According to the US Department of Public Safety, following another vehicle too closely is the fifth leading cause of auto collisions in the US. What’s more, following other vehicles too closely, or tailgating, is the primary cause of rear-end crashes. 1
Stopping distance is the distance your car travels from the moment you step on the brake pedal until your car stops moving. Following distance is the gap you should leave between you and the vehicle ahead of you to be able to stop safely in an emergency. But whether it’s stopping distance or following distance, you’re talking about the same thing: your ability to safely stop your vehicle if the vehicle in front of you should suddenly stop.
Calculating the stopping distance of your vehicle is a mathematical equation that’s best left to safety engineers. It takes into account variables like the speed of the car in meters per second, the car’s gravitational acceleration and the coefficient of friction between the wheels and the road. Here’s what the equation looks like:
Clearly, this isn’t the type of calculation that you can do while driving. Fortunately, calculating your car’s safe following distance is.
Following distance looks at stopping distance from a different perspective. It’s the gap you leave between your car and the car ahead of you. It’s measured in time rather than meters. The general rule is to maintain a safe following distance of at least three seconds behind the vehicle ahead. This should give you enough space to stop in an emergency, like if the car ahead of you stops abruptly. 3
Here’s how it works: Choose a fixed point that is even with the car in front of you. For example, a road sign, tree or a building. If you pass that same fixed point before three seconds (count 1-1,000; 2-1,000; 3-1,000), then you are driving too close to the car in front of you and you need to fall back a bit.
The 3-second rule only applies if road and weather conditions are ideal. Slow down and increase your following distance, preferably to six seconds, during adverse weather conditions or when visibility is reduced.4
Tip: Never drive at a speed at which the stopping distance required exceeds the distance you can see. 5
Some driving experts point out that the 3-second rule only covers the braking distance of your car. There’s also a human element to consider. Known as “thinking distance,” it’s the distance travelled while a driver perceives a hazard and applies the brakes. The average reaction time for an alert, defensive driver is 0.67 seconds.6
Rounding up, reaction time turns the 3-second rule into a 4-second rule.
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Keep this in mind every time you get behind the steering wheel. You are expected to be in control of your vehicle at all times. The only excuse, ever, for hitting the vehicle ahead of you is that your own car was hit from behind by someone else, and you were then knocked into the one in front of you. 7