Traffic speeds on interstates and freeways 10 years after repeal of national maximum speed limit

April 2008

Objectives: Essentially all published analyses of operational and safety outcomes related to enactment or repeal of the national maximum speed limit (NMSL) were based on data limited to the initial 1-3 years. The purpose of the present study was to collect and analyze longer term speed data. Method: In 2006, traffic speeds were surveyed at 26 locations on urban and rural expressways in five states (California, Montana, Nevada, New Mexico, and Texas) where speeds had been measured in 1996, immediately after repeal of NMSL. Most speed limits were unchanged during the approximately 10-year period. However, Montana introduced a numeric 75 mph limit for passenger vehicles in place of a "reasonable and prudent" limit. In Texas, urban freeway speed limits for passenger vehicles were reduced 5 mph, and truck limits were increased 5 mph. Results: On rural interstates without speed limit changes, travel speeds increased for both passenger vehicles and large trucks; the proportion of passenger vehicles exceeding 80 mph tripled. On rural interstates in Montana where speed limits were lowered for passenger vehicles, travel speeds decreased, even for large trucks whose speed limits had not changed. On urban freeways where speed limits did not change, travel speeds declined somewhat for both passenger vehicles and large trucks; during the study period there also were large increases in traffic volume and development of surrounding areas. On urban freeways in Texas where speed limits declined for passenger vehicles, travel speeds generally decreased, even for large trucks whose speed limits actually had increased. Conclusions: The data suggest that where traffic volumes allow, travel speeds still are increasing 10 years after repeal of NMSL, and many drivers are speeding. The study also shows that speed increases can be curbed and even reversed when speed limits are lowered. Increased enforcement is needed to curb speed increases and the increased risk of serious crashes.