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I-1 SECTION I Summary Introduction The AASHTO Strategic Highway Safety Plan identified 22 goals that need to be pursued to achieve a significant reduction in highway crash fatalities. Two of the goals within the plan include Keeping Vehicles on the Roadway (Goal 15) and Minimizing the Consequences of Leaving the Road (Goal 16). Several emphasis areas have evolved from these two goals: run-off-road (ROR) crashes, head-on crashes, crashes with trees in hazardous locations, and curve-related crashes. This guide focuses on the crash types prevalent on horizontal curves and provides objectives and strategies to improve safety on curves. Many of the strategies identified to improve the safety at horizontal curves are common to strategies to reduce ROR and head-on crashes. In cases where the guides dealing with the ROR and head-on crash emphasis areas provide thorough coverage of a particular strategy, the reader is directed to the specific sections of those guides for more detailed information. If particular issues that pertain specifically to horizontal curves are not covered in the ROR or head-on guides, these issues are discussed within the text of this guide. General Description of the Problem Statistics from the Fatality Analysis Reporting System (FARS) indicate that 42,815 people were killed in 38,309 fatal crashes on the U.S. highway system in 2002. Approximately 25 percent of these fatal crashes occurred along horizontal curves. These crashes occurred predominantly on two-lane rural highways that are often not part of the state DOT system. Considering these statistics and that the average accident rate for horizontal curves is about three times the average accident rate for highway tangents (Glennon et al., 1983), implementing strategies designed to improve the safety at horizontal curves will help achieve the overall goal of the AASHTO Strategic Highway Safety Plan. Approximately 76 percent of curve-related fatal crashes were single-vehicle crashes in which the vehicle left the roadway and struck a fixed object or overturned, whereas 11 percent of curve-related fatal crashes were head-on crashes. Thus, ROR and head-on crashes accounted for 87 percent of the fatal crashes at horizontal curves, and the strategies for improving safety at horizontal curves focus on reducing the frequency and severity of these types of crashes. These strategies may not eliminate crashes with other vehicles, pedestrians, bicyclists, and trains that may be directly in the path of the vehicle, but crash statistics do not indicate that these types of collisions are prevalent on curves. Objectives of the Emphasis Area The two main objectives for improving safety along horizontal curves are to ⢠Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve and ⢠Minimize the adverse consequences of leaving the roadway at a horizontal curve.
Strategies designed to fulfill these objectives are presented in Exhibit I-1. Because the AASHTO Strategic Highway Safety Plan is geared toward low-cost, short-term safety improvements, the list of strategies presented in Exhibit I-1 is arranged in general terms from low-cost, short-term treatments to high-cost, long-term treatments. SECTION IâSUMMARY I-2 EXHIBIT I-1 Objectives and Strategies for Improving Safety at Horizontal Curves Objectives Strategies 15.2 A Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve 15.2 B Minimize the adverse consequences of leaving the roadway at a horizontal curve 15.2 A1 Provide advance warning of unexpected changes in horizontal alignment 15.2 A2 Enhance delineation along the curve 15.2 A3 Provide adequate sight distance 15.2 A4 Install shoulder rumble strips 15.2 A5 Install centerline rumble strips 15.2 A6 Prevent edge dropoffs 15.2 A7 Provide skid-resistant pavement surfaces 15.2 A8 Provide grooved pavement 15.2 A9 Provide lighting of the curve 15.2 A10 Provide dynamic curve warning system 15.2 A11 Widen the roadway 15.2 A12 Improve or restore superelevation 15.2 A13 Modify horizontal alignment 15.2 A14 Install automated anti-icing systems 15.2 A15 Prohibit/restrict trucks with very long semitrailers on roads with horizontal curves that cannot accommodate truck offtracking 15.2 B1 Design safer slopes and ditches to prevent rollovers 15.2 B2 Remove/relocate objects in hazardous locations 15.2 B3 Delineate roadside objects 15.2 B4 Add or improve roadside hardware 15.2 B5 Improve design and application of barrier and attenuation systems
