Geometric design standards are mainly based on logically derived relationships and engineering judgements which are seldom validated by accident studies. Consequently, it is difficult to assess the likely safety consequences of departures from standards. This paper compares the results of studies in different countries and summarizes current international knowledge of the relationships between safety and the principal non-intersection geometric design parameters. In general, there is broad international agreement on these relationships. Many studies have attempted to relate geometric elements and accident rates, but only a limited amount of reliable information is available and quantifying the safety impact of marginal changes in the values of geometric design parameters is difficult. It appears that significant changes in the value of many geometric design standard elements are unlikely to result in large increases in accident rates and it is concluded that the available international information provides a good indication of the differences in accidents which would result from departures from design standards or from alternative route alignments.
Abstract