The speed limit is going up on Interstate 81 between the Washington County, Tennessee line and the Virginia state line. The limit for all vehicles will now be 70 miles per hour, according to a statement from TDOT on Wednesday. Previously, the speed limit had been 65 for passenger cars, 55 for big trucks. TDOT says recent surveys and crash analyses showed that a uniform speed limit for all vehicles would increase safety. New speed limit signs are going up next week.