CPHX TECHNICAL MEMORANDUM: ASSESSMENT OF ALTERNATIVE IMPROVEMENT STRATEGIES Note: This document presents a planning level assessment of the feasibility of various improvement strategies for consideration when developing MAG’s NextGen RTP. The RTP process would include further technical evaluation and vetting of the strategies with stakeholders and the public. © All Rights Reserved, 2010 TECHNICAL MEMORANDUM Central Phoenix Transportation Framework Study - Assessment of Alternative Improvement Strategies 1.0 Overview A preliminary evaluation system was developed for identifying potential locations in the Central Phoenix Transportation Framework (CPHX) study area for: . Conversion of existing interchanges to diverging diamond interchanges (DDIs) . New direct ramps facilitating access to/from high-occupancy vehicle (HOV) lanes at strategic locations . New arterial/freeway overcrossings/undercrossings at strategic locations . Two-lane (choice lane) exit ramps The evaluation system is intended to be a planning-level evaluation tool that is flexible enough for application to the entire freeway network. More detailed evaluations can be added in future iterations. The evaluation system was developed in three steps: . Identifying desirable characteristics for improvement strategies locations . Developing the approach for identifying and evaluating specific locations . Applying the framework and summarizing the results. Each step is described in the sections below. 2.0 Improvement Strategies 2.1 Diverging Diamond Interchanges DDIs are a variant of the standard diamond interchange, which has two signalized intersections with each serving a separate on/off ramp and FIGURE 1 TYPICAL TIGHT URBAN DIAMOND INTERCHANGE crossroad movements. Signal control is developed to minimize the (TUDI) number of vehicles standing between ramp signals, and signals generally are managed together as one signal. The tight urban diamond interchanges (TUDI) design is a compressed diamond interchange (Figure 1). The signalized intersections are more closely spaced and traffic flow, clearance times, and turning speeds are similar to a conventional at-grade intersection. Typically, a four-phase signal control procedure with overlapping is required at both intersections. The TUDI is most beneficial in urban and suburban areas where right-of-way (ROW) constraints exist and simplicity of operation is important, as it is easier for drivers to navigate. Unless otherwise indicated, all references to diamond interchanges in this paper mean the TUDI design. The DDI directs traffic on the crossroad to the opposite side of the road between the ramp terminal intersections (Figure 2). This design allows left-turning traffic on the crossroad to proceed without conflict with opposing traffic flows. Because no turning movements are allowed, this interchange operates with two-phase signal control. The two-phase signal control reduces delay time and results in a substantial increase in capacity. The DDI design works best where there are heavy left-turn movements onto or off freeway ramps and/or if thru movements are unbalanced during peak hours. The potential benefits of DDIs are as follows: 1 CENTRAL PHOENIX TRANSPORTATION FRAMEWORK STUDY . Simple/unopposed left- and right-turns from all directions, specifically including a “free-flow” left-trun from the crossing FIGURE 2 1 TYPICAL DIVERGING DIAMOND INTERCHANGE street to the freeway (DDI) . Increases left-turn lane capacity without needing more lanes . Better signal network synchronization; only two phases needed and shorter cycle lengths result in reduced delay . Lanes with multiple assignments in all directions . Better storage between the ramp terminals . U-turns from highway are accommodated well . Fewer conflict points . Better sight distances. For existing interchanges, DDIs have additional benefits related to retrofitting: . Existing bridge generally can be used . Additional right-of-way rarely needed (for standard diamond interchanges) . Offers the opportunity to add bus crossover lanes on the left-turn on-ramp to facilitate direct access to/from P&R/Transit Center located adjacent to the freeway . Construction time is reduced, as no bridge replacement is required . Maintenance of traffic is simplified during construction, as removal of the existing bridge is not required. Single-Point Urban Interchanges (SPUIs) have only FIGURE 3 one signal controlling all ramp and crossroad traffic TYPICAL SINGLE-POINT URBAN INTERCHANGE (SPUI) movements (Figure 3). A three-phase signal control commonly is used for the SPUI, accommodating leading left-turn and through movements for the crossroad and left-turn movements for the ramps. Right-turn movements for the ramps can be signal or yield controlled. The SPUI design reduces the number of conflict points to 24 compared with a TUDI, which has 30. The DDI notably only has two traffic crossing points (at the signals); therefore, traffic conflicts points are reduced to 14. The DDI increase in value as traffic volume increases, especially ramp volumes. This design is most beneficial when arterial volumes are lower and ramp volumes are heavy. A comparative study evaluated the DDI relative to the TUDI and SPUI utilizing Synchro 7, SimTraffic, and VISSIM 4.2 traffic modeling software. The study tested various traffic volume scenarios and resulted in the conclusion that “…the DDI far outperforms the diamond interchange and the SPUI under all scenarios, even with fewer lanes.”2 1 It is important to note that all turning movements may encounter the need to yield, depending on the number of lanes provided and presence of pedestrian/bicycle traffic. However, it is critical for the left-turn to be “free-flow,” as this is the defining concept of the DDI and its principal advantage. Refer to Missouri’s Experience with a Diverging Diamond Interchange, Lessons Learned, Missouri Department of Transportation, Rpt # OR 10-021, May 2010. 2 A Comparative Analysis of Diverging Diamond Interchange Operations, Steven B. Speth, PE, Oregon ITE. TECHNICAL MEMORANDUM ASSESSMENT OF ALTERNATIVE IMPROVEMENT STRATEGIES 2 CENTRAL PHOENIX TRANSPORTATION FRAMEWORK STUDY 2.2 Direct HOV Ramps Ramps providing direct entry to and exit from High-Occupancy Lanes (HOV) lanes (or main freeway lanes) of a freeway are referred to as Direct HOV (DHOV) ramps. Entry to the HOV lanes (or main lines) via DHOV ramps usually is accomplished as a left-on movement into traffic from the freeway center or median; exit to DHOV ramps from the HOV lanes (or main lanes) usually is accomplished as a left-off movement from traffic to the ramps in the freeway center or median. DHOV ramps generally are employed to provide freeway connections with major streets where motorists have local access to transit centers and park-and-ride (P&R) lots close to the freeway. This type of access also is especially beneficial in relation to high-intensity land uses like downtown commercial centers, sports stadiums, and shopping malls. The potential benefits of DHOV ramps include: . Effective way of extending an HOV facility into an off-line transit facility as well as serving P&R facilities . Enhanced/increased HOV/transit usage when coordinated with transit centers and P&Rs . Improved mainline operations – reduced weaving into and out of HOV lanes . Opportunities to control or enforce entering volumes and employ priority/freeway management tools . Alternate, more convenient access for high-intensity land uses . Increases travel time savings . Reduced traffic/congestion at adjacent intersections. The Figure 4 shows the DHOV ramps to/from I-10 (Papago Freeway) at E. 3rd Street in Phoenix. FIGURE 4 DIRECT HIGH-OCCUPANCY VEHICLE (DHOV) RAMPS AT RD LOOKING N. 3 STREET ON I-10/PAPAGO FREEWAY WEST HOV Lane DHOV Lane rd 3 St DHOV Lane rd Exit Ramp to 3 Street Overcrossing HOV Lane Image Sources: Google Earth In effect, DHOV ramps improve safety, reduce congestion, save time, and increase travel time reliability for motorists using HOV lanes and motorists traveling in the general purpose (GP) freeway lanes. Motorists (including operators of public transit vehicles) using the HOV lanes can experience difficulty merging left through traffic in the GP lanes to gain access to the HOV lane during congested periods. The weaving action creates a safety problem for all freeway users, whether the weaving action is to gain entry to the HOV lanes or exit from the HOV lanes. This particularly is true when buses, especially articulated buses, perform this merging action; buses, which cannot maneuver with the agility of automobiles, can cause congestion in the GP lanes as they move from outside lanes to the inside HOV lanes. By enabling carpools, vanpools, buses, and motorcycles to connect directly with HOV lanes, motorists avoid the need to weave across the GP lanes. Signal control also can be employed at the TECHNICAL MEMORANDUM ASSESSMENT OF ALTERNATIVE IMPROVEMENT STRATEGIES 3 CENTRAL PHOENIX TRANSPORTATION FRAMEWORK STUDY entry ramps to regulate access to the HOV lanes in the same manner that ramp signals are used to regulate entry to the GP lanes. 2.3 New Arterial Crossings New overcrossings/undercrossings of arterial streets between major cross streets can help reduce through traffic volumes at adjacent interchanges and alleviate congestion. Some potential additional benefits include: . Reduced travel time and distance for local trips . Improved circulation for bicycles and pedestrians . Improved operations at ramp terminal intersections – reduced through traffic volumes.