Access spacing and traffic safety

This paper reviews the many research studies that relate traffic safety to access spacing, presents results of specially conducted analyses of accident information obtained from eight states, and sets forth emergent guidelines for assessing safety impacts of access spacing. The literature review and safety analyses were performed as part of NCHRP Project 3-52, Impacts of Access Management Techniques. Accident rate indices, derived from the literature synthesis and safety analyses, show the relative increase in accidents that can be expected as the total driveway density in both directions increases. These indices suggest that doubling the access frequency from 10 to 20 access points per mile would increase accident rates by 40%. A road with 60 access points per mile would have triple the accident rate (200% increase) as compared with a spacing of 10 access points per mile. Each additional access point increases the accident rate by about 4%. The research results suggest a generally consistent relationship--the greater the frequency of driveways and intersections, the greater the number of accidents. While the specific relationships reflect variations in road geometry, travel speeds, and driveway and intersection volumes, the general relationship remains consistent. The access spacing implications are clear. Increasing the spacing between access points helps reduce the number and variety of events to which drivers must respond. In addition, wide access spacing gives drivers more time for perception, reaction and navigation.