VOLUME-TO-CAPACITY RATIO AND TRAFFIC ACCIDENTS ON INTERURBAN FOUR-LANE HIGHWAYS IN GREECE

Traffic volume is a measure of exposure widely used at present in traffic accident analysis. However, the same traffic volume on road sections with different capacities creates different operating conditions, and, therefore, different probabilities for accidents. Thus, the volume-to-capacity (V/C) ratio may offer a better measure of exposure than traffic volume per se. Recent work by the Transportation Planning and Engineering Department of the Faculty of Civil Engineering at the National Technical University of Athens on the relationship between traffic accidents and V/C ratio on an interurban four-lane undivided national highway in Greece is presented and compared with previous work by the department. It has been found that the rates for all traffic accidents at nonhazardous locations are almost constant, up to a V/C ratio of 0.65, and increase considerably for higher ratios, more than doubling when V/C is greater than 1.0. The same pattern of relationship between accident rates and V/C ratio also occurs at locations that are found to be hazardous by the quality control technique, and when only certain specific categories of accidents are analyzed, such as day and night, or dry and wet pavement accidents. Most of the accidnets occurring at higher V/C ratios were of the rear-end type, while at lower V/C ratios, head-on and out-of-control accidents prevailed.