Two second rule

The two-second rule (also known as the three-second rule in some states) is a rule of thumb by which a driver may maintain a safe trailing distance at any speed. The rule is that a driver should ideally stay at least two seconds behind any vehicle that is directly in front of his or her vehicle. It is intended for automobiles, although its general principle applies to other types of vehicles.

The two seconds is not a guide to safe stopping distance. It is more a guide to reaction times.

The two-second rule is useful as it can be applied to any speed. It is equivalent to one vehicle-length for every 8 km/h (5 mph) of the current speed, but drivers can find it difficult to estimate the correct distance from the car in front, let alone to remember the stopping distances that are required for a given speed, or to compute the linear equation on the fly. The two-second rule gets around these problems, and provides a simple and common-sense way of improving road safety.

The practice has been shown to considerably reduce risk of collision, and also the severity of an accident, if an accident occurs. It also helps to avoid tailgating and road rage for all drivers.

The risk of tailgating is largely caused by the accident avoidance time being much less than the driver reaction time. Driving instructors advocate that drivers always use the "two-second rule" regardless of speed or the type of road. During adverse weather, downhill slopes, or hazardous conditions such as black ice, it is important to maintain an even greater distance.

The two-second rule tells a defensive driver the minimum distance needed to reduce the risk of collision under ideal driving conditions. The allotted two-seconds is a safety buffer, to allow the following driver time to respond.

To estimate the time, a driver can wait until the rear end of the vehicle in front passes any distinct and fixed point on the roadway - e.g. a road sign, mailbox, line/crack/patch in the road. This should be done without looking up for more than one second, which would defeat the purpose. After the car ahead passes a given fixed point, the front of one's car should pass the same point no less than two seconds later. If the elapsed time is less than this, one should increase the distance, then repeat the method again until the time is at least two seconds.

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Wikipedia