The Effect of Ramp Type and Geometry on Accidents

This study was made to learn more about freeway ramps, to determine which geometric features play important roles in ramp safety, and to classify these features according to ramp type and relative safety merits. The study involved 722 freeway ramps. In a period of about 3 years, over 2 billion vehicles used these r amps, and during this time 1643 accidents occurred . The accident rates of onramps were consistently lower than off-ramp accident rates. (Accident rate = number of ramp accidents per million vehicles using the ramp.) The average onramp rate is 0. 59 Acc/MV (accidents per million vehicles), whereas the average off-ramp rate is 0. 95 Acc/MV. The onramp rates vary from about 0. 40 Acc/MV to about 0. 93 Acc/MV depending on the type of ramp . The off-ramp rates vary from about 0. 62 Acc/ MV to about 2.19 Acc/MV. Ramps were classified as to type (diamond, trumpet, cloverleaf, etc). Accident rates were determined for each ramp type and further subdivided by onor off-ramp, and by the relative freeway-to-ramp grades. Correlations were found between accident rates and r amp type, relative freeway-to-ramp grades, fixed objects, speedchange lane lengths, possible safe entrance speeds at on-ramp noses, and off-ramp radius. No correlation was found to exist, or the study was unable to determine if a correlation existed, between ramp accident rates and on-ramp curvature, ramp lighting, ramp traffic volumes, and the magnitude of the ramp central angle.