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6 Engineering countermeasures for roadway departure crashes have been deployed by state agencies for several decades, and new countermeasures continue to be developed by industry and state agencies. In order to establish the state of the practice among the states for use of engineering countermeasures, it was first necessary to identify those countermeasures. To that end, two key resources were reviewedâthe AASHTO Strategic Highway Safety Plan, including the NCHRP Report 500 series, developed under the direction of AASHTO, and the FHWAâs Office of Safety website. This chapter presents the results of this effort and concludes with a list of countermeasures included in the survey questionnaire. AASHTO Strategic Highway Safety Plan and NcHrP report 500 Series In 1998, AASHTO approved the AASHTO Strategic Highway Safety Plan, which identified 22 emphasis areas to pursue in order to significantly reduce highway crash fatalities. That plan was revised and updated in 2005 (4). The Highway Safety Plan serves as an overview document that presents the 22 key emphasis areas and the strategies within each that should be undertaken to achieve a safety goal of reducing the fatality rate on the nationâs highways. The plan has three emphasis areas directly relevant to the safety issue of roadway departure crashes: 1. Keeping vehicles on the roadway, 2. Minimizing the consequences of leaving the road, and 3. Reducing head-on and across-median crashes. The Highway Safety Plan references a series of guides, prepared under NCHRP with the over- all title of NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, corresponding to the emphasis areas. Each guide, identified as a volume in the series, includes a brief introduction, a general description of the safety problem, the strategies and countermeasures to address the problem, and a model implementation process. Three guides that are relevant to this synthesis are: â¢ Volume 3: A Guide for Addressing Collision with Trees in Hazardous Locations (5), â¢ Volume 4: A Guide for Addressing Head-On Collisions (6), and â¢ Volume 6: A Guide for Addressing Run-Off-Road Collisions (7). An initial listing of engineering countermeasures can be found in these guides. Table 1, prepared from Exhibit I-1 in Volume 6, lists several strategiesâa term used in these guides interchangeably with âcountermeasuresââfor three objectives for the run-off-road emphasis area. C H A P T E R 2 Engineering Countermeasures for Roadway Departure Crashes
Engineering Countermeasures for Roadway Departure Crashes 7 Volume 3 of the NCHRP Report 500 series discusses strategies for addressing collisions with trees in hazardous locations (5). The report presents two objectives for addressing this problem: (1) prevent trees from growing in hazardous locations, and (2) eliminate the hazardous condi- tion and/or reduce the severity of the crash. A âhazardous conditionâ is not explicitly defined in the guide, but it would likely include any area within the required clear zone for the road type, along the roadside within a horizontal curve section, or any location where there is a cluster of run-off-road crashes. The strategies for addressing these two objectives are found in Table 2. Objective Strategies Keep vehicles from encroaching on the roadside Install rumble strips Install edge-line âprofile marking,â edge-line rumble strips, or modify with narrow or no paved shoulders Install mid-lane rumble strips Provide enhanced shoulder or in-lane delineation and marking for sharp curves Provide improved highway geometry for horizontal curves Provide enhanced pavement markings Provide skid-resistant pavement surfaces Apply shoulder treatments: â¢ Eliminate shoulder drop-offs â¢ Widen and/or pave shoulders Minimize likelihood of crashing into object or overturning if vehicle travels off the shoulder Design safer slopes and ditches Remove/relocate objects in hazardous locations Delineate trees or utility poles with retroreflective tape Reduce the severity of a crash Improve design of roadside hardware Improve design and application of barrier and attenuation systems Table 1. Run-off-road objectives and strategies (7). Objective Strategies Prevent trees from growing in hazardous locations Develop, revise, and implement planting guidelines to prevent placing trees in hazardous locations Mowing and vegetation control guidelines Eliminate hazardous condition and/or reduce severity of a crash Remove trees in hazardous locations Shield motorists from striking trees Modify roadside clear zone in vicinity of trees Delineate trees in hazardous locations Table 2. Objectives and strategies for addressing crashes with trees in hazardous locations (5).
8 Practices for Preventing Roadway Departures Volume 4 of the NCHRP Report 500 series discusses strategies and countermeasures for addressing head-on collisions. A head-on crash typically occurs when a vehicle crosses a center- line or a median and crashes into a vehicle traveling in the opposite direction. As stated in the Volume 4 guide, the objectives of reducing the number of head-on fatal crashes are to: â¢ Keep vehicles from encroaching into the opposite lane, â¢ Minimize the likelihood of a car crashing into an oncoming vehicle, and â¢ Reduce the severity of crashes that occur. The strategies (also considered as countermeasures) suggested in that guide for these three objectives are shown in Table 3. FHWA Office of Safety The FHWAâs Office of Safety maintains a website that provides a wealth of information on all aspects of highway safety (8). Its comprehensive program, in part, focuses on three areas that have been identified as providing the greatest potential to reduce highway fatalities using infrastructure-oriented improvements: roadway departures, intersection crashes, and pedestrian/bicycle crashes. As noted earlier, roadway departure crashes alone account for over 50% of the fatalities, are related with intersections for another 4.4%, and are related with pedestrian/bicyclist crashes for 1.3%. A section of the website is devoted to roadway departure safety (1). In that section, the FHWA notes that its efforts are guided by the Roadway Departure Strategic Approach and Plan, which involves implementing countermeasures that address roadway departure crashes that fall into three main categories or objectives: 1. Keep vehicles on the roadway, 2. Provide for safe recovery, and 3. Reduce crash severity. The countermeasures that are included within these three categories are highlighted in the following. Objective Strategies Keep vehicles from encroaching into opposite lane Install centerline rumble strips for two-lane roads Install profiled thermoplastic strips for centerline Provide wider cross-sections on two-lane roads Provide center two-way, left-turn lanes for four- and two-lane roads Reallocate total two-lane roadway width (lane and shoulder) to include a narrow buffer median Minimize the likelihood of crashing into an oncoming vehicle Use alternating passing lanes on four-lane sections at key locations Install median barriers for narrow-width medians Table 3. Objectives and strategies for addressing head-on crashes (6).
Engineering Countermeasures for Roadway Departure Crashes 9 Keep Vehicles on the Roadway For this first category/objective, the following broad-level countermeasures are suggested by the FHWA: â¢ Adequate pavement friction, â¢ Rumble strips and rumble stripes, â¢ Horizontal curve safety, and â¢ Nighttime visibility. Poor pavement conditions, especially wet pavement, have been identified as one of the major contributing factors in roadway departure crashes. Therefore, the FHWA suggests that traditional friction courses or high-friction surface treatments should be considered for curves with numerous wet-weather crashes or for severe curves with higher operating speeds (1). The FHWA Office of Safety has a website dedicated to rumble strips and rumble stripes (where the edge line is placed over the rumble strip) (9). It references a recently completed project that provides a report by Himes et al. (10) that documents the policies and practices of several state DOTs for their use of rumble strips and rumble stripes; the report has an accom- panying Decision Support Guide (11). For the general countermeasure of horizontal curve safety, the FHWA notes that: . . . about three-quarters of curve-related fatal crashes involve single vehicles leaving the roadway and striking trees, utility poles, rocks, or other fixed objectsâor overturning. Most roadway departure countermeasures are effective when applied specifically at horizontal curves. A focus on horizontal curves can prove to be a cost- effective approach to reducing roadway departure crashes (1). At the website, the reader is pointed to a report entitled Low-Cost Treatments for Horizontal Curve Safety 2016 (12), which documents numerous countermeasures: â¢ Longitudinal pavement markings: â Centerline, and â Edge line. â¢ Delineators. â¢ Advance markings for curves: â Speed advisory markings in lane, and â Speed reduction markings (also known as optical speed bars). â¢ Basic signing countermeasures: â Advance warning signs, â Advisory speed plaques, â Combination curve/intersection signs, â Supplemental devices in a curve, â Combination horizontal alignment/advisory speed signs, â Chevron alignment signs, and â One-direction large arrow signs. â¢ Enhanced signing countermeasures: â Larger devices, â Doubling-up devices, â Retroreflective strips on sign posts, â High retroreflective and fluorescent sheeting, and â Flashing beacons. â¢ Dynamic curve warning systems.
10 Practices for Preventing Roadway Departures â¢ Skid-resistant pavement countermeasures: â High-friction surface treatments, â Pavement grooving, and â Superelevation. â¢ Shoulder countermeasures: â Shoulder widening, â Shoulder paving, â SafetyEdge, and â Rumble strips and rumble stripes. â¢ Roadside improvements: â Clear zones, â Slope flattening, â Roadside barriers, and â Delineation on barriers. For each of these countermeasures (with a few exceptions), the report discusses their design, application guidelines, safety effectiveness, and relative cost (low, medium, or high). For the nighttime visibility countermeasure, the FHWA has a separate Nighttime Visibility website that contains information about three areas that affect nighttime visibility: adequately maintained retroreflective signs, pavement markings, and roadway lighting (13). Specific counter- measures are not presented, however. Provide for Safe Recovery For the second objective, the FHWA notes that three general countermeasures are effective for assisting drivers in recovering safely: â¢ Shoulders, â¢ Safe pavement edges, and â¢ Clear zones. Shoulders are a common geometric design element for highways, and their design features are found in AASHTO design guides and state design policies and manuals. To mitigate vertical drop-offs at the pavement edge, the FHWA advocates installing SafetyEdgeâ a paving technique where the edge is shaped at approximately 30 degrees from the pavement cross slope. Its website has several pages devoted to this specific countermeasure, including case examples from several states. A clear zone is defined as an unobstructed, traversable roadside area that allows a driver to stop safely or regain control of a vehicle that has left the roadway. Design guidelines for the width of the clear zone can be found in AASHTO and state DOT design manuals. Within this counter- measure group is removal of or protection from trees and utility poles and other roadside hard- ware that is not considered crashworthy. Reduce Crash Severity Reducing the severity of a crash is the third objective of FHWAâs Roadway Departure Strategic Approach and Plan. As noted previously, providing an adequate clear zone for the road type should eliminate what could be an injury-producing roadway departure crash. However, road hardware such as sign and luminaire supports and delineator posts are often placed within the clear zone, and because of the terrain, often a sufficient clear zone cannot be provided within reasonable costs. In the former case, these devices are designed to be crashworthy, meaning that
Engineering Countermeasures for Roadway Departure Crashes 11 they are much less likely to cause an injury if hit. If they cannot be, then they are shielded by safety barriers (e.g., guardrails, concrete barriers) or crash cushions, which can be considered countermeasures. List of countermeasures for State Survey The previous discussion of countermeasures from two primary sourcesâthe NCHRP Report 500 series and the FHWA Office of Safetyâhas identified numerous countermeasures related to pre- venting roadway departure crashes and reducing their severity should they occur. From those, 20 countermeasures were selected for the survey of the statesâ practices; these are shown in Table 4. They are grouped under four categories: traffic control device, pavement improvement, roadside measure, and geometric design, and they are arrayed under three objectives: keep vehicle on the roadway, minimize the consequences of leaving the roadway, and reduce head-on and cross-median crashes. The questionnaire also provided the opportunity to identify any addi- tional countermeasures that were being used by the states. COUNTERMEASURE OBJECTIVE Type Description Keep Vehicles on Roadway Minimize Consequences of Leaving Roadway Reduce Head-On and Cross- Median Crashes Traffic control device Wider edge line Advance curve warning pavement marking Speed advisory marking in lane Speed reduction marking Dynamic curve warning system Flashing beacons on warning sign Shoulder rumble strip Edge-line rumble stripe Centerline rumble stripe Raised (profiled) pavement marking Pavement improvement SafetyEdge High-friction surface treatment Pavement grooving Roadside measure Cable median barrier Tree removal Increase clear zone Flatten side slope Geometric design Shoulder widening on curved section Increase sight distance on curve Superelevation improvement Table 4. Countermeasures used for three objectives for reducing the occurrence and severity of roadway departure crashes.