Several studies note that after a warranted median barrier is installed, accident severity decreases (AASHTO, 1984). It is further noted, however, that accident frequency generally increases, because space available for returning-to-the-road manoeuvres is decreased (AASHTO, 1984). The intent of this research is to examine the safety impact of Jersey-type median barriers. Specifically, the hypotheses that: (1) total accident frequency may increase, and (2) serious accident frequency should decrease, are examined using the California Department of Transportation's (CALTRANS) TASAS data base. TASAS (Traffic Accident Surveillance and Analysis System) is a sophisticated traffic record system which combines an Accident Data Base (AXDB) with a Highway Data Base (HDB) for all state-maintained roads. Using this data base, each research hypothesis was tested relative to a variety of both highway and accident characteristics, controlling for the type of barrier utilized as well as for the size of the associated median. This report is organized into six chapters. Chapter Two contains a review of relevant aspects of transportation system safety related to highway median design and the implementation of longitudinal median barriers. Chapter Three describes the TASAS data base, the selection of sample highway sections included in the analysis, and a description of highway characteristics for the selected highway sections. Chapter Four contains a longitudinal analysis which focuses on "before" and "after" studies involving installation of Jersey barriers to isolate the resultant safety effects. Analysis of Jersey barrier performance with a range of geometric design standards, such as median width, was supplemented with analysis relative to measures of traffic volume to capture congestion effects. Chapter Five contains an analysis of the economic value of Jersey barriers utilizing a benefit cost approach in combination with the results of the longitudinal analysis described in Chapter Four. Chapter Six contains the conclusions and comments on this research work, and provides directions for further study. (A)
Samenvatting