Braking distance
A perception-reaction time of 1.5 seconds,[2][3][4] and a coefficient of kinetic friction of 0.7 are standard for the purpose of determining a bare baseline for accident reconstruction and judicial notice;[5] most people can stop slightly sooner under ideal conditions.
The latter is a road alignment visibility standard that provides motorists driving at or below the design speed an assured clear distance ahead (ACDA)[6] which exceeds a safety factor distance that would be required by a slightly or nearly negligent driver to stop under a worst likely case scenario: typically slippery conditions (deceleration 0.35g[7][Note 3]) and a slow responding driver (2.5 seconds).
The theoretical braking distance can be found by determining the work required to dissipate the vehicle's kinetic energy.
The work W done by braking is given by: where μ is the coefficient of friction between the road surface and the tires, g is the gravity of Earth, and d is the distance travelled.
The braking distance (which is commonly measured as the skid length) given an initial driving speed v is then found by putting W = E, from which it follows that The maximum speed given an available braking distance d is given by: From Newton's second law: For a level surface, the frictional force resulting from coefficient of friction
These values incorporate the ability of the vast majority of drivers under normal road conditions.
[2] However, a keen and alert driver may have perception-reaction times well below 1 second,[11] and a modern car with computerized anti-skid brakes may have a friction coefficient of 0.9--or even far exceed 1.0 with sticky tires.
[12] The coefficient of friction may be 0.25 or lower on wet or frozen asphalt, and anti-skid brakes and season specific performance tires may somewhat compensate for driver error and conditions.
The actual total stopping distance may differ from the baseline value when the road or tire conditions are substantially different from the baseline conditions, or when the driver's cognitive function is superior or deficient.
A driver who has innate reflexes, and thus braking distances, that are far below the safety margins provided in the road design or expected by other users, may not be safe to drive.
[19][20][21] Most old roads were not engineered with the deficient driver in mind, and often used a defunct 3/4 second reaction time standard.
There have been recent road standard changes to make modern roadways more accessible to an increasingly aging population of drivers.
stopping distance = 5 squared = 25, add a zero = 250, divide by 2 = 125, sum 2*50 = 225 feet (the exact value can be calculated using the formula given below the diagram on the right).
At 50 km/h this corresponds to about 15 m. For higher speeds up to about 100 km/h outside built-up areas, a similarly defined 2-second rule applies, which for 100 km/h translates to about 50 m. For speeds on the order of 100 km/h there is also the more or less equivalent rule that the stopping distance be the speed divided by 2 k/h, referred to as halber tacho (half the speedometer) rule, e.g. for 100 km/h the stopping distance should be about 50 m. Additionally, German driving schools teach their pupils that the total stopping distance is typically:
