Despite differences in speed limits ranging from 60 to 75 mph, a new study has found that average truck speeds in four states vary by less than 2 mph.

Fort Worth, TX-based Speed Measurement Laboratories studied truck speeds in Texas, Kentucky, Tennessee and New Mexico, all of which had different limits for cars and trucks. State authorities cooperated with the study, which used laser guns to measure truck speeds. Trucks that had their speed affected by traffic or from large fleets known to use engine governors weren't counted.
The lowest average speed, 67.4 mph, was recorded in Texas, which has a maximum daytime speed limit of 60 mph for trucks on rural interstates. The highest average speed, 69.2 mph, was found in New Mexico, which has a speed limit of 75 mph. That put 9% of trucks in Texas at or below the speed limit, while 92% of trucks in New Mexico were obeying the limit.
Carl Fors, president of Speed Measurement Labs, says the study shows that people will drive the speed they feel is safe, regardless of the speed limit.
The study was a follow-up to a 1995 study of 40,000 cars. "We wanted to see if changing speed limits had any effect on driving behaviors," Fors says. "We found that regardless of where you set the speed limit, people would drive the speed they want to," he says.
The same is true of trucks - but trucks, Fors found, stayed closer to the speed limit than the cars. Although Fors said in states like New Mexico, that may be due partly to the vehicles being unable to go that fast, he also credited it to truckers being better drivers overall.