Following too closely (tailgating), along with inattention/distraction, is a major cause of rear-end collisions. The commonly suggested rule of thumb “keep a following distance of at least one car length per 10 mph” is actually not a good practice.


The basic avoidance requirement is this: when the vehicle in front of you brakes hard, you should be able to react and start braking before the front of your vehicle reaches the point on the road where the rear of the leading vehicle was when it started braking. This can be achieved only if you are following at a time interval greater than your reaction time.

Using the suggested rule of a gap of one car length (~ 15 feet) per 10 mph (~15 feet/second) results in a following interval of approximately one second. That is, your required reaction time is less than or equal to one second. Since the average reaction time in a simple situation for most people, most of the time, is approximately one second, failure would occur at least one-half of the time. Keeping a two second gap, in my opinion, would be a reasonable practice, but would still not guarantee success without careful attention to your driving task.

You might argue that, if you maintain a two-second gap in heavy traffic on a multi-lane highway, somebody will change lanes in front of you, thus eliminating the safe gap. Since this aggressive lane changer is probably going to maintain a time interval of less than or equal to one second, you would be required to drop back only about two seconds. If this happens ten times on your trip, you will increase the likelihood of a safe arrival, and will arrive at your destination a mere twenty seconds later.

Some irresponsible drivers will habitually move up to within one-half of a car length, in 60 mph traffic, a time interval of equal to or less than 0.10 seconds. Therefore, if the vehicle in front of them suddenly brakes hard, there is a zero possibility of avoiding a collision, regardless of their often-inflated sense of their driving skill.


Due to their poorer braking capability, large trucks, even those with ABS (anti-lock braking systems), should maintain at least a 3-second following distance. This photo illustrates the damage a tractor-trailer can inflict by rear-ending a passenger car.


Motorcyclists should allow a greater following distance given the disastrous consequences that can result from tailgating. This is particularly important if the motorcycle does not have anti-lock brakes since, contrary to popular belief, a non-ABS motorcycle actually requires a longer stopping distance than a passenger car.

Unfortunately laws against tailgating are rarely enforced by police patrol officers.

More information on related topics can be found in Highway Accidents: Investigation, Reconstruction and Causation available at: Information about the book and author is available at: