ACCESS SPACING AND SAFETY: RECENT RESEARCH RESULTS

Over the past 40 years, more than 20 studies have shown how accidents increase with decreasing access spacing. These results have been well documented. Within the past several years, a number of additional research efforts have provided a further analysis of this basic relationship. These efforts include: (1) the multi-state accident investigation reported in NCHRP Report 420, (2) an accident model prepared for Indiana highways, (3) a comprehensive analysis of accidents versus access spacing in Minnesota, and (4) a conceptual analysis based upon the product of conflicting traffic volumes. This paper compares the results of these recent studies, showing similarities and differences. In all studies, accident rates increase as access spacing is reduced. The volume-product approach and some of the empirical studies suggest that accident rates increase at approximately the square root of the increase in access points per mile.