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Treatment Descriptions 47 reduction in injury crashes. (Rodegerdts et al., 2007) The crash reduction is achieved because there are fewer conflict points at roundabouts than at conventional intersections and because speeds are reduced. (FHWA, 2000b) Single-lane and multilane roundabouts have different operational characteristics that affect their relative safety benefits. Although the number of crashes and the severity of injuries were reduced at both single-lane and multilane roundabouts, a smaller reduction in total crashes was observed at the multilane roundabouts. Both single-lane and multilane roundabouts exhibit sim- ilar reductions in injury crashes. (Rodegerdts et al., 2007) Roundabouts improve safety for bicyclists by slowing automobiles to speeds closer to bicycle speeds (thus reducing high-speed conflicts) and by simplifying turning movements for bicycles. (FHWA, 2000b) Crashes involving bicycles comprise approximately 1% of all reported crashes at roundabouts. (Rodegerdts et al., 2007) Roundabouts also decrease the risk of severe pedestrian collisions, due to reduced vehicle speeds and a reduced number of conflict points for pedestrians. A Dutch study conducted at 181 roundabouts found a 73% reduction in all pedestrian crashes and an 89% reduction in injury crashes for pedestrians. (FHWA, 2000b) Crashes involving pedestrians comprise approximately 1% of all reported crashes at roundabouts. (Rodegerdts et al., 2007) Bicycles do not experience the same safety benefit at roundabouts as vehicles and pedestrians. In some cases, roundabouts can be less safe for bicycles than conventional intersections, despite the speed reduction benefits. (FHWA, 2000b) 4.8 Approach Curvature 4.8.1 Overview No test sites provided documented applications for the high-speed intersection treatments dis- cussed in this section. The high-speed intersection treatments discussed in this section have been applied at roundabout approaches only. These treatment applications introduce horizontal curve geometry to induce lower speeds. Approach curvature can be applied to new roundabouts and may be especially effective on rural highways. Designs vary with curve geometry. Secondary effects and considerations should include right of way, grading, driver workload, and truck movements. 4.8.2 Applicability and Considerations Approach curvature is a geometric design treatment that can be used at high-speed intersection approaches to force a reduction in vehicle speed though the introduction of horizontal deflection. As shown in Exhibit 4-14, approach curvature consists of successive curves with progres- sively smaller radii. Research and applications of approach curvature have focused on (Credit: FHWA, 2000b) Exhibit 4-14. Approach curvature.
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48 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections roundabouts. However, this geometric design treatment has potential to be applied to con- ventional intersections. Appropriately designed curve radii and length of curve can reduce vehicle speeds at the entry of a roundabout or conventional intersection, as well as reduce certain types of crashes. If successive curves are installed in high-speed environments, introducing other speed reduction treatments prior to the approach curvature may reduce the speed changes needed between successive curves. (Arndt, 2000) The use of approach curvature at downhill approaches is discouraged. Experience with approach curvature suggests that this geometric treatment should be used in conjunction with reduced speed limit signs or advisory speed signs. Sight distance should also be considered at the intersection approaches to ensure appropriate visibility of pedestrians and bicycles crossing the intersection approach or traveling through the intersection. Excessive sight distance should be discouraged to focus driver attention to the immediate roadway. Raised medians to reinforce speed reduction may become problematic for maintenance, espe- cially in areas subject to snow plowing. 4.8.3 Treatment Layout/Design The length and curve geometry should be determined from the upstream segment operating speed and the target speed at the intersection. Design control information from the "Green Book" (AASHTO, 2004), as well as published documentation from Australian applications at roundabouts (FHWA, 2000b), may provide design guidance. It may be appropriate to introduce the curved geometry concurrently with additional treatments such as warning signs. The dominant feature of approach curvature treatments is the radius of the curve. When designing successive curves, each curve must be visible before the next curve to allow drivers adequate time to adjust speeds. The length of the approach curvature affects speed reduction. Adequate curve length must be provided to discourage drivers from traveling into adjacent lanes; however, excessive curve lengths can result in an increase of single-vehicle crashes. (Arndt, 2000) Super-elevation and side friction also affect vehicles traveling through the cur- vature. To provide required super-elevation on each curve, short tangent sections can be designed between each successive curve. Similar to roundabout design, approach curvature should be designed with an appropriate "design vehicle" to ensure that all trucks, buses, and emergency vehicles may traverse the approach without encroaching on the shoulder or adjacent sidewalks. 4.8.4 Speed Effects Current information about approach curvature and how it impacts speed and safety relates exclusively to roundabouts. Although no published research or testing results have been found that describe the impacts on speed and safety at conventional intersection approaches, similar safety benefits might be realized by applying this treatment to conventional intersections. The geometric design of roundabouts shows that the curve radius directly impacts vehicle speeds. As the radius of the curve decreases, a larger reduction in vehicle speed is required to negotiate the curve. Exhibit 4-15 shows the speedradius relationship for curves with +0.02 and -0.02 super-elevations.