Background Prior to Congress passing legislation to regulate the National Maximum Speed Limit (NMSL) to 55 mph in January of 1974, many States posted limits as high as 75 mph. Thirteen years later, in April of 1987, Congress passed another legislation to allow States to increase speed limits to 65 mph on certain interstate highway sections in rural areas where the population is less than 50,000. On November 28, 1995, Congress passed the National Highway System (NHS) Designation Act, which delegated the responsibility of speed limit designation to State governments. As a result, many States have raised speed limits on interstates and other roads in both rural and urban areas. The relationships between posted speed limits, actual travel speeds, and crash/fatality rates are complex.

Objectives Study the relationships between posted speed limits; enforcement levels; travel speeds of free-flowing traffic; average daily traffic (ADT) crashes, injury and fatality rates per mile, by crash type; and travel speeds of crash involved vehicles. Estimate the changes in crashes and fatalities when speed limits are raised or lowered, by road type. Examine crash trends at lower speeds in heavily congested metropolitan areas due to increasing number of registered vehicles and licensed drivers.

Proposed Approach Statistical analyses of FARS and State crash data, FHWA speed data and State ADT data. Speed limits, speeds, ADT and crashes will be tracked and compared for specific roadway segments over time. The evaluation may require 2-3 years, depending on the complexity of acquiring and assembling necessary data from existing sources. The study could be a cooperative effort with FHWA and/or FMCSA.