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Development of Clear Recovery Area Guidelines (2024)

Chapter: Chapter 2 - Clear Recovery Area Background

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Suggested Citation:"Chapter 2 - Clear Recovery Area Background." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 2 - Clear Recovery Area Background." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 2 - Clear Recovery Area Background." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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5   Up to the 1960s, little emphasis was placed on roadside safety design. The prevailing philosophy was that reasonable and prudent drivers did not inadvertently leave the travelway, and the penalty for doing so by others was acceptable. Studies by Stonex at the General Motors (GM) Proving Ground in the late 1950s and early 1960s showed that even professionally trained drivers strayed from the travelway and that measures to minimize risks of roadside encroachments were needed and warranted (4, 5). Work at the GM Proving Ground contributed significantly to the acceptance by many of the need for a “forgiving roadside.” This need was underscored by the alarming number of run-off-the-road, single-vehicle crashes and the high severity associated with these crashes. Recommended measures to minimize risks to errant motorists included providing (a) flat, unencumbered roadsides of sufficient width to permit an errant driver to safely bring their vehi- cle under control or to stop; (b) traversable sideslopes, preferably 1:6 or flatter, and safer ditch sections; (c) breakaway or yielding supports for signs and light poles; and (d) improved guardrail systems, including safer treatments at guardrail ends. Results of the GM studies also formed the basis for initial dimensions of recommended “recovery areas” (6, 7). These areas were later referred to as “clear zones” (1, 2), “clear recovery zones” (8), or roadside recovery distance (9). The GM studies provided probability data on lateral extent of vehicular movement for run- off-the-road crashes. Using these data, the American Association of State Highway Officials (AASHO), and subsequently AASHTO, suggested that, where feasible, a clear, unencumbered recovery area should extend 30 ft or more laterally from the travelway (6, 7). The GM studies indicated that the lateral extent of vehicular movement would not exceed 30 ft in approximately 80% of run-off-the-road crashes on high-speed highways. National guidelines continued to recommend a 30-ft clear zone distance up until 1977, although it was recognized that the 30-ft width was somewhat arbitrary and based on accident studies at the GM Proving Grounds where relatively flat roadsides were provided. The 1977 AASHTO Guide for Selecting, Locating, and Designing Traffic Barriers (Barrier Guide) contained clear zone recommendations that were dependent on design speed, the slope of the cut or fill section, and whether the hinge at the juncture of the shoulder with the sideslope was rounded (1). These guidelines indicated that the width of the clear zone should increase with increasing design speed and increasing steepness of fill slopes (width decreases with increasing steepness of cut slopes). For example, the recommended clearance for a high-speed roadway (60-mph design speed) with a fill section having a 1V:4H unrounded sideslope was approximately 43 ft. For the same example and a 40-mph design speed, the recommended clearance was approximately 18 ft. Clear zone criteria contained in the 1977 AASHTO Barrier Guide were developed by Ross et al. in a study sponsored by the Federal Highway Administration (FHWA) (10). In the study, the Highway- Vehicle-Object Simulation Model (HVOSM) computer program was used to determine the lat- eral extent of vehicular movement for encroachments on fill and cut roadside sections, rounded C H A P T E R   2 Clear Recovery Area Background

6 Development of Clear Recovery Area Guidelines and unrounded, at speeds of 40, 50, and 60 mph (11). The assumed driver response for the simulated encroachments included an emergency steer-back-to-the-travelway maneuver and emergency full braking. The 1989 AASHTO RDG (2) contained certain revisions to the clear zone criteria of the 1977 AASHTO Barrier Guide (1). In addition to the variables considered in the Barrier Guide, clear zone widths were also defined in terms of traffic volume, and greater ranges of design speed were adopted, but the effects of slope rounding were not considered. Clear zone criteria presented in the 1989 AASHTO RDG were derived from data in the 1977 Barrier Guide, in combination with state practices and the collective judgment of the task force that prepared the RDG. The following statement in the 1989 RDG is noteworthy. The numbers obtained from Figure 3.1 or Table 3.1 imply a degree of accuracy that does not exist. Again, the curves are based on limited empirical data, which was then extrapolated to provide data for a wide range of conditions. Thus, the numbers obtained from these curves represent a reasonable measure of the degree of safety suggested for a particular roadside, but they are neither absolute nor precise (2). Users of the 1989 Guide were reminded of the subjective nature of the clear zone recommen- dations and that engineering judgment was essential in their application. A comparison of clear zone recommendations in the 1977 Guide and 1989 RDG is shown in Table 1. This example assumes a 60-mph design speed and a 1V:4H foreslope. The 1989 RDG recommends essentially the same clear zone distance as the 1977 Guide for high-volume road- ways but recommends considerably lower clear zones for lower-volume roadways. The guidance found in the 2002 and 2011 editions of the AASHTO RDG (12, 13) was essentially unchanged from the 1989 RDG (2). NCHRP Project 17-11 It was recognized that updated guidelines were needed to aid designers in determining safe and cost-effective recovery areas, while also recognizing the constraints associated with building or improving the highway system. The objective of NCHRP Project 17-11, “Determination of Safe/ Cost Effective Roadside Slopes and Associated Clear Distances” was to develop relationships between recovery area distance and roadway and roadside features, vehicle factors, encroachment parameters, and traffic conditions for a range of highway functional classes that can subsequently be used to establish clear zone guidelines. It was recognized that the use of crash data for determining the statistics on the extent of lateral movement of vehicles encroaching onto the roadside is often limited by a vehicle striking a fixed object or rolling over. Therefore, any lateral extent of encroachment distribution derived from crash data will be a truncated distribution, and the full effect of sideslopes and other variables on lateral extent of encroachments is only partially observed. A research approach that combined crash data analyses with computer simulation results was developed to overcome this limitation. Use of computer simulation permits a detailed analysis of 1977 Barrier Guide (1) 1989 Roadside Design Guide (2) Clear Zone Clear Zone Design ADT Distance, ft Design ADT Distance, ft All 43 < 750 20–24 750–1500 26–32 1500–6000 32–40 > 6000 36–44 Table 1. Comparison of clear zone recommendations in 1977 Barrier Guide (1) and 1989 RDG (2).

Clear Recovery Area Background 7   vehicle trajectory and resulting vehicle kinematics for a wide range of variables for which data may not otherwise be available. When combined with real-world crash data, the results can be used to determine the influence of and develop relationships between various encroachment parameters. Clinical reconstruction and analyses of the National Automotive Sampling System (NASS) Crashworthiness Data System (CDS) data were used to develop probability distributions for the key encroachment parameters for different highway functional classes (14). The reconstructed crash data provided key encroachment parameters for ran-off-road crashes, including encroach- ment speed, encroachment angle, vehicle orientation at encroachment (i.e., tracking, non-tracking), and driver control input (i.e., steering, braking, or both). The probability distributions developed from the weighted NASS-CDS data were applied to each encroachment parameter used in the simulation matrix to obtain a probability for each simulation outcome. Exceedance curves were developed and used to create lateral extent of movement relation- ships that combine simulation and real-world crash data such that they are a function of multiple encroachment parameters. The exceedance curves can be used to determine the percentage of encroachments that will exceed a certain lateral distance. The researchers recommended that the guideline development process involve some form of cost-effectiveness or risk analysis procedure. NCHRP Project 17-11(02) The objective of NCHRP Project 17-11(02), “Development of Clear Recovery Area Guidelines” was to use the data and relationships generated under NCHRP Project 17-11 to develop guide- lines for roadside clear zones expressed in terms of key roadway and roadside design parameters. The research approach was to use the Roadside Safety Analysis Program (RSAP) to perform a benefit-cost analysis. RSAP is an encroachment probability-based analysis tool comprised of various analysis modules that include an Encroachment Probability Module, Crash Probability Module, Severity Prediction Module, and Benefit-Cost Module (15). The analytical methodology incorporated in RSAP uses a series of conditionally independent probabilities to represent a roadside encroachment event. This includes the probability of an encroachment occurring, the conditional probability of a crash given a roadside encroachment has occurred, the probable severity of a crash if one occurs, and the expected benefit-cost ratio of the roadside design configuration being investigated. The design alternative evaluated in the incremental benefit-cost analyses was varying recovery area distances for a given set of roadway and roadside conditions. The benefits of a design alternative are defined in terms of a reduction in crash cost derived from a decrease in crash frequency and/or severity, while the costs are defined by the direct construction and maintenance costs associated with the alternative. The encroachment exceedance curves and encroachment severity relationships developed under NCHRP Project 17-11 were intended to be used to update the RSAP Crash Prediction and Severity Prediction Modules, respectively. It was discovered that the crash scene diagrams supplied by the National Highway Traffic Safety Administration (NHTSA), and subsequently used by the researchers under NCHRP Project 17-11, were not to scale. Thus, the reconstructed encroachment parameters used to develop the marginal probabilities for weighting the simulation outcomes were not correct. Further research to develop reconstructed run-off-road crash data with various encroachment parameters includes NCHRP Report 665: Identification of Vehicular Impact Conditions Associated with Serious Ran-off-Road Crashes (16), performed under NCHRP Project 17-22 and NCHRP Web-Only Document 341 (3). The effort initiated under NCHRP Project 17-11(02) was reformulated under NCHRP Project 17-11(03) using this new crash data as well as improved vehicle dynamics simulation and analysis tools.

Next: Chapter 3 - Vehicle Dynamics Encroachment Simulations »
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The clear zone concept for roadside design emerged in the mid-1960s as a single distance for lateral clearance that reduced the likelihood of an errant vehicle striking a roadside obstacle. Subsequent recovery area guidance that evolved over the next two decades provided a variable distance expressed in terms of traffic volume, design speed, sideslope, and other roadway and roadside factors.

NCHRP Research Report 1097: Development of Clear Recovery Area Guidelines, from TRB's National Cooperative Highway Research Program, develops updated guidelines for roadside clear zones expressed in terms of key roadway and roadside design parameters. These updated guidelines can aid designers in better understanding the risk associated with roadside encroachments while recognizing and working within the associated design constraints.

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