The American Association of State Highway and Transportation Officials (AASHTO) defines design speed as “a selected speed used to determine the various geometric features of the roadway. The assumed design speed should be a logical one with respect to the topography, anticipated operating speed, the adjacent land use, and the functional classification of the highway.” The working definition for “target speed” is the operating speed that the designer intends for drivers to use. The topic of “design speed” versus “target speed” typically focuses on low-speed urban and suburban roadways, especially where the 85th percentile speed is higher than the posted speed limit. Research is needed to gain a better understanding of how roadway, roadside, and non-roadway elements influence the operating speed—the actual speed of the driver—in order to improve roadway designs and reliably achieve desired speed outcomes.
The objectives of this research are to (1) determine the effects of roadway, roadside, and non-roadway elements on operating speeds on roadways with a target speed between 30 and 40 mph and (2) develop recommendations on how the findings can be incorporated into the roadway design process.
Task 1. Conduct a critical review of relevant domestic and international literature, and if appropriate, conduct a survey of relevant domestic professionals. Include clarification of definitions of speed-related terms such as target speed, design speed, and operating speed. Summarize the known effects of roadway, roadside, and non-roadway elements on speed. Identify lessons learned, gaps, and challenges related to operating, design, posted, and target speeds. Identify successful practices from the United States and other countries related to the research methodologies, analysis, and the use of roadway, roadside, and non-roadway elements that influence target speed.
Task 2. Describe in detail the proposed analytical approach that will be used to develop the proposed research design and methods for target speed analysis. Describe the expected inputs, outputs, strengths, and limitations as well as implications for the data collection and analysis plan.
Task 3. Develop a data collection plan. The research team is expected to (1) develop a list of site types that reflect different roadway, roadside, and non-roadway elements and data sets throughout the country and (2) identify which approaches or sources will be used to collect data.
Note: The data collection plan shall include visual representation of study location types.
Task 4. Prepare an interim report that documents the work completed in Tasks 1 through 3. Include a detailed work plan for the work anticipated in Phase II.
Task 5. Develop a comprehensive list of potential study locations and categorization of the elements.
Note: NCHRP must approve the list of study locations.
Task 6. Conduct data collection and analysis according to the approved Phase II work plan.
Note: Include visual representation of selected study locations.
Task 7. Recommend a process by which appropriate roadway, roadside, and non-roadway elements should be considered to achieve a target speed.
Task 8. Develop the final deliverables.