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61 Table 21 Weaving Distances For Managed Lane Cross-Freeway Maneuvers Allow up to 10 mph Mainline Intermediate Ramp Recommended Design Year Speed Reduction for Managed (between freeway entrance/exit Minimum Weaving Volume Level Lane Weaving and managed lanes entrance/exit)? Distance Per Lane (ft) No 500 Yes Medium Yes 600 (LOS C or D) No 700 No Yes 750 No 600 Yes High Yes 650 (LOS E or F) No 900 No Yes 950 Source: Venglar et al. (2002). Note: The provided weaving distances are appropriate for freeway vehicle mixes with up to 10% heavy vehicles; higher percentages of heavy vehicles will require increasing the per lane weaving distance. The value used should be based on engineering judgment, although a maximum of an additional 250 ft per lane is suggested. Fitzpatrick et al. (2007) developed guidance materials on Electronic toll collection methods have improved capac- intermediate at-grade access to a buffer-separated managed ity at toll plazas, but there is still a need to accommodate the lane. They determined that compliance with access points anticipated volume of vehicles using the facility. McDonald was better for those with greater lengths (e.g., 1,500 ft), provides a detailed procedure for estimating the appropri- but that over 7% of observed access maneuvers involved ate number of queue lanes and queue length, depending on vehicles using the managed lane to pass slower-moving toll collection method, but he also provides a general rule vehicles. They also found "that when presented with the of thumb from Caltrans to provide 3.5 to 4 toll lanes per opportunity to enter a managed lane that is located very close approaching freeway lane and a minimum queue storage to an entrance ramp, drivers will attempt to cross multiple length between 200 and 250 ft (Leisch et al. 2005). lanes to do so." Providing sufficient weaving distance for cross-freeway maneuvers was therefore important to facil- Alternative Interchange Designs itate access to the managed lane. For the design of the at- grade access opening, they recommended the configuration An FHWA study (Hughes et al. 2010) examined two alter- shown in Figure 21. native interchange designs, reviewing characteristics related to geometric design, access management, traffic control Toll Facilities devices, and other features. The two designs included Double Crossover Diamond (DCD) and DLT interchanges. Addi- A particular type of managed facility is a tolled facility. Some tional studies have also evaluated these interchange designs tolled facilities are separate roadways on unique alignments, and others. Findings from those studies related to geometric whereas others are selected lanes on a concurrent alignment design are summarized in this section. with a general-purpose facility. Each has particular charac- teristics to consider when designing access points, whether Double Crossover Diamond/Diverging Diamond they are at-grade openings or full-fledged interchanges. In ITE's Freeway and Interchange Geometric Design Hand- The DCD interchange, also called a Diverging Diamond book (Leisch et al. 2005), McDonald describes details of geo- interchange (DDI), is a recent interchange design that is metric design elements for toll plazas. Although a number being considered as a viable interchange form to improve of the practices listed are influenced by traditional manned traffic flow and reduce congestion. Similar to the design of toll plazas where tickets and cash are exchanged, there are a conventional diamond interchange, the DCD interchange a variety of examples and principles that are also valid for differs in the way that the left and through movements unmanned electronic toll collection. navigate between the ramp terminals. The purpose of this interchange design is to accommodate left-turning move- Toll plazas typically have more lanes than adjacent sec- ments onto arterials and limited-access highways while tions of a freeway and require sufficient merge and diverge eliminating the need for a left-turn bay and signal phase at tapers to accommodate the added lanes. Similarly, a toll the signalized ramp terminals. Figure 22 shows the typical plaza or toll island on a ramp requires enough lanes to serve movements that are accommodated in a DCD interchange. the anticipated demand, necessitating the addition and/or The highway is connected to the arterial cross street by reduction of lanes on the ramp proper. Table 22 repro- two on-ramps and two off-ramps in a manner similar to a duces the information from the Florida Turnpike cited by conventional diamond interchange. However, on the cross McDonald for taper rates at toll plazas with traditional street, the traffic moves to the left side of the roadway payment collection. between the ramp terminals. This allows the vehicles on
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62 1300' Minimum - 1500' Desired Shoulder or Median Managed Lane See Detail A See Detail B See Detail C General Purpose Lanes Shoulder 4" Yellow 4" White Edge Line Lane Lines Intermediate Access Managed Lane 8" White Line 4' Typical 1" Gap 40' Max. - 20' Desirable 8" White Line Raised Pavement Markers Type I-C or Type II-C-R General Purpose Lane Detail A Managed Lane Raised Pavement Markers Type II-C-R 8" White Line 3' 12' 3' 12' 3' 12' 3' 45' General Purpose Detail B Lane Managed Lane 8" White Line 4' Typical Raised Pavement Markers Type II-C-R 3' 12' 3' 12' 3' 12' General Purpose 45' Lane Detail C Notes: All pavement marking materials shall meet the required Departmental Material Specifications as specified in plans. = Direction of travel. FIGURE 21 Design of intermediate at-grade access opening for buffer-separated freeway managed lane (Fitzpatrick et al. 2007). Table 22 the cross street that need to turn left onto the ramps to con- Desirable Taper Rates at Toll Plazas tinue to the on-ramps without conflicting with the opposing Number of Traditional Desirable Taper through traffic (Hughes et al. 2010). Plaza Type Payment Lanes at Plaza Rate Mainline Plaza Up to 8 lanes 25:1 10 to 14 lanes 20:1 The primary design element of a DCD interchange is the 16 or more lanes 15:1 relocation of the left-turn and through movements to the Ramp Toll Plaza All 20:1 opposite side of the road within the bridge structure. The turn- Source: Leisch et al. (2005). ing radii used at the crossover junction to displace these movements at an existing installation in Springfield, Missouri,
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63 FIGURE 22 Typical DCD interchange configuration (Hughes et al. 2010). are approximately 300 ft. FHWA advises that consider- · The noses of the median island should extend beyond ation should be given to designing radii at crossovers with the off-ramp terminals to improve channelization and heavy vehicles in mind. On rural locations where the minor prevent erroneous maneuvers. street has high-speed limits, the use of reverse curvature · Left- and right-turn bays should be designed to allow has been suggested. This may result in loon-like flare-outs for separate signal phases. at the ends of the bridge structure, and additional right-of- way may be required to widen the bridge or the underpass Bared et al. (2005) used simulation to compare the opera- structure. tional performance of a four-lane DDI with a conventional diamond. They concluded that performances for lower and Median width is also an important design element for medium volumes are nearly identical in both designs; how- a DCD interchange (Hughes et al. 2010). Greater median ever, their results from higher volumes showed that the con- width is required for the flaring needed for reverse curves. ventional diamond had lower throughput, higher average Designers are advised to obtain minimum median widths delay per vehicle, greater stop time, longer queues, and max- from the Green Book and to take into account the installa- imum off-ramp flows as compared with the DDI. Evaluation tion of post-mounted signs on medians on the bridge deck of a six-lane DDI at three scenarios with very high volume for safe and effective channelization of traffic. Appropriate indicated that the left-turn capacity of the DDI was twice that offsets for signs should be in accordance with the MUTCD. of the conventional diamond. The report states that driver simulator experiments on the Missouri DCD interchange, which included the use of glare screens, showed no erroneous maneuvers by tested subject Displaced Left-Turn drivers. Suggested design practices, based on input from Missouri DOT, include the following: The DLT interchange, also known as the continuous flow interchange, is an innovative interchange design that has sev- · The minimum crossing angle of the intersection should eral aspects similar to the at-grade DLT intersection and some be 40 degrees. aspects similar to the DCD interchange. It is a design treat- · The radius design should accommodate between 25 and ment that has been advocated as promising because it removes 30 mph. the conflict at the main intersection between left-turning and · Superelevation may not be needed because it could opposing through vehicles (Hughes et al. 2010). detract from any desired traffic calming effect. · Lane width should be approximately 15 ft. The main feature of the DLT interchange design is the left- · Design should accommodate WB-67 trucks. turn crossovers that are present on the cross-street approaches. · Adequate lighting should be provided. In a DLT intersection, the left-turning traffic is relocated at · Nearside signals should be considered. a location several hundred feet upstream of the first sig- · DCD interchange designs may only be appropriate nal-controlled ramp terminal of the diamond interchange, where there are high-turning volumes. shown at the right side of Figure 23. This left-turning traffic · Nearby intersections with long cycle lengths should be (shown as a dashed line) is crossed over the opposing through avoided. lanes. The traffic then travels on a new roadway that is situ- · Pedestrian crossings at free-turning movements should ated between the opposing through lanes and a roadway and be evaluated and pedestrian signals may be needed. that carries the right-turning traffic from the ramp. Drivers