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Treatment Descriptions 49 (Credit: FHWA, 2000b) Exhibit 4-15. Speedradius relationship. 4.8.5 Safety Effects Although the majority of research and testing includes approach curvature at roundabouts, similar safety benefits might be realized by applying this type of geometric treatment to conven- tional intersections. Approach curvature that includes successive curves reduces crashes at a roundabout and min- imizes single-vehicle crashes. A Queensland, Australia, study found that reducing the change in 85th-percentile speed on successive curves to 12 mph reduced single-vehicle and sideswipe crashes. Although decreasing the curve radius on an approach can generally minimize rear-end crashes, it may also increase the potential for single-vehicle crashes. (FHWA, 2000b) 4.9 Splitter Islands 4.9.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. However, the treatments in this section have been applied at round- about intersection approaches and conventional intersection approaches in New Zealand and France. Splitter islands slow, direct, and separate conflicting traffic. These treatments can be applied to stop- or yield-controlled approaches. Design variations include length, geometry, and landscaping. Secondary effects and considerations should recognize that splitter islands can pro- vide refuge for pedestrians crossing at the intersection. 4.9.2 Applicability and Considerations A splitter island is a raised or painted area on an intersection approach used to separate enter- ing and exiting traffic, to deflect and slow entering traffic, and to provide refuge for pedestrians crossing the road in two stages. (FHWA, 2000b) These islands can increase driver awareness by channeling the traffic on the minor approaches. In some cases, these treatments are called throat or fishtail islands; these are shown in Exhibit 4-16. The geometry of fishtail islands introduces deflection at the approach, thereby reducing vehicle speeds at the intersection.
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50 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-16. Throat and fishtail islands. Although splitter islands have generally been used at roundabout approaches, this treatment has been applied to the minor approaches of T-intersections and two-way stop-controlled inter- sections in France and New Zealand. Exhibit 4-17 depicts a splitter island at a T-intersection in France. Similar to a splitter island at a roundabout, splitter islands at conventional intersections can benefit pedestrians crossing at the intersections. The island shortens the crossing distance, cre- ates a refuge area, and allows pedestrians to cross the approach in two stages, if necessary. FHWA has sponsored a study to determine the effectiveness of combining splitter islands on minor approaches with other types of speed reduction treatments, such as centerline rumble strips or pavement markings, on major approaches at two-way stop-controlled intersections on rural highways. 4.9.3 Treatment Layout/Design The length and geometry of splitter islands vary significantly. Splitter islands can either take the form of a cross-section change, including either visually or physically narrowing the travel way, or create horizontal deflection. In Exhibit 4-18, Splitter Island A creates a cross-section change without directly creating horizontal deflection. Splitter Island B conceptually creates hor- izontal deflection. Form A could present a visual cue to drivers to slow before the intersection. Form B could potentially reduce speed by creating a deflected path, similar to a roundabout through movement. Exhibit 4-17. Splitter island at a T-intersection in France.
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Treatment Descriptions 51 Exhibit 4-18. Splitter islands. Exhibits 4-19 and 4-20 depict schematic layouts of signing and striping for teardrop splitter islands. The oval shape of the island aids in reducing vehicle approach speeds and increases inter- section visibility. This splitter island can be mountable to allow larger vehicles to maneuver through the inter- section. U.K. standards recommend that no obstacles, such as signs or signals, be placed on the splitter island; that the color of the median be uniform (no striping) but different than the pavement color to offer increased day and night visibility; and that the splitter islands be con- structed of a mineral treatment rather than turf or grassy compositions. (Ministry of Equip- ment, 1998) In U.S. applications, a sign is usually mounted on the splitter island to increase its visibility. 4.9.4 Speed Effects Much of the information about splitter islands and how they impact safety and speed relates specifically to roundabouts. There is little published research or test results to describe the impacts of approach splitter islands on speed or safety at conventional intersection approaches. Splitter islands create deflection at roundabouts, which reduces the speeds of vehicles travel- ing through the intersection. 4.9.5 Safety Effects The limited literature that is available on splitter islands shows that there is a safety benefit to applying this treatment to conventional intersections. However, there is no information to indi- cate to what extent the safety effects of splitter islands are due to speed reduction and to what extent they are due to the separation between traffic moving in opposite directions. A study of 134 intersections in New Zealand found that installing throat or fishtail islands resulted in crash reductions of 30%-60% for fatal, injury, and pedestrian crashes during both Exhibit 4-19. Example layout of a splitter island at a T-intersection.