Grade Separation

Adding Capacity Grade Separation GRADE SEPARATION Description Intersections handling a high volume of traffic and pedestrians (and possibly railroads) limit the capacity of the approaching roads. Grade-separating these conflict points allow an uninterrupted flow of traffic while also eliminating the safety threat posed by trains, pedestrians, or other vehicles. Three primary roadway improvement objectives are accomplished using grade separated intersections: . Increased capacity and uninterrupted flow. Increased safety. Reduced vehicle-train conflict and delay. Cost: Time: Moderate Overpasses increase the capacity of a roadway Impact: Spot by allowing uninterrupted flow in all directions. Who: City/State The intersection approaches do not come in direct contact but rather bypass each other. Hurdles: Funding Fewer signals are required to direct traffic, eliminating the queues caused by signals. Target Market Grade separation increases roadway safety by Arterial Intersections with High Traffic Volumes reducing the vehicle-vehicle and vehicle- Intersections are a large cause of congestion on pedestrian conflicts. The crossing traffic is arterial streets. Signal time given to each removed from the intersection, thus eliminating direction dramatically decreases a road’s the possibility of collisions between those capacity, increasing the possibility of congestion streams of vehicles. Pedestrians are given and queues. This planned stop-and-go condition greater protection from cars, as there will be reduces safety and increases travel time for all only one line of traffic to cross and more refuge drivers. Elevating one of the streets will reduce points can be provided at multiple locations. the conflict caused by intersecting roadways. Removing at-grade intersections with railroads The reduced interference will increase the road substantially increases speed for both trains and capacity. cars. Street traffic moves freely over or under Railroad Crossings railroad tracks, reducing wait times for a passing Railroad crossings pose a threat to the safety of train and increasing travel speed and capacity of street and rail traffic. At-grade crossings, the roadway. Most importantly, the incidence especially near intersections, create numerous rate of train-vehicle collisions is eliminated as conflict points for cars, trains, and pedestrians. the intersection no longer puts traffic in front of These crossings force both cars and trains to trains. reduce their speed, increasing travel time and congestion for cars and decreasing overall efficiency of the rail network. For more information, please refer to: http://mobility.tamu.edu/mip/strategies.php. How Will This Help? A project to install grade separations at railroad Grade-separated intersections substantially crossings has been proposed for FM 306 in New increase capacity by eliminating delay caused by Braunfels. A local citizens group1 states that the previous intersection or railroad. Traffic there are 32 daily trains that cross FM 306 at moves freely and any needed signal timing can grade, and these trains often cause significant be increased by the lack of a traditional delays, sometimes blocking the crossing while intersection, as signals may only be necessary they stop or back up. The FM 306 corridor not for accessing the exit and entrance ramps of the only serves local traffic, but it also is a school bus interchange. route that includes 98 daily school bus crossings over the tracks. Additional crossings by Elevating one portion of a street or rail crossing emergency response vehicles are also affected by improves safety by eliminating vehicle, train, and the railroad tracks. Local citizens worked with pedestrian conflicts. Crossing traffic is elected officials and TxDOT to propose two minimized; trains are separated from the grade separations at railroad crossings on this roadway; and pedestrians cross traffic less corridor; the project is tentatively scheduled to frequently—all decreasing the likelihood of a let in August 2012.2 collision. The American Association State Highway and Transportation Officials (AASHTO) Application Techniques and Principles Highway Safety Manual reports that converting Grade separated intersections should be used an at-grade, four-leg intersection to a grade- when other techniques (e.g., signal timing, lane separated interchange reduces injury crashes by additions) are unsuccessful in reducing 57 percent. Converting a signalized intersection congestion to an acceptable level or were into a grade-separated interchange reduces impossible due to external constraints. Grade injury crashes by 28 percent. separation works well when one or more directions experience heavy volumes and high Implementation Examples levels of congestion even after other methods Texas: Two examples of grade separations can (such as signal optimization or additional turn be found on US 281 north of San Antonio, at the lanes) have been exhausted. intersections with SH 46 and FM 1863. Aerial images of these locations are provided in the The congestion on all approaches of an figures below. intersection should be examined to find the best grade separation solution. For example, if These two locations provide mobility for congestion primarily occurs on one cross street, through traffic on US 281 while still permitting a two-level interchange is recommended. access to and from other roads on the state highway system. US 281 @ SH 46 (Google Earth) US 281 @ FM 1863 (Google Earth) For more information, please refer to: http://mobility.tamu.edu/mip/strategies.php. If congestion is significant on all approaches, a intersections are large. Road-user costs, three-level interchange better provides the such as fuel and oil usage, wear on tires, desired capacity. If the adjacent intersections repairs, delay to motorists, and crashes are also congested, a grade separation may only that result from speed changes, stops, move the problem, rather than solving it. and waiting, are well in excess of those for intersections permitting For road/rail intersections, a simple overpass uninterrupted or continuous operation. reduces the delays felt by both traffic and trains. Cars and trains lower their speeds when 6. Traffic volume warrant – A traffic approaching at-grade intersections. The lack of volume warrant for interchange interaction will increase speeds through these treatment may be the most tangible of intersections and eliminate the possibility of any interchange warrant. Although a collisions. specific traffic volume at an intersection cannot be used as the sole criterion to The AASHTO Green Book3 describes six warrant an interchange, it is an conditions, or warrants, that should be important variable, particularly when considered when determining if a grade combined with the traffic distribution separation or an interchange is justified at a pattern and the effect of traffic behavior. particular site: 1. Design designation – The Not all warrants for grade separations are determination to develop a highway with included in the warrants for interchanges. full access control between selected Additional warrants for grade separations locations becomes the warrant for include grade separations that would: providing grade-separations or . Serve local roads or streets that cannot interchanges for all intersecting practically be terminated outside the roadways crossing the highway. right-of-way limits of freeways. 2. Reduction of bottlenecks or spot . Provide access to areas not served by congestion – Insufficient capacity at the frontage roads or other means of access. intersection of heavily traveled routes . Eliminate a railroad-highway grade results in intolerable congestion on one crossing. or all approaches. Serve unusual concentrations of 3. Reduction of crash frequency and pedestrian traffic (for instance, a city severity – Some at-grade intersections park developed on both sides of a major have a disproportionate rate or arterial). frequency of serious crashes. If . Serve bikeways and routine pedestrian inexpensive methods of reducing crashes crossings. are likely to be ineffective or impractical, a grade-separation or interchange may . Provide access to mass transit stations be warranted. within the confines of a major arterial. 4. Site topography – At some sites, grade- . Provide free-flow operation of certain separation designs are the only type of ramp configurations and serve as part of intersection that can be constructed an interchange. economically. 5. Road-user benefits – The road-user costs from delays at congested For more information, please refer to: http://mobility.tamu.edu/mip/strategies.php. Issues project typically takes place over multiple years, Funding these relatively large projects remains to allow for appropriate design, right-of-way the single biggest implementation issue, though acquisition, any needed environmental the benefits are often well worth the cost. clearance, securing necessary funding, and Finding locations with available right-of-way and construction phasing. As mentioned above, the public approval sometimes proves difficult, FM 306 project is tentatively scheduled to let in especially in densely-developed areas. Sufficient August 2012,2 and the duration of the right-of-way should be acquired as the construction period has not been specified. The intersection is first built; acquiring additional concept of the project has been discussed since land dramatically increases cost. 2008, and design work took place in 2010.1 The acquisition of needed rights-of-way

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