Two-second rule

The two-second rule is useful as it can be applied to any speed. Drivers can find it difficult to estimate the correct distance from the car in front, let alone remember the stopping distances that are required for a given speed, or to compute the equation on the fly. The two-second rule provides a simpler way of perceiving the distance. 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. 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. One can count the duration of time simply by saying "zero... one... two". Some instructors suggest 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. 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. ==Three-second rules==

Some authorities regard two seconds as inadequate, and recommend a three-second rule. German law requires a minimum 0.9 second distance but when tested under relaxed conditions researchers found that their test subjects spent 41% of the test time at following distances under 0.9 seconds. The United States National Safety Council suggests that a three-second rule—with increases of one second per factor of driving difficulty—is more appropriate. Factors that make driving more difficult include poor lighting conditions (dawn and dusk are the most common); inclement weather (ice, rain, snow, fog, etc.), adverse traffic mix (heavy vehicles, slow vehicles, impaired drivers, pedestrians, bicyclists, etc.), and personal condition (fatigue, sleepiness, drug-related loss of response time, distracting thoughts, etc.). For example, a fatigued driver piloting a car in rainy weather at dusk would do well to observe a six-second following distance, rather than the basic three-second gap. ==See also==