Over the past decade, there has been a trend toward higher speed limits, especially on rural interstates and freeways. Eleven states have increased speed limits to 75 mph, with some of them raising the limit as high as 85 mph on freeways and 75 mph on other roads. The TRB National Cooperative Highway Research Program's NCHRP Research Report 1006: Guide to Understanding Effects of Raising Speed Limits provides a better understanding of the expected safety implications of increasing speed limits to 75 mph or higher. Supplemental to the report are a presentation and NCHRP Web-Only Document 328: Safety Effects of Raising Speed Limits to 75 mph and Higher.

Across the United States, maximum speed limit laws vary dramatically. In the mid-1990s, many states increased speed limits to 75 mph on select roads. More recently, some states have established speed limits greater than 75 mph. The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 328: Safety Effects of Raising Speed Limits to 75 mph and Higher is supplemental to NCHRP Research Report 1006: Guide to Understanding Effects of Raising Speed Limits.

The objective of this research was to examine the effects of raising and lowering posted speed limits on driver behavior for urban and rural nonlimited access highways. Sites selected for study were furnished by the participating States. The study was conducted during the period from October 1985 to September 1992, when the maximum speed limit was 55 mi/h (89 km/h) on nonlimited access highways. During this period, the States and localities lowered and raised posted speed limits on short roadway segments, typically less than 2 mi (3.2 km) in length.

TRB's National Cooperative Highway Research Program (NCHRP) Research Results Digest 303: Safety Impacts and Other Implications of Raised Speed Limits on High-Speed Roads explores the effects of raised speed limits from 55 miles per hour or greater on freeways and non-freeways in rural and urban settings. The effects considered included impacts on safety and operations, as well as socioeconomic and environmental effects. The full report is available on the TRB website as NCHRP Web-Only Document 90.