The safety performance of left-side off-ramps at freeway diverge areas was evaluated by comparing the corresponding areas of right-side off-ramps. Field observations and video recording of the traffic flow at the four left-side off-ramps were conducted. Data collected in the field include traffic conflicts, vehicle speed, traffic volume, geometry, pavement marking, and traffic signs. In addition, crash records at a total of eleven left-side off-ramps and sixty-three similar right-side off-ramps in Florida were collected. Based on the data collected, a traffic conflict study and a cross sectional comparison of traffic crashes between left-side and right-side off-ramps were conducted in this research. Four types of traffic conflicts were identified and counted. The average conflict rates near the ramp area were found to be approximately 10 per 1,000 conflicting vehicles. Approximately 5% of vehicles were observed to make lane change maneuvers at all three left-side off-ramps, among which nearly 15% were aggressive lane changes. The vehicle speed differences between exiting vehicles and throughtraffic on the optional lanes were less than 10 mph. This implies that the traffic flow remained in a stable state. Crash data were compared for the left-side off-ramps with right-side off-ramps by two exit ramp types: one-lane exit and two-lane exit with an optional lane, respectively. The comparisons indicate that the left-side off-ramp did have higher average crash counts, crash rate and percentage of severe crashes, but the differenceis only statistically significant for the percentage of severe crashes. Acrash prediction model for one-lane exit was developed to identify the factors that contribute to the crashes that have been reported for selected freeway segments. The results indicate the sign of variables: traffic volumes on freeway and ramps, left side of exit ramp, and deceleration lane length are all positive, which implies that number of crashes will increase when they increase.
Abstract