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Status Report, Vol. 43, No. 1 | SPECIAL ISSUE: SPEED | January 31, 2008 Subscribe

How posted limits affect speeds

A not so surprising thing happened in Montana once it reinstated a numeric daytime speed limit for passenger vehicles in 1999 after nearly four years without one on rural interstates. Travel speeds plunged.

"What's impressive is the huge drop between 1996 and 2006 in the percentage of vehicles going very fast," says Anne McCartt, Institute senior vice president for research. "The proportion of passenger vehicles exceeding 75 mph, the limit set in 1999, tumbled 45 percent. The proportion surpassing 80 mph plummeted 85 percent. Large trucks slowed, too."

Texas is another example of what happens when speed limits change. In 2002 the state lowered limits for passenger vehicles on urban freeways to 65 mph from 70. The result was a 66 percent decline between 1996 and 2006 in the proportion of passenger vehicles exceeding 70 mph.

While Texas lowered its urban interstate speed limit, in May 2006 it raised to 80 mph from 75 the daytime limit for passenger vehicles on sections of 2 western rural interstates, I-10 and I-20. On I-20 the proportion of passenger vehicles exceeding 80 mph rose to 10 percent in September 2007 from fewer than 1 percent before the limit changed. On I-10 the proportion surpassing 80 mph initially fell to 2 percent in August 2006 from 4 percent in May but rose to 7 percent in September 2007.

"The initial drop on I-10 may be because of its location near the U.S. border with Mexico, which is heavily patrolled by law enforcement," McCartt says. "Several highly visible border patrol initiatives coincided with posting the higher speed limit. The I-20 sites didn't have the same level of enforcement during the study, so drivers may have felt more at ease going faster."

Texas and Montana are among 5 states the Institute has monitored since the 1995 repeal of the national maximum speed limit. In 1996 California, Nevada, New Mexico, and Texas raised limits on interstates 10-15 mph, and travel speeds increased (see "Speeds up in two states that raised speed limits," March 22, 1997, and "Limits up, speeds up, deaths up," Oct. 11, 1997). Passenger vehicle speeds have continued to climb on rural interstates in the intervening years. Between 1996 and 2006, the proportion exceeding 80 mph tripled on rural interstates without speed limit changes. Speeds generally fell on urban interstates and freeways where the limits didn't change, with small increases on some roads.

"One reason could be increased traffic volume, which tends to tamp down speeds," McCartt notes. In Nevada, where limits remained constant and traffic speeds declined, the number of vehicles per hour increased by nearly half. Three California freeways where average speeds either declined or remained constant saw traffic counts rise by as much as 146 percent.

U.S. travel speeds rise

More than a decade after the repeal of the 55 mph national maximum speed limit, travel speeds generally have gone up around the country, and many drivers exceed posted limits.

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