Approximately 75% of fatal crashes in Hillsborough County, Florida, occur on roads with posted speeds of 40 mph and higher. Selecting an appropriate speed when planning or designing a roadway is critical to balancing safety and mobility. Historically, speed (e.g., posted, design) selection stems from the Federal Highways Administration’s (FHWA) Manual on Uniform Traffic Control Devices (MUTCD) and American Association of State Highways and Transportation Official’s (AASHTO) A Policy on Geometric Design of Highways and Streets (Greenbook) policies that rely heavily on the 85th percentile speed. However, the 85th percentile speed is based on driver behavior and is often much higher than the speed drivers SHOULD be traveling at on the roadway. As a result, both nationally and internationally, researchers and practitioners are attempting to approach speed selection differently, using context-based speed-setting approaches.
The first approach is Target speed, which is the highest speed at which vehicles should operate in a specific context. It reflects the planned or desired goal for the roadway from the long-range plan of the community it serves. Target speed often cannot be achieved in existing conditions but rather in future conditions that leverage multiple projects to achieve a desired speed through changes in roadway design, context/land use updates, and changes in user types.
The second approach is Design Speed, which is the speed at which vehicles can travel safely on the roadway and is influenced by geometric (e.g., functional classification, radii, etc.) conditions. Both speed approaches have limitations in that Target Speed is very subjective, while Design Speed is heavily focused on specific roadway design factors that can yield higher design speed selections. Hillsborough County aims to develop two methodologies to identify Target and Design Speeds for arterials and collectors. The methodologies and tools will help County staff and Practitioners provide less subjectivity when selecting Target Speeds and provide flexibility for selecting lower Design Speeds for projects on both existing and new roadways.
This presentation shares the practice in Hillsborough County to help planners and designers identify Target Speed and Design Speed with less subjectivity. Hillsborough County developed two tools to identify Target Speed and Design Speed for arterials and collectors using context-based speeding setting approaches.