After 18 months with a split speed limit for cars and trucks, Idaho reports that despite predictions to the contrary, slower-moving trucks have not caused the crash rate to change.

The drop in the interstate speed limit for trucks from 75 mph to 65 mph was approved in 1998 in exchange for a three-year experiment that allowed heavier trucks to be permitted on specific routes. Only nine permits have been issued to carry more than the standard limit of 105,500 pounds, and just four for the full 129,000 pounds.
Highway Operations Chief Charles Roundtree says there has been no statistically significant change in crash figures since the lower limit took effect. This is despite the fact that the average truck speed has dropped from nearly 68 mph to 66.4 mph, while car speeds have stayed at 73.5 mph, a 7-mph differential.