The clear zone concept for roadside design emerged in the mid-1960s, when the idea of a single lateral distance beyond which any potential roadside obstacle did not require removal or protection was introduced. However, acceptance of a single distance for lateral clearance has diminished over time. The Guide for Selecting, Locating and Designing Traffic Barriers (1977) and AASHTO's 1989 Roadside Design Guide provide guidelines for roadside recovery areas based on traffic volume, design speed, side slope, and other roadside conditions. Although these guidelines provide a more realistic approach than the application of a single distance, there are major concerns because the values are based on old studies that used relatively limited data and extrapolated numbers. Experience has also indicated that the recovery areas provided along highways is usually not completely clear of all objects and often has side slopes greater than desired. Further, transportation agencies frequently face difficulties in providing desirable recovery areas because of right-of-way constraints or construction costs. Consequently, current practice is to provide an area that provides a reasonable opportunity for a driver to regain a measure of control or to slow an errant vehicle. Updated guidelines are needed to aid designers in determining safe and cost-effective recovery areas, while recognizing the associated constraints.
NCHRP Project 17-11 developed relationships between recovery-area distance and roadway and roadside features, vehicle factors, encroachment parameters, and traffic conditions for a range of highway functional classes and design speeds. Baseline relationships have been developed for different functional classes of highways. These relationships are then adjusted for shoulder width, horizontal curvature, and ditch configuration.
Before Project 17-11(02) began, an error was found in the reconstructed crashes from Project 17-11. One of the objectives of NCHRP Project 17-22 was to develop a data base of run-off-the-road crashes by combining data collected under 17-22 with the data from 17-11 and the FHWA rollover study. The Project 17-22 researchers noted some major discrepancies between the 17-11 data and the data from the other two studies. Further investigation found that the diagrams in the electronic crash files provided by NHTSA to the 17-11 researchers had been compressed. This resulted in distances and angels being calculated incorrectly. The 17-22 research team has reconstructed all of the 17-11 cases to the correct distances and angels. However, prior to developing guidelines it will be necessary to redevelop the relationships previously developed under 17-11.
Objective: The objective of this study was to develop guidelines for roadside clear zones that can be incorporated into the AASHTO Roadside Design Guide.
Status: This project is complete; however, additional, final-stage work is being conducted under NCHRP 17-11(03).