Recent Roadway Geometric Design Research for Improved Safety and Operations (2012) / Chapter Skim
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Pages 35-57
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From page 35... ...
Research and applications of approach curvature previously focused on roundabouts. However, the report states that this geometric design treatment has potential to be applied to conventional intersections as well.
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From page 36... ...
Based on the results of the analyses, they concluded the following: • Adding left-turn lanes is effective in improving safety at signalized and unsignalized intersections. Installing a single left-turn lane on a major-road approach would be expected to reduce total intersection accidents at rural unsignalized intersections by 28% for four-leg intersections and by 44% for three-leg intersections, with corresponding reductions of 27% and 33% at urban unsignalized intersections.
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From page 37... ...
Delays to through traffic owing to right turns in the same situation on a four-lane arterial were substantially lower, in the range from 0 to 1 s per through vehicle. They concluded that pedestrians at unsignalized intersections or driveways can have a substantial impact on delay to through vehicles owing to slowing of right-turning vehicles yielding to pedestrians, but provision of a right-turn lane could reduce pedestrian-related delays to through traffic by as much as
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From page 38... ...
• Right-turn treatments on roadways with operating speeds equal to or greater than 40 mph." Where raised channelization is implemented at intersections, they also recommended that median and island curb sides and curb horizontal surfaces be treated with retroreflectorized markings and be maintained at a minimum luminance contrast level of 2.0 with overhead lighting or 3.0 without overhead lighting. intersectiOn sight Distance Where determinations of intersection sight distance (ISD)
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From page 39... ...
not be significantly larger than the circulatory path radius." Recommended maximum entry design speeds from the Guide are shown in Table 17. The design process described in the Guide states that a vehicle is assumed to be 2 m (6 ft)
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From page 40... ...
The report's authors add that these alignment considerations often compete with the fastest path speed objectives. inscribed circle Diameter In the first FHWA Roundabouts Guide (Robinson et al.
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From page 41... ...
NCHRP Report 672 also states that care be taken with entry widths greater than 18 ft or for those that exceed the width of the circulatory roadway, as drivers may mistakenly interpret the wide entry to be two lanes when there is only one receiving circulatory lane. intersection sight Distance Concerning ISD at roundabout approaches, the first edition of the FHWA Guide (Robinson et al.
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From page 42... ...
Researchers suggested that the critical headway estimate of 6.5 s in the first edition of the FHWA Roundabout Guide appeared to be somewhat conservative for design purposes for both single-lane and multilane entries. They recommended a lower value of 6.2 s for design purposes, which was approximately one standard deviation above the mean observed critical headway (Rodegerdts et al.
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From page 43... ...
This situation occurred most often when there was a queue of vehicles waiting to enter the roundabout or when the vehicle was already stopped for a prior pedestrian crossing event. • Did not yield: The motorist did not yield to a crossing pedestrian or a pedestrian waiting on the curb or splitter island to cross.
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From page 44... ...
They recommended designs that encourage bicycle users to merge into the general travel lanes and navigate the roundabout as a vehicle, explaining that the typical vehicle operating speed within the circulatory roadway is in the range of 15 to 25 mph, which is similar to that of a bicycle. Because multilane roundabouts are more challenging for bicyclists, additional design features may be appropriate for those locations.
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From page 45... ...
These drivers then turn right at the cross street. Drivers on the minor street desiring to turn left onto the major road must first turn right at the main intersection, execute a U-turn at the downstream median opening, and proceed back through the main intersection.
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From page 46... ...
The key difference between an MUT intersection and an RCUT intersection is that an MUT intersection allows through movements from the side street. An RCUT intersection has either no median openings at the intersection or has one-way directional median openings to accommodate traffic making left turns from the main street onto the side street.
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From page 47... ...
As with MUT intersections, designers are referred to the Green Book for specific design guidelines for minimum median widths, and much of the guidance in the FHWA report for crossover spacing for MUTs also is applied to RCUT intersections. The report states that several factors should be considered when selecting the appropriate spacing from a main intersection to a U-turn crossover.
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From page 48... ...
The considerations and trade-offs are similar to those between the main intersection and U-turn crossovers for an MUT or RCUT intersection. The distance needs to be sufficient to provide adequate vehicle storage and prevent spillback from one signal-controlled intersection to the next.
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From page 49... ...
Additionally, the capacity of the USC intersection was found to be approximately 50% greater than that of a conventional intersection with similar geometry under balanced traffic volumes. For highly unbalanced volumes, particularly when the intersection volumes were relatively low, they found that a conventional intersection outperformed the USC intersection.
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From page 50... ...
. FHWA's Signalized Intersections: Informational Guide (Rodegerdts et al.
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From page 51... ...
Based on turning radii and associated design speeds, they posited that this was the likely location where speeds of right-turning vehicles would be lowest. transit cOnsiDeratiOns TCRP sponsored a recent project to develop guidance for transit and highway agencies in the operations, planning, and functional designs of at-grade crossings of busways in FIGURE 19 Concept design of two-level signalized intersection (Shin et al.
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From page 52... ...
Researchers concluded that, overall, U-turns do not have the large negative effect at signalized intersections that many have assumed, as safety and operational effects were minimal. In NCHRP Project 17-21, researchers determined state and local agency design practices and policies related to unsignalized median openings for U-turns, such as those shown in Figure 20.
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From page 53... ...
• The effects of U-turn and left-turn volumes on median opening crash frequency cannot be separated, because a review of crash data for median openings found that crash report data do not distinguish clearly between crashes involving U-turn maneuvers and those involving left-turn maneuvers. • For rural unsignalized intersections: – They should have medians that are as wide as practical, as long as the median is not so wide that approaching vehicles on the crossroad cannot see both roadways of the divided highway.
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From page 54... ...
• Median opening lengths at rural divided highway intersections generally should be kept to the minimum possible. Increases in median opening length were found to be correlated with higher rates of undesirable driving behavior.
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From page 55... ...
– Provide appropriate sight distance and visibility for driver recognition of the intersection and conflicting users." • Maximum entering design speeds are based on a theoretical fastest path of 20 to 25 mph for single-lane roundabouts and 25 to 30 mph for multilane roundabouts (Rodegerdts et al.
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From page 56... ...
. • Double Crossover Intersections are found to have greater throughput than a conventional intersection, along with lower values for number of stops, average stop time per vehicle, average queue, and maximum queue length (Bared et al.
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From page 57... ...
– Median widths of more than 50 ft generally should be avoided at suburban, unsignalized intersections. • Median opening lengths at rural divided highway intersections generally should be kept to the minimum possible.
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