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45 did not yield to pedestrians who were crossing or waiting minated upstream of the entrance line. They recommended to cross, although in all but one case the pedestrians were designs that encourage bicycle users to merge into the gen- waiting to cross, so there was no imminent risk identified by eral travel lanes and navigate the roundabout as a vehicle, the research team. Researchers also observed only four con- explaining that the typical vehicle operating speed within the flicts in the 769 crossing events. Comparison with findings circulatory roadway is in the range of 15 to 25 mph, which is from a separate FHWA study (Carter et al. 2005b) indicated similar to that of a bicycle. Because multilane roundabouts that driver yielding at roundabouts was better than at uncon- are more challenging for bicyclists, additional design features trolled approaches, but not as high as at stop signs or traffic may be appropriate for those locations. signals. The researchers suggested that design changes could include reductions in exit radii, reductions in lane widths, and/or relocation of crosswalks (Rodegerdts et al. 2007). Innovative Designs A number of new, innovative, or otherwise unique intersection Bicycle Considerations designs were topics of considerable attention between 2000 and 2010. An FHWA study by Hughes et al. (2010) examined The first FHWA Roundabout Guide (Robinson et al. 2000) four alternative intersection designs, reviewing characteristics recommended that the "designer should strive to provide related to geometric design, access management, traffic con- bicyclists the choice of proceeding through the roundabout trol devices, and other features. The four designs included Dis- as either a vehicle or a pedestrian." The Guide stated that, placed Left-Turn (DLT), Median U-Turn (MUT), Restricted "in general, bicyclists are better served by treating them as Crossing U-Turn (RCUT), and Quadrant Roadway (QR) inter- vehicles; however, the best design provides both options to sections. Findings from that study and others related to the geo- allow cyclists of varying degrees of skill to choose their more metric design of those intersection types are summarized here. comfortable method of navigating the roundabout." Displaced Left-Turn According to the Guide, to "accommodate bicyclists trav- eling as vehicles, bike lanes should be terminated in advance The main feature of the DLT alternative intersection is the of the roundabout to encourage cyclists to mix with vehicle relocation of the left-turn movement on an approach to the traffic." Under this treatment, it was recommended that bike other side of the opposing roadway, which consequently lanes end 100 ft upstream of the yield line to allow for merg- eliminates the left-turn phase for this approach at the main ing with vehicles. This method is most successful at smaller intersection (Hughes et al. 2010). Traffic that would nor- roundabouts with speeds below 20 mph, where bicycle mally turn left at the main intersection first crosses the oppos- speeds can more closely match vehicle speeds. ing through lanes at a signal-controlled intersection several hundred feet upstream of the main intersection. Left-turning To accommodate bicyclists who prefer not to use the cir- vehicles then travel on a new roadway parallel to the oppos- culatory roadway, the Guide advised that "a widened side- ing lanes and execute the left-turn maneuver simultaneously walk or a shared bicycle/pedestrian path may be provided with the through traffic at the main intersection. The dashed physically separated from the circulatory roadway [i.e.,] line in Figure 12 illustrates a typical left-turn maneuver at a not as a bike lane within the circulatory roadway. Ramps or DLT intersection. The layout in Figure 12 is for a full ver- other suitable connections [could] then be provided between sion, which has DLT movements on all four approaches; this sidewalk or path and the bike lanes, shoulders, or road after the eastbound vehicles turn northbound, they must surface on the approaching and departing roadways." The travel through another crossover for southbound left-turning designer was advised to exercise care in locating and design- vehicles. This design reflects a shift of the through traffic ing the bicycle ramps so that they are not misconstrued by lanes into the median in an attempt to minimize the need pedestrians as unmarked pedestrian crossings, nor should the for additional right-of-way. At several locations where DLT exits from the roadway onto a shared path allow cyclists to intersections have been implemented as a retrofit to an exist- enter the shared path at excessive speeds. ing conventional at-grade intersection, the existing median has been preserved, and there is no shift in the through lanes. The second edition of the Roundabouts Guide (Rodegerdts DLT can also be installed at a three-legged intersection with et al. 2010) advises that, for nonmotorized users, one impor- the displacement on the major road in only one direction. tant consideration during the initial design stage is to main- tain or obtain adequate right-of-way outside the circulatory A study by Jagannathan and Bared (2005) investigated roadway for the sidewalks. All nonmotorized users who the design and operational performance of the DLT, then are likely to use the sidewalk regularly, including bicyclists in also known as the crossover displaced left-turn (XDL) or the situations where roundabouts are designed to provide bicycle continuous-flow intersection. The researchers' purpose was access to sidewalks, should be considered in the design of the to provide a simplified procedure to evaluate the DLT's traffic sidewalk width. Report authors recommended that bicycle performance and compare it with conventional intersections. lanes not be provided through the roundabout and be ter- Using microsimulation, they modeled typical geometries
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46 intersection and on costs involved in constructing a left-turn storage area for the crossed-over left-turn movement. Radii of the crossover movements can range from 150 to 200 ft, whereas the radius of the next left-turn movement at the main intersection is dependent on the turning movement of the design vehicle. Lane widths at the crossover reverse curve need to be wider than 12 ft to accommodate larger design vehicles. Consideration could also be given to having wider lane widths (e.g., up to 15 ft) for the receiving crossroad. The angle between the DLT intersection left-turn lanes and the main through lanes is referred to as the crossover angle and is influenced by the median width and the alignment of the mainline lanes; a recommended range of values for this angle is 10 to 15 degrees. To minimize the footprint of the intersection, Hughes et al. (2010) stated that median widths can be reduced, but they still need to be adequate to accommodate signs. Designers are referred to the Green Book for minimum median widths, but caution is advised to also take into account the possi- bility of installing post-mounted signs in these medians for safe and effective channelization of traffic. Offsets for signs should be in accordance with the MUTCD. FIGURE 12 Left-turn movement on a typical DLT intersection approach (eastbound to northbound). Results from an analysis by El Asawey and Sayed (2007) indicated that the capacity of a XDL intersection was higher than that of a conventional intersection by about 90%, and it over a wide distribution of traffic flow conditions for three outperformed conventional and upstream signalized cross- different design configurations or cases. They concluded that over intersections under all of their unbalanced-volume scenar- their comparisons with conventional intersections showed ios. They concluded that, for locations where right-of-way is considerable savings in average control delay and average not an issue, the XDL will be recommended for implementa- queue length, as well as an increase in intersection capacity. tion because of its superior performance compared with the They also concluded that their models provided an accessible other two intersections. tool for the practitioner to assess average delay and average queue length for those configurations. Median U-Turn Simmonite and Chick (2005) conducted a similar study in the United Kingdom to evaluate a displaced right-turn The MUT has been used in Michigan and other states as a intersection. They concluded that (1) intersection capacity treatment to balance intersection congestion and safety prob- can increase with a footprint similar to a large roundabout lems (Hughes et al. 2010). The MUT intersection design and only a small increase in costs, and (2) pedestrians and involves the elimination of direct left turns from major and/ cyclists can easily be provided for without compromising the or minor approaches (usually both). Drivers desiring to turn capacity using "Walk with Traffic" facilities. They suggested left from the major road onto an intersecting cross street must that the concept was an appropriate intersection type for use first travel through the at-grade main intersection and then on the U.K. highway network, providing operational benefits execute a U-turn at the median opening downstream of the where there were heavy right turns, full provision for non- intersection. These drivers then turn right at the cross street. motorized users, and an expected accident record unlikely to Drivers on the minor street desiring to turn left onto the major differ from other large signalized intersections. road must first turn right at the main intersection, execute a U-turn at the downstream median opening, and proceed back Hughes et al. (2010) stated that removal of conflict through the main intersection. Figure 13 shows the left-turn between the left-turn movement and the oncoming traffic at movements of a typical MUT geometric design. The opti- the main intersection is the primary design element in a DLT mum directional crossover spacing was recommended to be intersection. The DLT vehicles typically cross the opposing 660 ft (±100 ft) from the main intersection. Elimination of through traffic approximately 300 to 400 ft upstream of the left-turning traffic from the main intersection simplifies the main intersection under the control of another traffic signal. signal operations at the intersection, which accounts for most Research referenced in the report indicated that the appropri- of the intended benefits. The MUT intersection is typically a ate upstream distance is dependent on queuing from the main corridor treatment applied at signalized intersections. How-
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47 Minor street Minor street Major street Major street a) Major Street Movements b) Minor Street Movements FIGURE 13 MUT left-turn movements (based on Hughes et al. 2010). ever, the concept has also been used at isolated intersections typically 36 ft wide. The FHWA report refers to the Green to alleviate specific traffic operational and safety problems. Book for minimum median widths, and it presents alterna- tives for locations with restricted right-of-way. The FHWA report states that the MUT intersection per- formed well on arterials that have sufficient median width Restricted Crossing U-Turn to accommodate the U-turn maneuver. Because of Michi- gan experience with these intersections, the report discussed Hughes et al. (2010) refer to RCUT intersections as a prom- typical design values from the Michigan DOT. In general, ising solution for arterials with more dominant flows on the Michigan corridors with MUT intersections have median major road. Also referred to as superstreet intersections, they widths ranging from 60 to 100 ft. This design is used as a cor- are described as having the potential to move more vehicles ridor treatment in Michigan, although it has also been used efficiently and safely than roadways with comparable traffic for isolated intersections. volumes that have conventional at-grade intersections with minimal disruptions to adjacent development. The RCUT At an MUT, the design of the main intersection is similar intersection redirects left-turn and through movements from to the design of a conventional intersection, except that the the side street approaches. Instead of allowing those move- main intersection is designed for larger volumes of right-turn ments to be made directly through the intersection, as in a movements than a conventional intersection serving the same conventional design, an RCUT intersection accommodates total volumes because the left-turning vehicles become right- those movements by requiring drivers to turn right onto the turning vehicles. With this in mind, the intersection must be main road and then make a U-turn maneuver at a one-way designed with right-turn bays of sufficient width and length median opening 400 to 1,000 ft downstream. Figure 14 shows to accommodate the volume of turning vehicles. Depending a conceptual diagram of an RCUT intersection. This configu- on the right-turn volume, dual right-turn lanes or an exclusive ration shown is generally intended for higher-volume major right-turn lane and an adjacent shared-use through and right- roads in suburban and rural areas, especially at intersections turn lane may be needed. Channelized right turns at an MUT with relatively low through traffic volumes entering from intersection are rarely used, because they may require even the side road. For this type of intersection, left turns from more right-of-way, present a multistage pedestrian crossing, the main road are similar to conventional intersections, made and create a more difficult driving maneuver for a driver from left-turn lanes on the main road directly onto the side turning right from the minor street and weaving over to use road. For this type of RCUT intersection design, pedestrians the U-turn crossover. At some MUT intersections (e.g., at cross the main street in a diagonal fashion, going from one partial MUT intersections), left turns from the side road are corner to the opposite corner. An RCUT design that does not allowed as well as left-turn bays provided on the minor road permit direct left-turns is shown in Figure 15; this design approaches. The MUT intersection has secondary intersections channels all turning traffic to the crossovers on either side of at each of the crossover locations. One-way crossovers with the intersection. deceleration/storage lanes are highly recommended. The key difference between an MUT intersection and an MDOT has developed design guidelines for directional RCUT intersection is that an MUT intersection allows through median crossovers. In Michigan, the report states, it is cus- movements from the side street. An RCUT intersection has tomary for drivers of passenger vehicles to queue side-by- either no median openings at the intersection or has one-way side in a 30-ft wide crossover and treat it as if it had two directional median openings to accommodate traffic making lanes. However, large trucks and other heavy vehicles typi- left turns from the main street onto the side street. Similar to cally use the entire width of the crossover. MDOT uses the MUT intersection, the median width is a crucial design striped two-lane crossovers (with two lanes of storage lead- element for an RCUT intersection. The report states that ing up to the crossover) in some places. These crossovers are desirable right-of-way widths needed to accommodate large
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48 Side street Main street FIGURE 14 Conceptual RCUT configuration with direct left turns from the major road (based on Hughes et al. 2010). trucks without allowing vehicles to encroach on curbs or all travel time by removing left-turn movements (Hughes shoulders, assuming 12-ft-wide lanes and 10 ft of shoulder, et al. 2010). A QR intersection can reportedly provide other range from approximately 140 ft for four-lane arterials to benefits as well, such as improving pedestrian crossing time, approximately 165 ft for eight-lane arterials. For this same and a QR intersection can be among the least costly of the situation, desirable minimum median widths between 47 and four alternative intersections to construct and maintain. 71 ft are typically needed. As with MUT intersections, design- ers are referred to the Green Book for specific design guide- At a QR intersection, all four left-turn movements at a lines for minimum median widths, and much of the guidance conventional four-legged intersection are rerouted to use in the FHWA report for crossover spacing for MUTs also is a connector roadway in one quadrant. Figure 16 shows the applied to RCUT intersections. connector road and how all four of the left-turning move- ments are rerouted to use it. Left turns from all approaches The report states that several factors should be considered are prohibited at the main intersection, which consequently when selecting the appropriate spacing from a main intersec- allows a simple two-phase signal operation at the main inter- tion to a U-turn crossover. Longer spacing between the main section. Each terminus of the connector road is typically sig- intersection and crossovers decreases spillback probabili- nalized. These two secondary signal-controlled intersections ties, providing more time and space for drivers to maneuver usually require three phases. into the proper lane and read and respond to highway signs. Shorter spacing between the main intersection and cross- Key features in the geometric design of a QR intersection overs translates into shorter driving distances and travel are choosing a quadrant in which to locate the connecting times. AASHTO recommends spacing from 400 to 600 ft roadway; determining the number of connecting roadways; for MUT designs based on signal timing, whereas MDOT and designing the main intersection, the secondary inter- established 660 ± 100 ft as the standard spacing (Hughes sections, and the horizontal alignment and cross section of et al. 2010). the connecting road. In choosing a quadrant for the connecting roadway, common considerations are available right-of-way, Quadrant Roadway construction cost, and effect on left-turn movements; for the latter, the quadrant is typically placed so that the movement According to FHWA, the primary objective of a QR inter- with the highest volume is least affected by the new design of section is to reduce delay at a severely congested intersection the intersection. That is, the left turn with the highest demand of two busy suburban or urban roadways and to reduce over- is the one that receives the most direct path. Discussion of Side street Main street FIGURE 15 Basic RCUT intersection with no direct left turns (based on Hughes et al. 2010).
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49 Arterial Arterial Cross street Cross street Qu a d r a n t Qu a d r a n t ro ro a dw a y a d wa y a) Left Turn Pattern From The Arterial b) Left Turn Pattern From A Cross Street FIGURE 16 Left-turn movements at a QR intersection (based on Hughes et al. 2010). QR designs with multiple quadrant connectors is also pro- Synchronized Split-Phasing/ vided in the FHWA report. Double Crossover Intersection Bared et al. (2005) studied the operational characteristics of a For the main intersection, the design would be similar to synchronized split-phasing intersection, also called a double that of a conventional intersection with turn prohibitions. crossover intersection (DXI). An example of a DXI is shown Appropriate pavement markings or median designs are to in Figure 17. In this example, eastbound traffic crosses over be employed to convey the message to drivers that no left to the left side at signalized Intersection A (small circle on the turns or U-turns are allowed. Right-turn lane criteria are the left of the figure), whereas the right-turners use the dedicated same for a QR intersection as a conventional intersection right lane before reaching A. The crossed traffic will cross except for the right turns in the quadrant with the connect- over back to the right side at signalized Intersection C (small ing roadway. Right-turn demands do not change at the main circle on the right). Westbound traffic also crosses over in a intersection in the other three quadrants. Through volumes at similar way. At Intersection B (large circle in the center of the main intersection are higher in all four directions than at the figure), there is one through lane and one shared (through a conventional intersection because of rerouted left-turning and left-turn) lane. No dedicated left-turn lanes are provided. traffic. Pedestrian crosswalks would normally be provided Right-turn lanes are required for eastbound and westbound across all four approaches at the main intersection. traffic. Merging lanes for the northbound and southbound right-turn movements are required. Radii of crossover move- Hughes et al. (2010) state that the distance from the main ments can range from 150 to 200 ft, and the radius of the intersection to the secondary intersections is critical to the left-turn movement at B is 100 ft. Movements can be better success of a QR intersection design. The considerations and understood by following the arrow markings in the figure. The trade-offs are similar to those between the main intersection northbound and the southbound traffic are similar to the cor- and U-turn crossovers for an MUT or RCUT intersection. responding movements at a conventional intersection, with The distance needs to be sufficient to provide adequate vehi- one left-turn lane, one through lane, and one shared (through cle storage and prevent spillback from one signal-controlled plus right-turn) lane. The length of left-turn lane is 450 ft. intersection to the next. It is also necessary to provide enough spacing for adequate signing and to ensure that each set of signal controls is visible. Longer distances lead to higher costs for right-of-way, construction, and maintenance of the connecting road. Longer distances may restrict progres- sion from one signal to the next on the main streets and can translate into more vehicle-hours of travel. Considering all of those factors, a minimum spacing of 500 ft from the center of the main intersection to the center of the secondary inter- sections is presented as adequate for many situations. The horizontal alignment of the connecting roadway is key to providing proper access to both roadways as well as any driveway connections. The authors recommend using the relevant geometric design data from the AASHTO Green Book for a design speed of 30 mph to determine the appropri- ate superelevation, radius, and runoff length. FIGURE 17 Double crossover intersection (Bared et al. 2005).
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50 Results of traffic simulation were presented that showed secting roadway. Eyler evaluated a selection of design varia- input flow and throughput for DXI were similar, whereas the tions using VISSIM modeling, and he determined that the throughput was approximately 1,000 veh/hr lower than input overall capacity was near 75% of a four-lane freeway. He flow for a conventional design. For peak volumes, the aver- also conducted a generalized cost comparison and found that age delay per vehicle for conventional design was 219 s/veh, while the new designs were more expensive than traditional compared with 87 s/veh for the DXI. The authors also noted at-grade intersections, the annual travel time savings would that the numbers of stops, average stop time per vehicle, offset the construction cost within three years. His conclu- average queue, and maximum queue length were lower for sion was that these designs merited further consideration as the DXI than the conventional design. Finally, they con- alternatives to both conventional at-grade intersections and cluded that including a pedestrian phase for the DXI pro- typical expansion of arterials to freeways. duced lower delay than the conventional intersection, and that the left-turn capacity in a DXI was more than twice that of a conventional design. Alternatives for Turning Movements at Rural Intersections A variation on the DCI is the Upstream Signalized Cross- The purpose of NCHRP Project 15-30 (Maze et al. 2010) over (USC) intersection. Sayed et al. (2006) investigated signal was to investigate alternative safety improvements at rural optimization strategies for USC intersections and identi- expressway intersections, to identify their relative effective- fied selected operational issues. They used microsimulation ness (if data were available), and to report any experiential to model and analyze a USC intersection and, for compari- information from those agencies that have tried the alterna- son, a conventional intersection. Their analysis revealed that, tives. After reviewing existing guidance and literature, the for relatively balanced volumes, a USC intersection could research team conducted case studies to investigate and docu- significantly reduce average vehicle delays, particularly when ment the effectiveness of ten treatments. Using that informa- the volumes entering the intersection are relatively high. tion, researchers recommended improvements to rural median Additionally, the capacity of the USC intersection was found intersection design guidance provided in the Green Book and to be approximately 50% greater than that of a conventional the MUTCD for high-speed (50 mph or greater) expressways intersection with similar geometry under balanced traffic (divided highways with partial or no access control). The volumes. For highly unbalanced volumes, particularly when research team recommended that the next update to the Green the intersection volumes were relatively low, they found that Book include design guidance for rural expressway intersec- a conventional intersection outperformed the USC inter- tion designs that eliminate or reduce far-side conflict points section. Overall, they concluded, the USC intersection showed (e.g., J-turn intersections and offset T-intersections) or those considerable potential for situations in which one or more that address the issue of gap selection for minor road drivers of the following conditions existed: (1) intersection volumes (e.g., left-turn median acceleration lanes and offset right-turn were balanced and near or over the capacity of a conven- lanes). Examples of those designs are shown in Figure 18. tional intersection; (2) traffic volumes were somewhat unbal- Studies of the J-turn and offset-T designs revealed reduc- anced, but the overall entering volumes were too high to tions in crashes between 40% and 92%. Definitive results be accommodated with a conventional intersection; or from the turning movement accommodations were unavail- (3) the intersection had heavy left-turn volumes that caused able because of a small set of crash data, but the authors were excessive delays. optimistic about the treatments' ability to reduce preventable crashes. El Asawey and Sayed (2007) also concluded that the capacity of a simulated USC intersection was approximately 50% higher than that of a conventional intersection. They Two-Level Signalized Intersection noticed that with an increase in left-turn percentage from Shin et al. (2008) presented an unconventional intersection 20% to 30%, there was a relatively constant increase in delay design (used in China) known as the Two-Level Signalized for the USC, between 1 and 4 seconds. Intersection (TLSI) that completely separated east-west and north-south traffic. The TLSI, shown in Figure 19, also enabled Arterial Interchange the use of directional separation and leading, lagging, or over- lapping lefts on both upper and lower levels. They described Eyler (2005) discussed a family of interchange designs that the TLSI as a design consisting of two independently operat- were developed for arterial roadways: split-level single- ing intersections that is able to operate signals with flexibility point, left-hand windmill, hybrid (half single-point, half according to changing traffic conditions. windmill), and partial cloverleaf. Each of the four designs had one consistent requirement, that each at-grade inter- The results from their simulation modeling indicated that, section in the interchange was the junction of only one turn- compared with other innovative intersection types, the TLSI ing movement and one through movement. There was never had the shortest delay times in most evaluation scenarios as a location where traffic crossed both directions of an inter- well as the least sensitivity to variations in traffic volume.