One good rule says at least one second for each 10 feet of vehicle length at speeds below 40 mph. At greater speeds add one second for safety.

For example, if you are driving a 40-foot vehicle, you should leave 4 seconds between you and the vehicle ahead.

In a 60-foot rig, you’ll need 6 seconds. Over 40 mph, you’d need 5 seconds for a 40-foot vehicle and 7 seconds for a 60-foot vehicle.

To know how much space you have, wait until the vehicle ahead passes a shadow on the road, a pavement marking, or some other clear landmark. Then count off the seconds like this: “one thousand-and- one, one thousand-and-two” and so on, until you reach the same spot. Compare your count with the rule of one second for every 10 feet of length.

If you are driving a 40 foot truck and only counted up to 2 seconds, you’re too close. Drop back a little and count again until you have 4 seconds of following distance (or 5 seconds, if you’re going over 40 mph). After a little practice, you will know how far back you should be. Remember to add one second for speeds above 40 mph. Also remember that when the road is slippery, you need much more space to stop.