The Rule of Seconds – Calculating Safe Following Distances

The Virginia Commercial Driver’s Manual outlines the information truck drivers must know to pass Virginia’s exam to obtain a commercial driver’s license (CDL). The manual describes a technique for a truck driver to use to calculate a safe following distance. Known as the “rule of seconds,” practicing this technique could help to save your life.

Tractor trailers and cars

We have all looked in the rear view mirror only to see a tractor trailer bearing down on us. It can be frightening because we know that in an accident, trucks and buses most often strike the vehicle directly in front of them because their heavy weight prevents them from stopping quickly. If the vehicle in front of a large truck is a smaller, more agile vehicle like a car, that vehicle could decelerate at a much faster rate than the truck, then forcing the truck to slam into the car’s rear end before it can stop.

The “Rule of Seconds”

How can we determine whether the truck behind us is following too close and what can we do about it? That’s where the “rule of seconds” applies.

At highway speeds, say 60 miles per hour, a vehicle is traveling about 88 feet per second. Most commercial truck drivers are taught to allow one second’s worth of travel distance per ten feet of the length of their truck, plus one extra second for safety, between themselves and the vehicle in front of them.

Although you may not know the exact length of the truck following you, most big tractor trailers are about 65 feet in total length (tractor and trailer). With a few exceptions, that’s the maximum length allowed under Virginia law (see Va. Code § 46.2-1112). So if the truck driver is following the “rule of seconds”, he should be 660 feet behind you when traveling 60 miles per hour (6.5 seconds plus one second is 7.5 seconds, times 88 feet equals 660 feet). That’s more than two football fields!

You can use a similar calculation for other size trucks. If a truck is 50 feet long, leave 5 seconds space between the truck and car. A truck that is 60 feet long should leave 6 seconds space. Of course, this rule applies in good weather, during reasonable traffic flow, and on a good road surface. Under more dangerous conditions, it’s important for a truck driver to add extra seconds to the following distance.

How can you use the “rule of seconds” to help you be a safer driver? Using the formula for safe following distance, if you notice a tractor trailer following you closer than it should, try to change lanes and get out of his way. The truck driver may be the one following too close, but in most impacts between a tractor trailer and a car, you can guess which one loses.

The next time you are on the highway, remember the “rule of seconds”. It just may save your life.