The minimum following distance you should leave behind a truck depends on the truck's length and your speed. The Federal Motor Carrier Safety Administration (FMCSA) recommends maintaining at least one second of following distance for every 10 feet of the truck's length when driving below 40 mph. For example, a 40-foot truck requires a 4-second gap at lower speeds. At speeds over 40 mph, you should add an extra second for safety, making it 5 seconds for a 40-foot truck. In adverse conditions like rain, fog, or poor roads, doubling this distance is advised to ensure safe stopping and reaction time
. To put it simply:
- For trucks under 40 mph: 1 second per 10 feet of truck length (e.g., 4 seconds for a 40-foot truck).
- For trucks over 40 mph: Add 1 extra second (e.g., 5 seconds for a 40-foot truck).
- In bad weather or poor conditions: Double the recommended following distance.
This ensures enough space for the truck to stop safely and react to sudden changes, considering trucks have longer stopping distances and slower reaction times
. Additionally, a common practical method to measure this distance is to pick a roadside landmark and count the seconds between when the truck passes it and when you reach it. If the count is too short, increase your following distance immediately
. In summary, the minimum safe following distance behind a truck is generally around 4 seconds at speeds under 40 mph for a typical 40-foot truck, increasing with speed, truck length, and adverse conditions.
