Two-second rule
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The two-second rule is a rule of thumb by which a driver may maintain a safe following 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 the driver's vehicle. It is intended for automobiles, although its general principle applies to other types of vehicles.
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 dramatically reduce risk of collision, and also the severity of an accident should an accident occur. 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 or hazardous conditions such as black ice, it is important to maintain an even greater distance of three or four seconds.
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. However, don't take your eyes off the vehicle for more than a second or that would defeat the purpose. As you count to yourself the elapsed time in seconds, the front of your car should pass the same point no less than two seconds later. If the elapsed time is less than this, increase the distance, then repeat the method again until the time is at least 2 seconds.
One can count the duration of time simply by saying "zero... one... two" but for greater accuracy, it is suggested that drivers say "only a fool breaks the two-second rule". At a normal speaking rate, this sentence takes approximately two seconds to say, and serves as a reminder to the driver of the importance of the rule itself.
The TailGuardian distance advisory decals recently adopted by Stagecoach Buses in the UK use the two second rule in their calibration[3]. Advisory Decals for 30, 50 and 70 mph are calibrated to be invisible outside those safe distance, only rendering themselves visible once the car following has entered the safety zone for the speed that they are travelling.
