National Academies Press: OpenBook

Practices for Preventing Roadway Departures (2018)

Chapter: Chapter 2 - Engineering Countermeasures for Roadway Departure Crashes

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Suggested Citation:"Chapter 2 - Engineering Countermeasures for Roadway Departure Crashes." National Academies of Sciences, Engineering, and Medicine. 2018. Practices for Preventing Roadway Departures. Washington, DC: The National Academies Press. doi: 10.17226/25165.
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Suggested Citation:"Chapter 2 - Engineering Countermeasures for Roadway Departure Crashes." National Academies of Sciences, Engineering, and Medicine. 2018. Practices for Preventing Roadway Departures. Washington, DC: The National Academies Press. doi: 10.17226/25165.
×
Page 7
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Suggested Citation:"Chapter 2 - Engineering Countermeasures for Roadway Departure Crashes." National Academies of Sciences, Engineering, and Medicine. 2018. Practices for Preventing Roadway Departures. Washington, DC: The National Academies Press. doi: 10.17226/25165.
×
Page 8
Page 9
Suggested Citation:"Chapter 2 - Engineering Countermeasures for Roadway Departure Crashes." National Academies of Sciences, Engineering, and Medicine. 2018. Practices for Preventing Roadway Departures. Washington, DC: The National Academies Press. doi: 10.17226/25165.
×
Page 9
Page 10
Suggested Citation:"Chapter 2 - Engineering Countermeasures for Roadway Departure Crashes." National Academies of Sciences, Engineering, and Medicine. 2018. Practices for Preventing Roadway Departures. Washington, DC: The National Academies Press. doi: 10.17226/25165.
×
Page 10
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Suggested Citation:"Chapter 2 - Engineering Countermeasures for Roadway Departure Crashes." National Academies of Sciences, Engineering, and Medicine. 2018. Practices for Preventing Roadway Departures. Washington, DC: The National Academies Press. doi: 10.17226/25165.
×
Page 11

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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: • Adequate pavement friction, • Rumble strips and rumble stripes, • Horizontal curve safety, and • 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 objects—or 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: • Longitudinal pavement markings: – Centerline, and – Edge line. • Delineators. • Advance markings for curves: – Speed advisory markings in lane, and – Speed reduction markings (also known as optical speed bars). • Basic signing countermeasures: – Advance warning signs, – Advisory speed plaques, – Combination curve/intersection signs, – Supplemental devices in a curve, – Combination horizontal alignment/advisory speed signs, – Chevron alignment signs, and – One-direction large arrow signs. • Enhanced signing countermeasures: – Larger devices, – Doubling-up devices, – Retroreflective strips on sign posts, – High retroreflective and fluorescent sheeting, and – Flashing beacons. • Dynamic curve warning systems.

10 Practices for Preventing Roadway Departures • Skid-resistant pavement countermeasures: – High-friction surface treatments, – Pavement grooving, and – Superelevation. • Shoulder countermeasures: – Shoulder widening, – Shoulder paving, – SafetyEdge, and – Rumble strips and rumble stripes. • Roadside improvements: – Clear zones, – Slope flattening, – Roadside barriers, and – 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: • Shoulders, • Safe pavement edges, and • 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 SafetyEdge— 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 FHWA’s 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 sources—the NCHRP Report 500 series and the FHWA Office of Safety—has 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 states’ 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.

12 Survey Questionnaire For this synthesis, a questionnaire was sent to DOTs in all 50 states and the District of Colum- bia. The questionnaire, shown in Appendix A in a condensed form, was organized into five parts. Part I was used to obtain information about the person who responded. Parts II through V contained 34 questions, grouped as follows: • Part II related to how state DOTs identify roadway departure crash problem locations and programs for selecting and implementing engineering countermeasures. • Part III had questions relevant to the states’ use (or non-use) of each of the 20 counter- measures listed in Table 4, plus questions relevant to identifying new countermeasures, states’ evaluations of countermeasures, and their need for additional research. • Part IV had two questions to explore how states were addressing vehicle-based technolo- gies, including vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), and autonomous vehicles, as they relate to preventing roadway departure crashes. • Part V had two questions—one to inquire if the state had a particular countermeasure that representatives wanted to feature as an effective countermeasure and another to provide an opportunity to make any further comments related to the application of countermeasures. Forty-one states responded to the survey questionnaire. A complete tabulation of all the responses by state for each question is provided in a series of tables in Appendix B. For many of the questions, there are hyperlinks to files provided by the state. These files have various content, including policies, specifications, drawings, reports, and other relevant technical material, all of which add to the information library for the roadway departure crash issue. They were reviewed, and relevant information was extracted and included in the discussion that follows. The remainder of this chapter will present the primary findings obtained from the state responses to the questions within Parts II through V. Part II. Roadway Departure Problem Identification and Implementation Programs (Questions 1 Through 4) Part II had four questions related to how states identify roadway departure crash problem locations and programs for selecting and implementing engineering countermeasures. These questions asked: 1. If the state had prepared a roadway departure safety implementation plan, 2. If the state had compiled and analyzed roadway departure crash data, 3. If the state had developed any SPFs for roadway departure, and 4. Which implementation strategy (i.e., hot spot, systematic, or systemic) the state followed. C H A P T E R 3 Survey of the State of the Practice

Survey of the State of the Practice 13 The responses to these questions are shown in Tables B1 through B4 in Appendix B and are summarized in the following sections. Roadway Departure Safety Implementation Plans (Question 1) The FHWA has a safety initiative entitled “Focused Approach to Safety” whereby it provides resources to eligible high-priority states to address the nation’s most critical roadway safety challenges, one of these being roadway departure crashes. Since 2009, as part of this initiative, the FHWA, through a contractor and in conjunction with the states, has developed Roadway Departure Safety Implementation Plans (RDSIPs) for 18 participating states. The state-tailored plans include systemic implementation of low-cost engineering treatments aimed at specific crash sub-types. The plans include recommended roadway departure countermeasures, a set of strategies with deployment levels, and an estimate of the funding needed to achieve a substantial and cost-effective annual reduction in roadway departure fatalities (see https://safety.fhwa.dot. gov/fas/ for further explanation and a sample RDSIP). The first question posed to the states was to determine whether they had prepared an RDSIP; they were asked to provide a link to it if one was available. Twenty-one states responded posi- tively, with 11 providing hyperlinks to their RDSIPs or a similar document (see Table B1 in Appendix B). These states were those from the list of 18 participating states noted previously. This synthesis does not summarize and analyze the contents of these RDSIPs. However, they do serve as a resource of engineering countermeasures that those states felt should be imple- mented to address their data-driven crash problems for roadway departure crashes. For exam- ple, Arkansas’s RDSIP identified the following countermeasures to be deployed to address its roadway departure crash problem: • Enhanced signs and markings, including: – Oversized advance curve warning signs mounted on both left and right, – Chevrons, – Advisory speed plates beneath the advance warning signs, – Additional strategies to reduce high-end approach speeds (e.g., speed feedback signs, periph- eral transverse pavement markings), – Raised thermoplastic markings, and – Wider edge lines. • Centerline, edge-line, and shoulder rumble strips. • Alignment delineation. • Wet-weather treatments, including: – High-friction surfaces, and – Pavement grooving. • Guardrail upgrades (40). Each of the states’ RDSIPs were reviewed to see if any state included a countermeasure not already mentioned; none had. Roadway Departure Crash Data (Question 2) The intent of question 2 was to ascertain whether (and if so, how) state DOTs have compiled and analyzed crash data related to roadway departures. Table B2 in Appendix B shows the responses of each state to this question. Thirty-six of the 41 states responding replied “yes” (meaning that they had compiled roadway departure crash data), and of those, 12 provided a link to their crash data and, if available, analysis and report. While the roadway departure crash analyses of several states could be used as examples, the analysis from Massachusetts is provided in Appendix C.

14 Practices for Preventing Roadway Departures An initial task of analyzing roadway departure crash data is to extract the relevant crashes from the statewide total crash database. This requires defining what constitutes a roadway departure crash and the data elements that capture the relevant crash records. Page 8 from the Massachusetts’s analysis (see Appendix C) shows its lane departure definitions and those from the Fatality Analysis Reporting System maintained by NHTSA. Safety Performance Functions for Roadway Departure Crashes (Question 3) SPFs are statistical models used to estimate the average crash frequency for a specific site type (with specified base conditions) based on traffic volume and roadway segment length. SPFs are developed through statistical regression modeling using historical crash data. They could also be developed for a crash type such as roadway departure crashes. Across the country, SPFs have been developed for a variety of analysis purposes. The predicted number of crashes calculated using SPFs is instrumental for a number of activities in the project development process, includ- ing (1) network screening, (2) countermeasure comparison, and (3) project evaluation. More information about SPFs can be found in a number of resources, including the Highway Safety Manual (2) and the website for the CMF Clearinghouse (3). Ten states responded that they had developed SPFs that could be used for the three purposes stated previously relevant to roadway departure crashes; their responses are shown in Table B3 in Appendix B. However, only four states provided hyperlinks to documents that provide that information. And, upon review of those documents, none of the states had developed an SPF specifically for roadway departure crashes. Programmatic Problem Identification and Implementation Strategies (Question 4) At the national level, the Highway Safety Improvement Program (HSIP) has encouraged a traditional approach of improving roadway safety at specific high-crash locations by identifying and analyzing individual crashes at the locations, defining crash patterns, determining appropri- ate countermeasures to reduce future crash potential, and implementing those countermeasures. This approach is frequently referred to as the hot-spot (meaning high-crash-frequency or crash- rate) approach. Two additional approaches were being used to complement the traditional hot-spot approach: • In the systematic approach, the first step is to identify low-cost countermeasures applicable to certain crash types. Then the crash data system is searched to identify highway sections that have targeted crashes at or above a crash threshold that would ensure cost-effective deploy- ment of these countermeasures. • The systemic approach involves widely implemented improvements (i.e., countermeasures) based on high-risk roadway features (e.g., no or narrow shoulders) correlated with specific severe crash types (e.g., roadway departure). It involves identifying the problem, screening and prioritizing candidate locations, selecting countermeasures, and prioritizing projects. It begins by looking at system-wide crash data to analyze and identify systemic safety problems. It then moves to a micro-level risk assessment of locations across the network, which then leads to selecting relevant countermeasures most appropriate for broad implementation. An example of how one state—Arizona—applied a systemic approach on two-lane rural highways with higher potential for run-off-road crashes is presented in Appendix D. This is an example of developing a performance-based practical design for shoulder width and superelevation, two countermeasures discussed in this report.

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 515: Practices for Preventing Roadway Departures identifies and summarizes practices being used by state departments of transportation (DOTs) to prevent roadway departure crashes and to identify the data-driven advantages and disadvantages of these practices. More than half of all traffic fatalities result from roadway departure crashes. This type of crash occurs after a vehicle crosses an edge line or centerline or otherwise leaves the traveled way. A variety of engineering strategies, often referred to as countermeasures, have been implemented by state and local agencies to prevent roadway departure crashes and reduce the severity of injuries if crashes do occur. Appendices A through F have been combined into one file and are available separate from the report.

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