Criteria for setting speed limits in urban and suburban areas in Florida

Current methods of setting speed limits include maximum statutory limits by road class and geometric characteristics and speed zoning practice for the roads where the legislated limit does not reflect local differences. Speed limits in speed zones are set based on 85th percentile speed, and they need to be adjusted based on such factors as crash experience, roadside development, and roadway geometry. However, reflecting these factors into the posted speed limit is likely to rely on practitioner's subjective decision-making. The purpose of this study was to develop mathematical models to set speed limits using more objective approaches. This study focused on nonlimited-access arterial roads in urban and suburban areas in Florida. These roads are characterized by a great variation in geometry, roadside development, and traffic movements, and therefore, the legislated speed limit may not be appropriate. For this project, traffic, geometric, and roadside information was collected at 104 sites with low crash occurrence, 85th percentile speed near the posted speed, and uniform traffic flow. Those variables were converted into adjustment factors that were applied to an ideal speed, chosen as the maximum statutory speed corresponding to the selected facility type. Accordingly, the ideal speed was reduced to a reasonable posted speed limit based on actual conditions at the selected site. The adjustment factors developed in this study are for such variables as access density, road class, lateral clearance, lane width, and signal spacing. It was found that the model developed in this study predicted speed limits more realistic than using 85th percentile speed solely. In addition, subjectiveness in adjusting the 85th percentile speed can be diminished by using the engineering based model. Results of this study may help the Florida Department of Transportation and its districts to quantify the speed limits and provide more objective justifications for setting speed limits.

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