier.

The figure below shows a major street approach to the continuous through lanes of a CGT intersection.

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Several studies have consistently shown the operational benefits to implementing this intersection form at three-leg locations when compared with a conventional signalized T intersection. These benefits include reduced delay, fuel consumption, and emissions.

Two T-intersections in Grand Junction and Durango, CO were experiencing a high incidence of crashes, particularly angle crashes and many with injuries, due to limited stopping sight distance. In response, the Colorado DOT implemented CGT intersections to reduce the number of angle crashes, while also improving the efficiency of these intersections.

The rural intersection of US-50 and SH 141 in Grand Junction, CO (photo below) serves an annual daily traffic (ADT) of approximately 16,800. Prior to conversion, the east-west highway (US-50) had two lanes in each direction and a posted speed limit of 45 miles per hour (mph). The north-south highway (SH-141) had one lane in each direction and a speed limit of 35 mph. In 2004, the traffic control on eastbound (EB) US-50 was converted from a fully-signalized intersection to a CGT to eliminate the substandard sight distance problem. The two right lanes were designed to carry continuous through traffic while a separate left-turn deceleration lane was provided for exclusive left-turning movement onto SH-141 (the mainline). Similarly, an acceleration lane was provided from SH-141 onto EB US-50 (the stem of the T- intersection). The acceleration and deceleration lanes are channelized, and pavement markings and a 4-foot-wide concrete median separate the continuous through lanes. After the conversion of this intersection to a CGT, angle crashes decreased from 16 to 0; injury crashes decreased from 12 to 2; and total crashes decreased from 16 to 7.

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The rural intersection of US-160 and US-550 in Durango, CO has an ADT of about 30,000 and both the east-west highway (US-160) and the north-south highway (US- 550) have a posted speed limit of 50 mph. The traffic control at the intersection of westbound (WB) US-160 and US-550 was converted to a CGT in 1996 to reduce the number of injury and angle crashes. After conversion, US-160 (the main line) had a single through lane running WB with a separate deceleration lane; and two lanes running EB. US-550 (the stem of the T-intersection) had one acceleration lane for WB movement. Pavement markings and a 4-ft-wide median separate the continuous through lane and the adjacent left turning lane. After the CGT conversion, angle crashes decreased from 15 (including 1 fatality) to 1; injury crashes decreased from 8 to 4; and total crashes decreased from 19 to 7.

Recent research sponsored by FHWA investigated the safety performance of the CGT compared to conventional signalized T intersections using sites in Florida and South Carolina. Although the sample size was too small to produce statistically significant results, the study data suggested a potential reduction in crash frequency with crash modification factor (CMF) estimates for total, fatal and injury crashes as 0.958, 0.846, and 0.920, respectively. The potential reduction in crash frequency, along with the demonstrated traffic operation performance associated with the CGT intersection relative to the conventional T signalized intersection, makes the CGT a viable candidate alternative intersection form when conditions exist to effectively implement.

A major considered for implementation is the design for non-motorists. Anecdotal feedback indicates that pedestrians and bicyclists find it challenging to cross the continuous flow through lanes on the major street approach when traffic volumes limit the number or size of available gaps.

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Intentionally Blank

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Module 6- Diverging Diamond Interchange (DDI) What is a DDI and how is it different?

The Diverging Diamond Interchange (DDI) or Double Crossover Diamond (DCD) is a diamond interchange form that uses directional crossover intersections to divide and transpose the directions of traffic on the cross street within the interchange ramp terminals. Crossing the through movements to the opposite side replaces left-turn conflicts with same-direction merge/diverge movements and eliminates the need for exclusive left-turn signal phases to and from the ramp terminals. All connections from the ramps to and from the cross street are joined outside of the cross- over intersections, and these connections can be controlled by two-phase signals, have stop or yield control, or be free flowing.

What are the benefits of a DDI?

The DDI offers several potential advantages in comparison to other interchange types in terms of traffic operations, safety, and cost.

Improved Traffic Flow (Reduced Intersection Delay):

By separating the two directions of traffic on the crossroad and transposing them between the crossover intersections, the separate left-turn movements are eliminated from the signal phasing. Traffic signals at DDIs operate with two-phase intervals compared to three or more phases at conventional diamond or partial cloverleaf interchanges. This reduction in phase intervals improves overall throughput on the local road and for left-turning movements to or from the freeway. Simply stated - the signals at a DDI operate more efficiently and reduce delay.

Safety:

By transposing the directions of traffic on the crossroad between the crossover intersections, vehicles making left-turns at the on- or off-ramps do not conflict with vehicles approaching from other directions. Fewer conflicts means a reduction in the likelihood of a collision. The geometrics of a DDI encourage lower traffic speeds (desirably 35 miles per hour or less) which combined with fewer conflict points has proven to reduce the number of crashes and crash severity. DDIs also may reduce the number of conflict points between vehicles and non-motorized users. Pedestrian crossing distances are typically shortened at a DDI compared to other interchanges due to a reduction in turn lanes and because only one direction of traffic is crossed at a time.

Reduced Construction Cost:

In comparison to conventional diamond interchanges, where left-turning vehicles are stored between the two ramp terminals, DDIs require less storage because of the

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improved efficiency for handling left-turns onto the freeway entrance ramp. Since fewer lanes are needed to accommodate the traffic demands, the required structure width is reduced resulting in significant cost savings. At existing conventional diamond interchanges, it may be advantageous to convert the interchange into a DDI. Retrofitting to a DDI may be less costly than options involving widening the crossroad and the bridge structure.

What are the operational benefits?

The signals at a DDI utilize two-phase signal operations, which offers great flexibility to accommodate varying traffic patterns.

Source: MnDOT Traffic Signal Timing and Coordination Manual At traditional diamond interchanges, it is common to add “protected” left-turn phases as traffic volumes increase. Adding more phases essentially “steals” time away from the major through movement and can increase overall intersection delays.

Fewer phases is generally better! A common feature to many innovative intersection designs with signals is to reduce the number of traffic signal phases by strategically handling left turn movements differently.

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Overall operations of a DDI may be greater compared to a conventional signalized diamond interchange due to shorter cycle lengths, reduced time lost per cycle phase, reduced stops and delay, and shorter queue lengths.

A traditional diamond with signalized intersections typically operates with three phase intervals.

Because the signals at a conventional diamond are relatively close to one another, problems with demand starvation are common.

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What are the safety benefits?

Conflict points at intersections are defined as locations where the paths of various traffic movements and modes cross, including motor vehicles, bicycles, and pedestrians. The number of conflict points and the volumes of conflicting traffic can serve as a surrogate measure of intersection safety. There are also three different types of vehicle-to-vehicle conflicts: merging, diverging, and crossing. Crossing conflicts pose the greatest risk for higher severity crashes as they denote the potential location for the more severe angle collisions to occur. Although traffic control devices such as signals are intended to allocate the right-of-way for conflicting traffic movements, when drivers violate a traffic control device or make an error in judgement, a collision can result from the conflict. By reducing conflict points, the propensity for crashes is also reduced.

The diverging diamond interchange has fewer total conflict points compared to a traditional diamond, and most importantly, much fewer of the more severe crossing conflicts.

Field evaluations conducted by FHWA and state Departments of Transportation have shown very impressive safety benefits for converting interchanges into DDIs.

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An evaluation conducted by the Missouri DOT found that converting conventional diamonds into DDIs reduced total crashes by over 40% and reduced Fatal & Injury crashes by over 60%.

Report available at: https://library.modot.mo.gov/RDT/reports/TR201406/cmr15-006.pdf

When should DDIs be considered?

DDIs have been implemented in a variety of traffic demands. The DDI generally offers higher capacity for left-turn movements compared to comparable conventional diamond interchanges. A smaller structural footprint may be possible with a DDI compared to a traditional diamond since fewer lanes may be able to accommodate the movements under the more efficient signal operation. The DDI may be a more cost effective alternative for new interchanges because of the generally narrower structure widths required. The DDI can also be an excellent candidate for the retrofit of existing interchanges. Several DDI retrofit projects in the range of $3-5 million have been implemented by utilizing the existing bridge structures to keep project costs low.

With fewer conflict points and growing evidence on the safety benefits of DDIs, another reason for possibly considering the DDI is as a safer option than other interchanges.

Although DDIs have a broad range of conditions for which they may be a good alternative, they are not a universal “one-size-fits-all” solution that can work everywhere. Sound engineering and careful evaluation should be utilized to determine if a DDI is the best solution for the unique conditions at a given location. As is the case for most large transportation system investments, a comprehensive assessment to explore and identify viable alternatives is suggested. Oftentimes other alternatives may be available than can offer greater benefits at comparable cost or with reasonable additional expense.

When might DDIs not be a good option?

Close Adjacent Signalized Intersections

The interaction of DDIs with adjacent signalized intersections has perhaps been the most significant concern in operating DDIs. Close adjacent signals can be problematic for any interchange configuration; but is of great consequence to the DDI because the efficient two-phase signals allow higher vehicle throughput that can exacerbate operational problems at the nearby adjacent signals typically operating under less efficient multi-phase cycles. Adjacent signalized intersections that do not have sufficient storage for stopped traffic can create queue spillback into the DDI and possibly the exit ramps. To the public, the DDI could be deemed a failure and wasted effort when in fact the DDI is operating as intended, but the adjacent intersections are now the bottleneck.

Accommodating Frequent Over-Height Loads

The DDI form does not readily accommodate the occasional “up and over” maneuver of an over-height vehicle exiting the freeway to bypass a vertical clearance constraint created by a structure over the freeway to then enter immediately back on. Also, this type of movement may be used to reroute freeway traffic during bridge maintenance or emergency situations and would not be available with a DDI configuration.

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Constraints Along the Crossroad

To construct adequate channelization that achieves a sufficient angle of crossing within the directional crossover intersections, widening the footprint of the crossroad on the approach to the DDI may be necessary. This could create additional project costs and impacts compared to a conventional diamond. Reducing the footprint by using a shallow angle of intersection could increase the potential for wrong-way movements on the crossroad. At locations involving the retrofit of an existing interchange, designers must balance the trade-offs associated with minimizing right-of-way costs and project impacts with the safety and operational performance needs to develop crossover intersections that provide clarity to motorists for the proper direction for movements through the DDI.

Summary of Potential DDI Pros and Cons

Pros Cons

Operations Two-phase signals typically reduce delay and Challenging to coordinate signal progression increase capacity for through traffic in both directions

Ability to coordinate signals for the left turns Operational challenges at nearby adjacent from the freeway exit ramp can reduce intersections with more complex signal phasing potential for queue spillback onto freeway mainline

Safety Fewer conflict points (especially crossing Potential for wrong-way maneuvers at conflicts) crossovers

Observed crash reductions from field Unusual conditions for driver sight lines when evaluations making turns off exit ramps onto crossroad Non-motorized Users Fewer conflicts between vehicles and peds Using a center walkway may be unfamiliar to first time users Shorter pedestrian crossings Directions of vehicular traffic may be counterintuitive to pedestrians

All crossings can be signalized within 2-phase operations Ability to cross to other side of crossroad Opportunities to provide bike lanes or shared use paths Costs and Impacts Generally lower construction cost due to Some additional ROW width may be needed to narrower structures compared to other accommodate crossovers at tight diamonds interchange forms due to reverse curves

Retrofit projects may have opportunity to utilize May require relocating or removal of adjacent existing bridge structures streets driveways to accommodate crossover and reverse curves

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What is the history behind the DDI?

Source: www.divergingdiamond.com

In the Fall of 2000, Gilbert Chlewicki as a graduate student at the University of Maryland, developed a term paper on new intersection designs. Inspired by the unusual configuration of the former interchange in Maryland where I-95 meets I-695 northeast of Baltimore, Chlewicki thought about if an intersection design could use this configuration. Chlewicki sketched up a design that he called the "criss-cross interchange" and wrote the term paper on the concept. The next semester, Chlewicki wanted to further explore the design but his advisor suggested he consider renaming to something more technical (and pointed out the similarity to the 80's pop singer Christopher Cross). So Chlewicki renamed the design the "diverging diamond interchange" due to the multiple diverging points throughout the interchange.

After completing his spring semester, Chlewicki made a trip to Europe visiting his sister who was studying there. His last stop on a month-long tour was to Paris, France. While there, he took an excursion to the Palace of Versailles. As his tour bus left the motorway in route to the palace, Chlewicki noticed that they were entering an interchange that had the diverging diamond design! He stood up in the bus in total amazement and shock - happy that the concept he thought up was good enough to be in use, but also a little disappointed that his idea was not original after all. In fact, there are two other interchanges in France that utilize this concept.

Chlewicki was now confident that the theory behind the design was sound and analyzed the design using actual traffic volumes at selected locations in Maryland to compare operational performance. To his amazement, the innovative design exceeded his expectations in terms of how well the intersections operated. Chlewicki wrote a paper entitled “New Interchange and Intersection Designs: The Synchronized Split-Phasing Intersection and the Diverging Diamond Interchange” that he presented at the Transportation Research Board Urban Street Symposium in Anaheim, California in July 2003. Several people at the Symposium were very impressed with the presentation while others were skeptical.

One person who was impressed was Joe Bared, PhD, PE from the Federal Highway Administration (FHWA). Bared had specialized in investigating new designs and saw potential in the concept. So, after the conference Bared started to examine these designs even more. Bared evaluated the DDI under a high, medium, and low volume scenario and the results were even more promising with significant improvements over conventional designs at high volumes. The findings were published in a 2005 paper “Design and Operational Performance of Double Crossover Interchange and Diverging Diamond Interchange” that appeared the Transportation Research Record #1912. This paper further created awareness and excitement over this innovative design.

A few transportation departments across the country started to discover the DDI concept and started to explore it for application on a project. But, concerns over a new and unproven design led to several false starts.

Don Saiko, PE, a project manager in the Springfield, Missouri District of Missouri DOT (MODOT), got word of the DDI concept and wanted to investigate it as an alternative for

69 Innovative Intersections and Interchanges Participant Notebook improving the existing interchange at I-44 and Kansas Expressway (SR 13) which had been experiencing traffic problems and safety issues due to the limited left turn storage areas for the turning movements onto the freeway entrance ramps.

Prior conventional diamond configuration of I-44 and SR 13 interchange in Springfield, MO

The simulations for utilizing the DDI design looked very promising to fix the traffic and safety problems at this location. It was also a very cost effective solution with the DDI only costing about $3 million. The DDI alternative was going to utilize the existing bridge structure that had five lanes with no sidewalk that could be converted into a DDI with four lanes and a sidewalk down the median.

Because no major construction was needed on the bridge, the project construction time was only going to take 6 months instead of two years with the other options. The design plans were approved and construction started in January 2009. The first DDI outside of France (pictured to right) was opened on June 21, 2009.

The Missouri DOT then opened a second DDI approximately a year later also in Springfield at the interchange of US 60 and National Avenue. Later in 2010, the Utah DOT opened a DDI at the I-15 interchange with Main Street in American Fork.

The current number and locations of existing DDIs in the country are described on the following website: http://www.divergingdiamond.com/

For a comprehensive list of available references and resources, visit the FHWA web site at: https://safety.fhwa.dot.gov/intersection/innovative/crossover/

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When a DDI is being studied as an alternative, there are several key operational and safety considerations that need to be evaluated early in the analysis process. These considerations may greatly influence several geometric design elements, the signal design, and impact how the proposed DDI should be assessed or modeled in a traffic analysis. Some of these issues, such as queue spillback from adjacent signalized intersections, require that the preliminary assessments extend along the crossroad beyond just the immediate physical area of the interchange.

Meeting demands for operational and safety needs are common goals for interchange projects. Most transportation projects are developed with the objective of providing adequate highway capacity to meet the needs of future traffic conditions. Various procedures and models are used by transportation planners to develop future year traffic forecasts. Those forecasts are usually stated in terms of an average daily traffic, with conversions then made to derive a Design Hour Volume (DHV).

Traffic data, and specifically DHV, is the foundation for making many crucial project design decisions. The alternatives development process requires making some initial assumptions regarding the number of travel lanes and traffic control at intersections, and then assessing traffic operations to determine if those conditions satisfy the goals. This is done in an iterative process to then adjust the assumptions and reassess operations until the desired results are obtained. Improving operational performance can usually be achieved by adding more lanes or adjusting the intersection traffic control scheme. Adding more lanes may require expanding the “footprint” of the interchange and incur additional costs and impacts.

Great effort is made to develop optimal designs that satisfy the project goals based on the future DHV within cost and other constraints. However, designers need to recognize the potential variability in future traffic volumes and that actual conditions may not turn out as the future traffic models indicated. Better project decisions can be made if designers do not think of DHV as an “absolute”, but rather as a guide to determining the probable future traffic conditions the facility will be operating under. Evaluating alternatives over a range of DHV can offer a “sensitivity test” of the alternatives under varying traffic conditions. Using this approach to consider alternatives under a range of DHV rather than just singular values is suggested for all interchange projects, but is also advantageous when evaluating the DDI as an alternative.

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The figure below shows a conceptual relationship for using traffic analysis in a process to develop the initial design parameters for a DDI as an alternative to evaluate for consideration.

Signal Operations at a DDI

The traffic signal operations at DDIs is different from conventional diamond interchanges. At DDIs the signals operate with “split phasing” to allow both crossover movements to proceed independently. This presents some special considerations for the traffic operational analyses and signal design early in the alternative development stage.

At the signalized DDI crossover intersections, there is no “side-street” movement as the major street essentially intersects with itself. For any signalized ramp turning movements, they run concurrent with the same 2-phase operation for the through movements. Most DDIs operate in “pre-timed” rather than “actuated” control since there is a desire to achieve a level of traffic progression for both directions of traffic to the extent possible. A pre-timed signal assures that the green bands and progression opportunities are maintained across multiple cycles. Certain levels of “actuated” control at a DDI may be beneficial for potential conditions such as an unusually heavy movement that may need more time on occasion or an unusually low-demand movement that can gap out without much impact on progression. When a very heavy movement is combined with an unusually low movement in the opposing direction, actuated control may provide opportunities to enhance flow for the predominant movement. Actuated control is also appropriate for any “pedestrian-only” signals at the DDI (e.g. for pedestrians crossing the signalized left turn onto the freeway). If the pedestrian movement has low and/or intermittent demand, using actuated signals will reduce the delay to vehicular traffic.

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It is possible to control the signals at both crossover intersections with one signal controller with one ring in the NEMA timing sequence dedicated to each crossover. The figure below shows a simplified DDI signal phasing with Ring 1 alternating between two phases for one crossover while Ring 2 alternates between two phases for the other crossover.

The transitions can occur at different points in the cycle to improve progression between the crossovers. Assignment of the phase numbering can vary based on agency preference or capabilities of the controller. Any signalized right-turn and left-turn movements from the freeway would run concurrent with the nonconflicting through traffic.

Greater efficiency can be achieved by introducing additional phase intervals (such as dummy phases and overlap phases) into the timing sequence to minimize loss time during all-red intervals.

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Favoring Cross Street versus Exit Ramps

With the DDI two-phase signal operation, there are two basic options of either favoring the cross street through traffic or the left turns off the freeway. Favoring the cross street allows through traffic to pass through both crossover signals in one movement (no stopping between the crossovers). Favoring the exit ramps allows the left turns off the freeway to proceed through the downstream crossover intersection in one movement. While most DDI operations tend to favor the through traffic on the cross street, the option of favoring the exit ramps is available for sites or time periods of heavy freeway to arterial demand to reduce the queuing down the exit ramp.

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Progression along Crossroad

The directional nature of the signal operations at the two crossover intersections can impede achieving progression through multiple signals in both directions along the crossroad. Signal progression may be obtainable in one direction of travel. The signal offsets could be shifted to favor the opposing direction in other periods.

DDIs that operate with signal cycle lengths consistent with those of the adjacent signals 9that typically operate with more phases) may be hampered with a cycle length longer than needed and suffer from inefficiency. Using “half-cycles” at the signalized DDI crossovers may provide for improved progression by opening the green band more often and reduce driver and pedestrian frustration caused by long wait times.

When the spacing between the crossover intersections is long (1,000 feet or more), it may be possible to achieve coordination in both through directions on the crossroad through both crossover intersections for a reasonable bandwidth.

Exit Ramp Traffic Control

The turning movements from the freeway exit ramp at DDIs can operate with either signal or yield control (or possibly free-flow into an exclusive receiving lane on the cross street). Yield control may be an option under low to moderate volume levels when turns can be accommodated when the conflicting through traffic is stopped at the nearby crossover. When the turning volumes are heavier, especially when more than one turn lane is needed, signalization is typically used. In addition to volume considerations, there are several advantages to signalizing the turning movements off the exit ramps:

• Signal protection for pedestrian crossings • Lane changing (weaving) within the interchange area may be easier thus reducing conflicts caused by traffic turning at downstream driveways or intersections • Enhanced safety due to driver sight lines and expectations for the location of approaching traffic

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The last bullet item is particularly important, especially for considerations to signalize the exit ramp right-turn movement. At some DDI locations, it has been observed that drivers tend to look to the wrong side of the street for approaching traffic. This contributed to a high frequency of “failure to yield” crashes. The figure below shows the differences between where drivers may expect to look for oncoming traffic (based on expectations established from conventional diamonds) and where oncoming traffic is actually approaching from at a DDI.

If signalized, consideration for not allowing the right-turn-on-red (RTOR) should be made based on this same concern. If the right turn exit ramp movement is high, an assessment of the queue possibly backing up the exit ramp should be made.

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If the left turn movement is also signalized, the DDI allows an opportunity to use left- turn-on-red (LTOR) operations, provided state laws allow this maneuver and there are not safety concerns related to sight distance.

Lane Configuration / Utilization

The lanes along the crossroad at a DDI can be dedicated for certain movements or be shared to accommodate more than one movement. There are also choices regarding how lanes are added or dropped in the interchange area. These options can include having lanes dedicated for left or right turns, shared through/left lanes, or exclusive through lanes.

It is generally advantageous to add auxiliary lanes in advance of the crossover to increase capacity and provide better signing that positions drivers into the proper lane to reduce lane changing between ramps. Effective advance signing allows drivers to select appropriate lanes ahead of time, reducing lane changes and confusion through the interchange.

The figure below shows two different options for accommodating left turns onto the freeway entrance ramp. In the top example, the leftmost lane is shared for both left turning traffic and the through movement. In the bottom example, the leftmost lane is a dedicated lane to serve the left turn traffic demand. Traffic demand volumes for the various movements is obviously one major consideration when assessing how to allocate lanes along the crossroad, but there are other important considerations such as the type and location of advance guide signing, the utilization of shared lanes and the impacts of stopped vehicles in a shared lane.

In the top example, an important consideration is what happens when through traffic is stopped and queues begin to form. The ability for traffic to turn left will be blocked when the queue extends beyond the ramp terminal.

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Unbalanced lane utilization has been observed at DDIs with two left turn lanes onto the freeway entrance ramp. Turning drivers tend to preposition in one lane in advance of the interchange, often leaving the other lane under-utilized. The lane utilization is a function of the relative demands of the turning movements, the type of advance signing, and crossover spacing. Some of the lane imbalance is likely attributable to drivers choosing the left-most lane out of concern that the shared left turn and through lane may become backed up at the second crossover intersection.

Queue Spillback from Adjacent Signalized Intersections

The proximity of adjacent signalized intersections along the crossroad to the DDI crossovers may greatly impact the operational performance of a DDI. If an adjacent signal is too close and the queue storage length is inadequate, the traffic spillback may inhibit the movement of traffic along the crossroad, and potentially block traffic at the DDI crossover intersections and from the freeway exit ramps. Since DDIs operate with two- phase signals, they typically provide significantly higher vehicle throughput than the adjacent signalized intersections downstream that commonly operate with eight or more phases.

Modifications to adjacent signalized intersections along the crossroad may be necessary to maintain the overall signal progression along the corridor and reduce potential effects of queue spillback. Although this consideration is not unique to the DDI, the potential operational benefits of the DDI ramp intersections may be overshadowed by poor operational performance of nearby signals on the crossroad. In addition to adding capacity at the downstream intersections, other strategies for improving the traffic flow along the corridor include:

• Eliminating the adjacent signal by grade separating the intersection or making it right-in/ right-out only • Relocating the intersection shifting it further away from the DDI crossover • Reducing the number of signal phases at the adjacent signal

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Weaving Between DDI and Adjacent Signalized Intersection

Queuing at the downstream intersection may also prevent the traffic turning right from the DDI off-ramp to be able to change lanes (weave) along the crossroad to access the left-turn lane(s) of the downstream intersection. This can result in the vehicle remaining at the ramp terminus, blocking traffic behind until the maneuver can be accomplished.

Demand Starvation

Demand starvation occurs when a traffic signal indication is green, but not being utilized because traffic is held at an upstream signalized intersection. When demand starvation occurs, the actual vehicle throughput at the intersections is less than the potential capacity. At a DDI, demand starvation is most likely to occur when an adjacent signal controls the traffic able to approach the first DDI crossover. Typically, the inbound signal receives traffic at the first DDI crossover from three movements at the upstream intersection (mainline through, side street right turns and side street left turns). Since this adjacent signal typically operates with more phases (and therefore more lost time and less green for the inbound traffic to the DDI), the capacity of the inbound DDI movement is may be underutilized.

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Queue Storage between Crossovers

The spacing between the crossover intersections is a key consideration as this will impact signal design and operations. The skew angle between the crossroad and freeway can impact the spacing typically with larger skews needing greater intersection spacing. The figure below shows the distinction between the crossover distance (measured from center to center of the crossover intersections) and the queue storage distance (effective length to store vehicles between the stop bar and upstream left turn from exit ramp). For retrofit interchanges, these distances are usually governed by existing site conditions of the ramp alignments and other design constraints. For new interchanges, the crossover distance can be “designed” to optimize traffic operations within the site conditions.

The range of crossover distances on DDIs built to date typically fall between 400 and 1300 feet. Shorter distances between the crossovers have greater risk for operational problems due to insufficient queue storage. Longer distances ranging from 750 to 1400 feet may provide better operations and flexibility in signal operational schemes.

Insufficient queue storage within the crossovers may create situations where stopped traffic blocks the left turn movement off the exit ramp or “blocks the box” within the upstream crossover effectively “locking” traffic within the DDI. Consequently, the queue storage distance needs should be assessed and the DDI designed to provide storage for the critical movement. The Utah Department of Transportation DDI Guidelines suggest a minimum crossover distance of 850 feet (measured from center to center of crossover intersections) for new construction DDIs to provide adequate operational distance. Some studies suggest that DDIs with less crossover spacing have reduced capacity, while others indicate that shorter spacing (and therefore less travel distance between the crossovers) offers better coordination of traffic progression with better overall operations.

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Ramp Metering

Ramp meters are utilized to control on-ramp traffic demands to mitigate turbulence impacts on the freeway mainline. Those ramp meters can have significant impacts on arterial performance if on-ramp demands exceed the available storage capacity on the ramp; it is not clear at this point whether those impacts might be larger or smaller than standard diamond interchanges. As left turns and through traffic on the cross- street often share lanes within the DDI, any queue spillback from ramp metering can have severe impacts on arterial performance.

Pedestrian and Bicycle Safety

Historically, interchanges have been challenging to non-motorized users and many older existing interchanges were designed in ways that do not reasonably serve these modes. Retrofits of existing interchanges to DDIs can offer opportunities to improve these facilities and newly constructed interchanges can integrate elements that serve the many types of users at an interchange. In comparison to other interchange types, DDIs offer several potential benefits to pedestrians, including:

 Two-phase DDI signals better serve pedestrian movements by typically providing more crossing time per phase and/or a shorter cycle length can reduce pedestrian wait time  Shorter crossings since DDIs typically have fewer lanes for a pedestrian to cross at any crossing  Simplification of conflicts to crossing only one-directional vehicular traffic at a time  With fewer lanes needed for vehicles, retrofits can offer opportunities for installing pedestrian paths through interchanges that currently do not have them

Potential challenges to pedestrians at a DDI include the unfamiliarity for first time users due to:

 Altered travel paths if travel is down the center of the interchange (between vehicular lanes)  Traffic approaching from unexpected directions

The reduced number of signal phases can make it easier to serve non-motorized movements compared to a multi-phase signal. At multi-phase signals, the need to provide adequate pedestrian clearance may result in the pedestrian movement controlling the phase lengths, leading to longer cycle lengths and greater pedestrian delay. In contrast, vehicle movements typically control phase length at two-phase DDI signals, which can often provide more time per phase to serve pedestrians. Since DDIs typically have fewer lanes than other interchanges (no left-turn pockets within the interchange), opportunities may be available to install multimodal facilities in the existing footprint.

Although pedestrian crossings at the crossovers are signalized, pedestrian crossings of the turn lanes to and from the freeway may not be signalized. These potentially uncontrolled crossing locations require special attention and consideration.

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A fundamental challenge in developing any interchange design is deciding how to best provide for pedestrian and bicycle movements, and anticipating the desire lines between different origins and destinations for these modes (e.g., how they travel through the intersection or interchange). Forecast travel patterns and volumes for non-motorized users are rarely available. At interchanges, land use development can sometimes lag interchange construction, resulting in pedestrian and bicycle needs that are not apparent on opening day. But early consideration and provision for pedestrian and bicycle movements should be a priority consideration for any DDI, and should be accounted for in even early design concepts.

In general, pedestrian safety and comfort can be enhanced by:  Improving sight distances between drivers and pedestrians . Appropriately locating the crosswalks  Reducing vehicle turning speeds across the crosswalks . Design the channelizing islands with appropriate curve radii . Providing appropriate traffic control at ramp terminal

A vehicle-pedestrian conflict point is where a pedestrian walkway crosses vehicular travel lanes. The number and type of conflict points that exist at an interchange may be taken as a reasonable surrogate for expected safety performance. At DDIs, the number of vehicle-pedestrian conflict points will vary with whether the path is on the outside or down the center. An important distinction is whether the conflict point is at a location where vehicles are stopped by a traffic control device (such as signals) or are free- flowing. Pedestrian crossings of free-flowing vehicle movements can be particularly challenging for promoting proper driver yielding behavior and can greatly reduce pedestrian comfort and overall safety.

There are two fundamental choices for locating the pedestrian path – either on the outside perimeter of the crossroad or down the center median. Having the path run along the outside perimeter may place pedestrian crossings at locations with limited visibility and across potential free vehicle movements. Having the pedestrian path down the center median improves the line of sight for both pedestrians and vehicles and allows left turns to the entrance ramps to run freely without conflicting with pedestrian crossings. Also, by employing the median crossing strategy, pedestrians cross at the signalized crossover intersections, consistent with expectations of both drivers and pedestrians. For these advantages, the center median crossing option is generally preferable if conditions allow.

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Separated pedestrian crossings that relocate pedestrian movements away from the interchange altogether should also be considered. Even though separated pedestrian crossings are costly, they offer numerous advantages.

First time encounters with center walkways at DDIs may seem unusual since typical street design has pedestrian facilities placed outside of vehicular traffic. Also, at some DDIs the channelization islands separating the right- and left-turning movements can be quite large, and pedestrians need clear guidance on where they should and should not walk, and where they should and should not cross. Cut-through island designs should be considered to enhance positive guidance to pedestrians as to where the walkway and crossing locations are located. A cut-through walkway can guide the pedestrian directly to the intended crossing point and can be angled to support pedestrians in viewing oncoming vehicular traffic and potential conflicts. The cut-through walkway should be at least eight feet wide to comfortably accommodate pedestrians, including those with wheelchairs and other mobility devices. The actual curb ramp landing should be aligned perpendicular to the street centerline, which minimizes crossing distance and orients pedestrians to access ramps. As an alternative to the cut-through design, landscaping (grass or gravel) can be used to define the boundaries of the pedestrian walkway.

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The general principles for designing accessible intersections are also applicable to DDIs. Specific guidance for designing “Accessible DDIs” is not yet available, however, some special considerations are noteworthy so that practitioners can design DDIs to meet the needs of pedestrians with disabilities such as vision or mobility impairments.

- Provide accessible pedestrian signals (APS) with pushbutton locator tones at all signalized crossings - Locate push buttons to be accessible by wheelchairs and adjacent to the crossing with a minimum separation of 10 feet - Provide audible speech messages to communicate directionality of traffic (from left or from right) at all crossing points

The 2009 MUTCD provides specifications for pushbutton locations and for APS in sections 4E.08 – 4E.13. The APS devices for different crossings should be installed on two separate poles.

Potential wording after activating the APS push-button (see MUTCD 4E.13, par 9 & 10): "Wait to cross eastbound lanes of Airport Road. Traffic approaching from your left."

During the walk interval: “Walk sign is on to cross eastbound lanes of Airport Road.”

The principles for designing bicycle-friendly interchange areas place a focus on trying to minimize bicycle–vehicle conflicts, providing adequate lateral space between vehicles and bicycles, minimizing the speed differential between bicycles and vehicles, and managing any bicycle–pedestrian conflicts. There are three basic options for accommodating bicyclists through a DDI:

1. Provide a marked bicycle lane through the DDI (pavement markings can reinforce that the bicycle lane is not a shoulder) 2. Provide a separated bicycle path (or shared-use path) 3. Shared-lane bicycle accommodations (bicyclists use the vehicular travel lane) with optional signing and/or sharrow markings

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The choice of these options should depend on agency policy or the context of the facility and likely demand by bicyclists. Without a bicycle lane, experienced bicyclists are likely to use the vehicular travel lanes while recreational bicyclists are more likely to use the sidewalk. When bicycle lanes are provided, it is generally preferred to locate them to the right of motorized vehicle traffic, consistent with generally expected bicyclist behavior.

DDI Design Issues

DISCLAIMER: The information in this module should not be interpreted as a standard or rule. DDI design is an iterative process requiring the consideration and balancing of various objectives within the project context and site specific constraints. Ranges of typical design values are presented for several geometric elements, however, deviating from these values does not automatically create a flaw or unsafe condition.

Several key design elements of a DDI are interrelated, and the overall design should collectively consider the combinations of the related dimensions for application to specific sites. Appropriate choices for design elements such as design speed, reverse curve radii, lane widths, median widths, and other features will vary depending upon the context and constraints associated with the project. As with most modern designs, flexibility is encouraged to utilize design parameters appropriate for the conditions.

Design choices may also be influenced by whether the project is a retrofit of an existing interchange or new construction. In either case, the conditions and

89 Innovative Intersections and Interchanges Participant Notebook constraints of the project will influence the appropriate design choices. Right-of-way opportunities and constraints may greatly influence the practical geometric design options available to the designer. As is the case with most interchange projects, the design of a DDI necessitates the balancing of multiple factors including safety, capacity, and cost. This module will focus on the geometric design issues pertaining to developing the crossover intersections, perhaps the most defining characteristic of a DDI. The geometry of the ramp terminals with the cross street is also presented. The figure below summarizes the major components of a DDI interchange.

Source: Utah DOT DDI Guidelines

Overpass or Underpass

Design options for a DDI may differ based on whether the crossroad passes over or under the freeway facility. On retrofit projects and even most new interchanges, the decision to go over or under the limited access facility is essentially predetermined by existing conditions. For DDIs with the crossroad over, even when there is an existing bridge in place, considerations for a dual bridge design or single bridge on new alignment may offer great advantages for maintaining traffic during construction. For retrofit projects, the ability to use the existing bridge structure can offer great cost advantages. If additional lanes are needed within the DDI crossovers, it may be possible to utilize the existing bridge and build a parallel structure. Having a center pedestrian walkway is an option for both conditions, but may be more viable for an overpass DDI since columns do not present an obstacle.

Design Vehicle

An appropriate design vehicle should be selected based on agency policy for the location context of the DDI. The design vehicle will determine several DDI dimensions for accommodating swept paths and turning paths. Consideration for the assemblage

90 Innovative Intersections and Interchanges Participant Notebook of lane width choice, channelization configuration, and curve radii will determine if the operational needs of the design vehicle are met, especially at the left and right turn ramp terminals. AASHTO’s Green Book provides turning path templates for a variety of design vehicles.

Consideration for provisions for oversize vehicles should be given where oversized vehicles regularly use the interchange. It may be appropriate to select different design vehicles for different approaches and turns at the interchange. For example, there may be oversized vehicles travelling certain routes or directions through the interchange. Certain movements could be designed with these larger vehicles in mind with the remainder of the movements designed to serve the more frequent design vehicle size.

Design Speed

The DDI crossover intersections use channelization to divide and then transpose the local road traffic to the opposite side between the crossover intersections, and then cross back on the second crossover into the departure zone of the DDI. The channelization and curve radii used in the reverse curves approaching the crossovers should promote slower speeds and encourage the safe transition of vehicles into their proper directional movements. The selected design speed will control the minimum curve radii used through the crossovers. With the general desire for slower speeds through the crossover intersections, design speeds at DDIs are typically lower (≤35 miles per hour) than at conventional interchanges.

The FHWA DDI Informational Guide recommends a design speed on the crossroad that ranges from 25 to 35 miles per hour, to promote reduced and consistent speed through the crossover intersections. These relatively low design speed values are intended to

91 Innovative Intersections and Interchanges Participant Notebook provide a horizontal alignment of reversing curves at the crossover intersections with curve radii ranging from about 175 to 400 feet. It may be desirable to use horizontal curves with even tighter radii for turns at the ramp terminals near pedestrian crossings to lower speeds further.

Missouri DOT’s DDI experience document advises that for passenger cars moving through the crossover intersections a 20-30 mph speed is desirable without encroaching upon an adjacent lane. The document also recommends that for WB-67s, MoDOT’s typical designated design vehicle, that the crossover be designed to proceed through at 20 mph without encroachment and that all turning movements to and from the ramps be accommodated at 15 mph. MoDOT’s DDI designs use design speeds of about 25 mph through the crossovers.

The current practice of the Utah DOT for determining the design speed of the crossover area is to use a speed that is 10 mph less than the design speed of the approaching facility; however, the guideline doesn’t apply if the resulting design speed is less than 25 mph. This rule of thumb acts as a traffic calming measure, allows for more flexible signal operations, and permits smaller reverse curve radii. It also reduces sight distance requirements and increases progression potential.

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The existing DDIs that UDOT designed have crossover speeds ranging from 25 to 40 mph. It may be appropriate or necessary to lower the posted speed limit on the crossroad in advance of the crossover area or post an advisory speed with advance warning signs to advise drivers as they approach the crossover.

The reverse curves at the crossovers should have an appropriate combination of radius and length, as geometry that is too abrupt can make it difficult, especially for large vehicles, to maintain a natural driving path in their own lane. Providing a tangent alignment between the crossover intersections assists drivers in maintaining the desired vehicle tracking and the curve-tangent-curve sequence promotes driving at the desired target speed.

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Natural Path (Tangent) Through Crossover

Trying to minimize potential for vehicle path overlap within the DDI crossover intersection is similar to the principle applied at multi-lane roundabouts. A well-designed crossover with a path alignment that is natural for the driver can reduce the potential for driver confusion and improve the overall safety of the DDI. Utilizing a tangent before, through and after the crossover that connects the reverse curvature can help reduce the conflict that occurs when the natural path of adjacent lanes cross one another.

Sight constraints may limit the ability to achieve the desired length of tangent and crossing more than two opposing lanes can increase the challenge. The top priority is achieving a tangent through the crossover intersection itself with the next priority of trying to get 15-25 feet of tangent before the crossover.

For most crossover designs, a tangent of about 100 feet is satisfactory, with 15 to 25 feet of that tangent prior to the stop bar at the entry and another 10 to 15 feet past the crossover.

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If developing a tangent is not possible, the design should try to balance the reverse curvature by having the point of reverse curvature near the center of the crossover.

Crossover Angle

The crossover angle, or angle of intersection, is an important design criteria that is somewhat different for DDI design. The crossing angle is defined as the acute angle between lanes of opposing traffic within the crossover based on the tangent sections or lines perpendicular to the radii at points of reverse curvature. At traditional intersections, it is common for the intersection angle to be at or near 90 degrees, with general discouragement of intersection angles less than 75 degrees. However, for the crossover intersections at a DDI achieving angles even close to 75 degrees is not typically feasible, nor desirable. Although a large crossover angle may help the intersection to appear more like a “normal” intersection of two different cross routes (and decrease the likelihood of a driver making a wrong-way movement), greater crossing angles generally result in larger footprints not feasible in a DDI retrofit of an existing interchange. Also, larger crossing angles in combination with sharp reverse curves can increase the potential for overturning of vehicles with high centers of gravity and cause excessive driver discomfort through the crossovers.

The FHWA Informational Guide recommends an intersection angle of 45 degrees or more. The Missouri DOT experience document recommends using “the largest crossing angle possible while balancing each of the horizontal geometric crossover aspects” and indicates that MoDOT has used crossing angles ranging from 40-50 degrees. The Utah DOT Guidelines identify a 30- degree recommended minimum crossover angle.

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Lower crossover angles generally increase the risk of wrong-way movements on the crossroad. Angles as small as 25 degrees have been utilized due to space constraints. Designing an appropriate crossover angle within the constraints of the location is an example of where design flexibility is needed. Following the general principle of trying to attain the largest crossing angle feasible that is in balance with other geometric parameters and site constraints is the wise and prudent strategy. The crossover angle at most DDIs designed to date fall within the range of 30 to 50 degrees. As the number of lanes within the crossover intersection increases, greater crossover angles may be needed to develop a median wide enough to accommodate a pedestrian pathway.

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The design of the “eyebrow” portion of the channelization island separating the directional roadways near the crossover can help reduce the risk of wrong-way movements. Raised channelization islands with enhanced delineation, or installing barriers along the eyebrow have been utilized by some agencies. Channelizing islands with raised curb close to the travel lanes is the most common treatment for the eyebrow. Having shoulders between the travel lane and channelization island can negate some of the channelizing properties that direct drivers into the proper path. Therefore, shoulders are not suggested within the eyebrow areas. Raised barriers have been utilized by Utah DOT at some DDIs with lower crossover angles (approximately 30 degrees). Although no empirical evidence has been gathered, the barriers do appear to provide the benefit of enhanced channelization. Utah DOT recommends that these barriers be as low as possible (not to exceed 42 inches) to accommodate reasonable sight lines through the DDI.

An emerging concept in DDI design is a check of the theoretical “Direct Through Path” for drivers to go the wrong way at a DDI crossover intersection. This useful design check has similarities to the “fastest speed path” test used in roundabout design. This design check assesses the risk for an inattentive driver approaching the crossover to proceed in the wrong direction by examining if it is physically possible for a driver approaching the crossover intersection to ignore the pavement markings and proceed straight (without turning) going the wrong way into the opposing lanes on the cross street.

A well-designed eyebrow should break-up a direct through path and provide a physical obstruction discouraging a wrong way maneuver between the two opposing approach legs of the crossover. To check the wrong-way through-path, draw a line from the median curb of one direction’s approach to the median curb of the other direction’s approach (see figure below). Ideally, the pass-through distance (the distance between the eyebrow curb and the line), should be negative or a zero overlap. Even if there is a direct path, if it is only a small gap it should still make it rather obvious to drivers that they should not pass through.

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The “Pass Through Distance” (or lack of it), is directly influenced by the angle of crossing. With smaller angles of intersection, designs that block the wrong-way through path become more difficult to achieve. The median width, number of lanes, lane and shoulder widths will also influence. Achieving the desired negative Pass Through Distance (or overlap of the eyebrow curb into the direct wrong way path), requires an iterative design process that balances other geometric features to providing a well- designed transition through the crossover intersection.

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The use of sharp curb corners in the design of the eyebrow area of the channelization island (as opposed to rounded corners), may help emphasize to drivers that a turn should not be made. Painting the vertical face of the concrete curb white (to match the edge line color) along the eyebrow may also help improve conspicuity of the channelization and guide drivers into the proper path.

In summary, designing the geometric layout of the crossovers is more than simply laying out a series of reverse curves. A well-designed crossover requires consideration of the “sum of the parts” to include the crossing angle, length of tangent, and the eyebrow design. The appropriate “assemblage” of these crossover design elements is very site specific.

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The crossover angle also influences the sight line for drivers turning onto the cross street from the exit ramps. The crossover intersections create a situation where the location of approaching traffic is not where drivers would typically expect and drivers may instinctively look down the wrong direction for a gap. The crossover angle influences the angle at which drivers must look for approaching traffic, as shown in the figure below. It is desirable to design the exit ramp intersection with the crossroad such that from the driver’s perspective, the location of approaching traffic is in the same general view location as a typical intersection. An angle close to 90 degrees is preferable, with a maximum of 110 degrees in consideration of drivers having difficulty turning their neck.

The approach angles and sight lines for each movement should be considered independently since each has a different conflict with opposing movements. If yielding, RTOR, or LTOR options are being considered at a DDI off-ramp, more acute angles should help with proper sight lines when looking upstream. Acute angles reduce the potential for drivers to look down the wrong approach but can produce undesirable viewing angles depending on the vehicle type and where the driver stops.

The geometry and sight lines for the ramp terminal intersection with the crossroad should also be consistent with the type of traffic control used and the design of the pedestrian crosswalk across the ramp (if one exists). Because this issue is of critical importance for the design of the pedestrian crosswalk, it is covered in detail in the next module.

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Alignment Alternatives

Although most DDIs constructed to date have utilized a symmetrical alignment, there are other alignment options that may be more favorable under various conditions or to accommodate certain constraints. One option is to shift the alignment of one of the cross street travelled ways asymmetrically to achieve the crossover angle. This option may be advantageous for situations that use the existing cross road bridge while a parallel bridge is constructed for the DDI.

In a symmetrical alignment, the reverse curves and providing tangents between the reverse curves increases the overall spacing requirements between the crossovers. Eliminating some of the curvature may allow shorter spacing and reduces driver “work load” through the DDI (possibly allowing faster speeds on the crossroad). With a wider median, the degree of reverse curvature can also be reduced. In new construction or reconstruction where sufficient width exists, a wider median between crossover intersections can minimize the alignment deflection (amount reverse curvature) and the number of curves.

The alignment options used to develop the crossover include: symmetrical alignments using reversed curves; offset alignment (where one direction is held basically as-is and the other is deflected with curves); and shifted alignments (where both directional alignments move sideways to perhaps avoid a constraint or better facilitate staged construction). The considerations for determining the best suitable alignment for a specific site include: • Constraints for needing to minimize the cross-section under or over a bridge (common in retrofit situations) • Desire to minimize the distance between crossovers or match existing ramp spacing on the crossroad (new and retrofit situations) • Desire to minimize the amount of reverse curvature • Right-of-way constraints and constructability issues

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Lane Width

Lane width decisions are typically based on accommodating the needs of the design vehicle. Oftentimes the lane width at the crossovers will be wider to provide for large vehicle off tracking through the curvature. The degree of vehicle off tracking is based on the horizontal geometrics such as curve radius, crossover angle, and length of tangent sections. Design models for vehicle turning paths should be used to assess potential off- tracking of the design vehicle through the site-specific curvature. Generally, curves with smaller radii will need larger lane widths for the crossover (perhaps 15-16 feet) while designs with modest curvature may allow lane widths ranging from 12-14 feet. Most DDI designs to date have widened out the lane width for the crossover to 14-15 feet.

Designers should consider the project context when making decisions on lane width. When considering vehicle off-tracking, normally the design should allow for all passenger cars to stay in their own lane through the crossover. If truck volumes are light, consideration can be given to the degree of off-tracking expected (encroachment into adjacent lane) and perhaps accepting a modest degree of occasional off-tracking. When heavy truck traffic is present, wider lane widths are desirable and consideration must be given to the potential for two trucks to navigate side-by-side through the interchange.

The normal lane width along the crossroad (11-12 feet) is typically tapered wider (14-15 feet) prior to, and just after, the crossover curves in consideration for the design vehicles to travel side by side through the crossover area. Lane width decisions should depend on site context. Any additional lane width should be tapered out after the curvature and not continued between the two crossovers.

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Shoulders

Shoulder considerations are different at DDIs. On most roadways, when a vehicle needs to pull over, driver expectation is to use the outside shoulder (along the driver’s right- hand side). However, within the crossover intersections at a DDI the right-side shoulder becomes the “inside” shoulder. Within the two crossover intersections, it is generally not desirable to have the “outside” (left-side) shoulder serve as the refuge area. The left lanes between the crossovers are used for left-turn movements and potentially experience more traffic and weaving. While it is not desirable for vehicles to pull over between the crossovers, the right (or inside) shoulder is generally considered the better place to do so if needed.

If shoulders are to be provided within the crossover intersection, it is recommended to maintain consistency in the right and left side shoulder widths even though this is contrary to what may be thought of as the “inside” and “outside” shoulders.

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Providing a shoulder at the eyebrows of the crossover intersection may not be desirable since they could increase the wrong-way “Pass Through” gap. Having a narrow, or no shoulder, between the right-most lane and the raised curbing of the channelizing island may better guide vehicles through the crossover and discourage wrong-way movements.

Shoulder width should also be considered in conjunction with the provisions for bicycle movements. Cyclists always have the option to share the lane with motor vehicles, and because of the slower vehicular speeds at the crossovers, may be an option chosen by cyclists at a DDI more so than other interchange types. Marked bicycle lanes can also be provided through a DDI and is discussed in the next module. Having the right-side shoulder area for bicycle movements is also an option. Bicycle movements through a DDI are similar with motor vehicle traffic in that they perform the same crossover movements as other vehicles.

Marked bicycle lanes can be provided through a DDI with bicycle movements the same as motor vehicle traffic performing the same crossover maneuvers.

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Vertical Alignment

When the crossroad bridges over the freeway, the vertical profile of the bridge structure will influence the sightline drivers have approaching the downstream crossover intersection.

It is desirable that drivers have an unobstructed view of the upcoming curvature and the traffic signals. Insufficient sight distance to the curvature and signals could contribute to sideswipe and rear-end collisions. As a minimum, the crest vertical curvature should provide stopping sight distance (SSD) at all points along the cross road.

If visibility to the traffic signals is restricted, consideration for installing supplemental signal heads should be considered.

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Signing, Markings, Signals & Lighting

The proper application of signing, markings, signals and lighting is critical to move drivers correctly through a DDI. The Manual on Uniform Traffic Control Devices (MUTCD) is the primary source for guidance and standards related to traffic control devices in general. This module contains information and guidance to apply existing MUTCD content for application to DDIs. The MUTCD (or its state-level equivalent or supplement) should be the primary and overruling source for traffic control device guidance. No special traffic control devices beyond what is currently contained in the MUTCD is needed (or appropriate) for DDIs.

Source – UTAH DOT DDI Guidelines

At several of the initial DDIs implemented, concerns over the unusual traffic pattern at a DDI perhaps led to choices to install numerous signs and an abundance of pavement markings. However, too many signs and too many markings can lead to driver confusion. Careful attention is needed to design traffic control devices that provide the critical driver information but balanced with the need to not overwhelm the motorist with excessive less important information.

Guide Signs

Practices regarding the use of guide signs along the crossroad facility vary greatly. Ideally, guide signs should provide information to drivers well upstream of any queues allowing for good decisions on lane choice and to maintain lane discipline through the DDI. Guide signage is commonly provided overhead using a gantry or mast arm system, but in some locations using ground mounted signs may be appropriate. Choices on guide sign design should consider the preview distances that drivers need to make proper lane choices prior to entering the DDI and the contextual conditions of the crossroad such as approach speeds, number of lanes, and possible obstructions to a driver’s view.

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Regulatory Signs

Regulatory signs are intended to instruct road users on what to do (or not do) under the given set of circumstances. Common regulatory signs used at DDIs include: - Turn prohibition signs - Keep Left, Keep Right signs - Lane Control signs - Do Not Enter and One Wat signs

Do Not Enter signs are used at some DDIs to supplement the No Right and Left Turn signs and Stay Left and Stay Right signs installed at the crossovers. Wrong-Way signs can also be installed to supplement Do Not Enter signs related to the crossovers and exit ramps. In some instances, the Do Not Enter sign was installed with a One-Way sign at the exit ramp left turn. Per the MUTCD, Wrong-Way signs are supplemental to Do Not Enter signs.

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Warning Signs

Warning signs call attention to conditions on, or adjacent to, a highway or street that are hazardous to traffic operations. These signs are used particularly when the hazard is not obvious or cannot be seen by the motorist. The signs covered in this section include: - Advance traffic control, Yield Ahead (W3-2) and Advance Signal Ahead (W3-3) - Reverse Curve (W1-4) - Lane Split (W12-1)

Pavement Markings

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Signal Design Sight distance to the signal heads is a very important consideration for the placement of signals. There are generally two strategies for signal placement at the crossovers. One strategy is to install the signal on the back side of the crossover, which could offer enhanced guidance into the appropriate lanes, but it may be difficult to see the signals from a distance along the cross street due to the crossover curvature. Supplemental near side signals can be used to alleviate this issue. Supplemental signal heads are recommended when the visibility of the overhead-mounted signal heads at the crossover is limited to approaching traffic due to the horizontal and/or vertical curvature. For the outbound movement, the supplemental signal head is typically installed on the left-hand side of the street.

The second strategy is to install a single mast arm with signals in both directions. This option may provide better signal visibility to motorists when they approach the crossover. However, a mast arm assembly of one approach can block another assembly at a different approach.

Green up arrows can be used to clearly indicate the proper direction of travel and reinforce that no turns are permitted at the crossover intersections.

There are options for the signal head displays: - Red, yellow, and green balls - Red ball, yellow and green arrows - Red, yellow, and green arrows

Lighting Design

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Module 7: Assessment & Evaluation Process Learning Outcomes

 Define Intersection Control Evaluation (ICE)

 Describe a potential process for assessing and evaluating intersection alternatives

Transportation planners and designers should utilize an open and consistent process for developing and evaluating alternative designs for intersections and interchanges.

Historically, many intersection problems were “solved” by installing a traffic signal or adding more lanes.

Intersection Control Evaluation (ICE) is a performance-based framework that utilizes a consistent and transparent process and objective metrics to analyze intersection geometry and control alternatives and consider all users. ICE aligns with promoting innovative intersections and interchanges.

The Minnesota Department of Transportation (MnDOT) first implemented ICE in 2007, and several more States have developed ICE policies since then. Although ICE policies differ among States, each shares several similar attributes that seek to determine the “best value” geometric design and traffic control for a given intersection weighing safety, operational, multimodal, environmental, ROW, and cost. The process involves giving design alternatives a preliminary screening and providing a short list of alternatives with the highest potential effectiveness that are carried forward to help shape an appropriate project scope.

Components of an ICE Process:

 All reasonable alternatives are considered with an initial screening for feasibility  Technical Analysis . Safety . Capacity/Operations . Costs – Benefit Costs . Right of Way impacts  Public/Political Considerations . Documentation of preferred alternative in report subject to available funding, other projects, project readiness, etc.

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ICE fits within the existing project development process

Several key operational & safety considerations need to be evaluated early in the analysis process since they may greatly influence geometric design, signal design, and impact how to properly model traffic.

Step 1 – Define the Problem

Having a clear understanding of the current problems is fundamental to developing project alternatives.

“What are the problems and what factors are contributing to these problems?”

Potential sources for problem reports:

 Local resident complaints  Safety analysis of high-crash locations  Observations from agency field staff  Observations from law enforcement agencies  Developers seeking traffic control for proposed new intersections  Potential problems based on future traffic projections

Gather information to quantify the existing (or anticipated) extent of the problem:

 Crash data  Traffic volumes  Field observations

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 Roadway conditions  Traffic control conditions

Step 2 – Establish Objectives

Specific objectives or goals for the project should be established based on input from the stakeholders. The established objectives allow for a basis of comparison among alternatives and for the prioritization and weighting of factors for different alternatives. For example, at an urban intersection near activity centers, pedestrian mobility and safety could be assigned significantly higher priority compared to an intersection improvement project at a remote rural location in an undeveloped area. If stakeholders identify an overriding objective, such as pedestrian mobility, then the full range of intersection alternatives should be assessed first (screened) with respect to this criterion. This helps simplify the initial screening of alternatives to assess how an alternative satisfies (or does not satisfy) that objective.

Step 3 – Establish Key Design Parameters

• Roadway Context Classification

• Design Hour Turning Movement Volumes

• Control & Design Vehicle

When considering intersection alternatives, integrate pedestrian, bicycle, and transit needs at an early stage of the project planning process. Characteristics of different intersection options produce variations in the physical geometry and traffic control schemes which can introduce both benefits and challenges to pedestrians, bicyclists and transit users.

Step 4 – Identify Key Constraints

• Right-of-way limitations and opportunities

• Major utilities / Railroads

• Access issues to adjacent properties

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Step 5 – Initial Screening

Conduct a planning level OPERATIONAL ANALYSIS

• Critical phase volume method

• Capacity Analysis for Planning of Junctions (CAP-X)

• VDOT Junction Screening Tool (VJUST)

• Highway Capacity Manual (HCM) Analysis

Conduct a planning level SAFETY ANALYSIS

• SPICE

• Conflict Point Analysis

• Surrogate Safety Measures

Step 6 – Detailed Traffic Analysis

While it may be desirable to conduct a traffic simulation of all alternatives, often there is neither the time nor the budget to do so. Consequently, an initial screening of which alternatives to advance based on sketch level analysis tools is more efficient. A more detailed analysis of the most promising alternatives can then be conducted.

Step 7 – Life-Cycle Benefit Cost Analysis Historically, comparisons of alternatives with regard to cost have been based on lowest construction and right-of-way cost and expedient design. However, many transportation agencies are increasingly considering costs throughout the life of the project in making investment decisions. Intersection design can significantly impact the maintenance and other costs that accrue after construction. In addition to direct agency costs, societal costs are significant at intersections - particularly crashes, delays, and emissions. There can also be economic impacts on nearby businesses.

A life-cycle cost analysis is useful in bringing together the multiple factors that extend beyond initial construction cost that are important to consider in the design of the intersection. A life-cycle cost analysis can greatly help inform the design decisions related to an intersection.

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Interpreting “Value”

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Learning Outcomes

 Define Intersection Control Evaluation (ICE)

 Describe a potential process for assessing and evaluating intersection alternatives

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