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52 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 4-20. Example layout of a splitter island at a four-legged intersection. day and night. The majority of the reduction in injury crashes involved vehicles crossing paths at the intersection. Most of the treated intersections were four-legged intersections in urban areas on local roads. (LTSA, 2001; FHWA, 2006) Implementations in France and New Zealand resulted in a total crash reduction of 30% and a reduction of angle and crossing crashes by 30%. (FHWA, 2006) Although no evidence was found to support this claim, it is possible that installing these types of islands at intersections could increase collisions with obstructions. (LTSA, 2001) 4.10 Speed Tables and Plateaus 4.10.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. These applications create vertical deflection and alert drivers to the need to slow down. These treatments can be applied to stop-controlled approaches. Design variations include geometry and materials. Secondary effects and considerations should include snow removal and driver expectancy. 4.10.2 Applicability and Considerations Speed tables are speed humps with a flat section on top, allowing the entire wheelbase of a pas- senger car to rest on the raised, flat section. The flat section of the speed table can be aesthetically treated (see Exhibits 4-21 and 4-22) with decorative surface material or constructed with brick or other textured materials. Speed tables have gently sloped ramps on both ends, which allow slightly higher vehicle speeds and a smoother transition than speed humps. (ITE, 2007) A plateau is an alternative speed table design that has been used in the Netherlands and installed on facilities with speeds ranging from 60 to 80 km/h (35 to 50 mph). In applications of this treatment, plateaus are generally installed on all intersection approaches and the minor approaches are yield-controlled. Advance warning devices are also recommended on all approaches. (Schermers and van Vliet, 2001) Although speed tables are generally applied on low-speed facilities in the United States, they may have applications on approaches to high-speed intersections where low speeds are desired.
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Treatment Descriptions 53 (Credit: Fehr & Peers, 2007) Exhibit 4-21. Speed table combined with textured pavement in Naples, FL. Speed tables can provide benefits for multimodal users when user needs are considered in design and application. Speed tables that are used at crosswalks can eliminate the need for curb ramps, increase pedestrian visibility, and provide more sidewalk area for multimodal users. Detectable warnings are recommended for speed tables used at crosswalks to accommodate visu- ally impaired users. (ITE, 2007) When designed with a gentle rise, speed tables have little impact on bicyclists. (Cochituate Rail Trail, 2007) Vertical deflection devices such as speed tables are sometimes prohibited on emergency response routes because of the friction they create. Snow and ice removal may require special attention when plowing roadways. Speed tables should be constructed downstream of storm sewer inlets and should have a tapered edge along the curb line to allow for drainage. (ITE, 1997) Additional or new street lighting may be necessary to improve the visibility of speed tables. 4.10.3 Treatment Layout/Design Speed tables and plateaus should be placed at a location where vehicles will not abruptly encounter them at a high speed. A report from the Institute of Transportation Engineers (ITE, 1993) suggests that the first speed table in a series be placed no more than 200 ft from a stop sign or horizontal curve, and not within 250 ft of a traffic signal. Both horizontal and vertical sight distance should be considered to determine the placement of a speed table. Many jurisdictions have standard placement guidelines for the location of speed tables. (Hallmark et al., 2002) (Credit: Fehr & Peers, 2007) Exhibit 4-22. Speed table combined with striping and colored concrete in Charlotte, NC.