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Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction between the tires and the road surface, [Note 1] and negligibly by the tires' rolling resistance and vehicle's air drag.
d MT = braking distance, m (ft) V = design speed, km/h (mph) a = deceleration rate, m/s 2 (ft/s 2) Actual braking distances are affected by the vehicle type and condition, the incline of the road, the available traction, and numerous other factors. A deceleration rate of 3.4 m/s 2 (11.2 ft/s 2) is used to determine stopping sight distance. [6]
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.
British Railway Class 90 infobox showing brake force Brake force to weight ratio of the Class 67 is higher than some other locomotives. In the case of railways, it is important that staff are aware of the brake force of a train so sufficient brake power will be available to bring the train to a halt within the required distance from a given speed.
In the case of automobile traffic, the key consideration in braking performance is the user's reaction time. [6] Unlike the train case, the stopping distance is generally much shorter than the spotting distance. That means that the driver will be matching their speed to the vehicle in front before they reach it, eliminating the "brick-wall" effect.
When braking suddenly or during collisions, passengers whip forward with an initial acceleration that is larger than during the rest of the braking process because muscle tension regains control of the body quickly after the onset of braking or impact.
The braking distance will be approximately 250 m (820 ft) at 100 km/h (62 mph) and 600 m (2,000 ft) at 160 km/h (99 mph). High-speed trains are usually equipped with a magnetic track brake, which can give about 0.3 m/s 2 extra, and give braking distances of about 850 m (2,790 ft) at 200 km/h (120 mph) and 1,900 m (6,200 ft) at 300 km/h (190 mph).
The stopping distance s is also shortest when acceleration a is at the highest possible value compatible with road conditions: the equation s=ut + 1/2 at 2 makes s low when a is high and t is low. How much braking force to apply to each wheel depends both on ground conditions and on the balance of weight on the wheels at each instant in time.