Traffic Operation and Safety Analyses of Minimum Speed Limits on Florida Rural Interstate Highways

Traffic operation and safety characteristics were analyzed in relation to the posting of the minimum speed limit of 40 mph on Florida rural interstate freeways. The operational analysis results showed that 57% of the recorded vehicles exceeded the maximum speed limit. In addition, while only 0.14% of recorded vehicles had speeds below the 40 mph posted minimum speed limit, safety analysis results revealed that 9% of crash-involved vehicles were estimated to have occurred at speeds below 40 mph. The overinvolvement of slow moving vehicles in the crash data suggests that even a small proportion of vehicles traveling under 40 mph can have negative impacts on safety. Thus, regulation of vehicle speeds at the lower end of speed distribution is important. Further, the second-order crash risk model developed to estimate the crash risk of a vehicle on the freeway as the function of the deviation from the mean traffic speed indicated that the minimum risk occurred when the pre-crash driving speed is 8 mph above the mean speed, which is equvalent to the 85th percentile speed observed in the field.