What is the minimum following distance for a commercial vehicle traveling at 40 mph in good conditions?

The FMCSA rule of thumb is one second of following distance for every 10 feet of vehicle length at speeds up to 40 mph. A typical tractor-trailer is around 40 to 50 feet long, requiring four to five seconds of following distance. Above 40 mph, add one additional second. The 'one second per 10 feet' rule accounts for the much longer stopping distance of a loaded commercial vehicle compared to a passenger car. You measure following distance by picking a fixed object on the road such as a sign, and counting the seconds between when the vehicle ahead passes it and when you pass it. One second is dangerously close at any speed for a commercial vehicle. Increase following distance in rain, fog, snow, or when the road is slick.