The American Driver and Traffic Safety Education Association suggests using the “three-second rule” to help prevent rear-end accidents.
The “three-second rule” accounts for your reaction time to the movements of the vehicle ahead and your vehicle’s stopping distance.
You should add more time if the road is slippery, if you’re being crowded by a tailgater, if you’re towing a trailer or if you’re driving a large truck.
The three-second rule:
When the vehicle ahead of you passes a stationary object, start counting: 1,001 … 1,002 …
The first second is your reaction time; the next two seconds account for your braking distance
You should not reach the object before you count to … 1,003. If you do, you are following too closely.
At a vehicle speed of 55 mph, the three-second rule creates a gap of 243 feet between cars.