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Suggested Citation:"Summary." 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:"Summary." 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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1   S U M M A R Y 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. However, these values are based on studies from the 1950s through the 1980s that used relatively limited data and extrapolated numbers. User agencies recognized a need for updated guidelines to aid designers in better understanding the risk associated with road- side encroachments while recognizing and working within the associated design constraints. The objective of NCHRP Project 17-11(03), “Development of Clear Recovery Area Guide- lines” was to develop updated guidelines for roadside clear zones expressed in terms of key roadway and roadside design parameters. It was recognized that the use of crash data for determining 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 the lateral extent of encroachments is only partially observed. Furthermore, detailed information on the roadway and roadside design variable of interest is often lacking in most databases. A research approach that combined crash data analyses with computer simulation results was developed to overcome this limitation. Use of computer simulation permitted a detailed analysis of vehicle trajectory and resulting vehicle kinematics for a wide range of variables for which data may not otherwise be available. The simulation matrix consisted of over two million unique vehicle encroachment simulations performed using a state-of-the-art vehicle dynamics code. The variables in the simulation matrix included vehicle type, encroachment speed and angle, vehicle orientation at departure (i.e., tracking or non-tracking), driver input (e.g., steering and/or braking), horizontal curvature, vertical grade, shoulder width, foreslope ratio, foreslope width, ditch bottom width, backslope ratio, and backslope width. Simula- tion output included lateral distance traveled, vehicle stability outcome, trajectory data, and velocity data. Real-world crash data were used to develop probability distributions for the selected encroachment variables, such as encroachment speed, encroachment angle, vehicle orientation at point of departure from the traveled way (i.e., tracking or non-tracking), and driver input applied during the encroachment (i.e., steering, braking, or a combination of both). The results were used to determine marginal probabilities for the values of the encroachment variables used in the simulation matrix that were applied as weighting factors to the simulation results. These weighting factors essentially define the probability of occurrence for a given set of simu- lated encroachment conditions and the resulting outcome of the encroachment. A probability Development of Clear Recovery Area Guidelines

2 Development of Clear Recovery Area Guidelines matrix for vehicle type was developed using vehicle sales data by combining sales percentages for the vehicle makes and models corresponding to the platforms of the simulated vehicles. Encroachment relationships in the form of statistical models were derived in terms of sig- nificant roadway and roadside design variables to assist with the determination of the prob- ability and severity of an impact given an encroachment has occurred. Models were developed for lateral distance traveled by the encroaching vehicle, longitudinal distance traveled by the encroaching vehicle, rollover probability, speed versus lateral offset, and vehicular angle or ori- entation versus lateral offset. These relationships were developed for two categories of posted speed and two facility types. The encroachment relationships were incorporated into a risk analysis tool to estimate the probability of a fatal or serious injury crash [P(K+A)]. An encroachment-based analysis meth- odology estimates the conditional probability of a crash given a roadside encroachment has occurred and the probable severity of the crash. Sensitivity analyses were performed using the risk analysis tool to evaluate the sensitivity of the design variables to the estimation of risk and the relative importance of the variables to the overall determination of P(K+A). The results of the sensitivity analyses were used to determine which variables to retain or exclude from the clear zone guideline development process. Analyses were parametrically executed using the risk analysis tool to cover combinations of facility type, posted speed limit, roadway and roadside design variables, clear zone distance, and hazard conditions beyond the clear zone edge. A relative risk approach was used to define a clear zone distance for a given design configuration that has a P(K+A) risk equal to that of the guardrail. The final clear recovery area guidelines were expressed in both chart and equation form in a format that could be considered for incorporation into the American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide. The guidelines can be used to determine a recommended clear recovery distance for a given set of roadway and roadside characteristics. The guidelines also consider the nature of the hazard that exists beyond the clear zone edge.

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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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