The Rule of Seconds – Calculating Safe Following Distances

  • February 23, 2009
  • Blog

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 rule for a truck driver to use to calculate a safe following distance. Known as the “rule of seconds,” being aware of it may help save your life.

Tractor trailer trucks and cars

All of us have looked in the rear view mirror only to see a big 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. Why? If the vehicle in front of a large truck is a smaller vehicle like a car, the car can probably stop faster than the big truck. When it does, the truck will 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” comes in. Drivers of big commercial vehicles are taught that if they are operating a 40 foot vehicle traveling at under 40 mph, they should leave 4 seconds between the truck and the vehicle in front of them. That’s one second for each ten feet of vehicle length. If they are driving a 40 foot vehicle at a speed over 40 mph, then the truck driver should leave 5 seconds between the two vehicles – one second for each 10 feet of vehicle length plus one extra second for safety.

Use a similar calculation for longer trucks: What if it’s a 50 foot truck? Leave 5 seconds space between the truck and car. A 60 foot truck? Leave 6 seconds space, but always add an extra second if the truck is traveling faster than 40 mph. Of course, this rule applies in good weather, with reasonable traffic and 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? 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). At highway speeds, say 60 miles an hour, a vehicle is traveling about 88 feet per second. So if the truck driver is following the “rule of seconds”, he should be 660 feet behind you (6.5 seconds plus one second is 7.5 seconds, times 88 feet equals 660 feet). That’s more than two football fields!

So if you notice a tractor trailer following you closer than that, he’s too close. 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.