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. For rubber tyres on cars, the coefficient of friction () decreases as the mass of the car increases. Additionally, depends on whether the wheels are locked or rolling during the braking, and a few more parameters such as rubber temperature (increases during the braking) and speed. Rules of thumb In a non-metric country the stopping distance in feet given a velocity in MPH can be approximated as follows: take the first digit of the velocity, and square it. Add a zero to the result, then divide by 2. sum the previous result to the double of the velocity. Example: velocity = 50 MPH. 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). In Germany the rule of thumb for the stopping distance in a city in good conditions is the 1-second rule, i.e. the distance covered in 1 second should at most be the distance to the vehicle ahead. 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: In the UK, the typical total stopping distances (thinking distance plus braking distance) used in The Highway Code are quoted in Rule 126 as: 20 mph: 40 feet (12 metres) 30 mph: 75 feet (23 metres) 40 mph: 118 feet (36 metres) 50 mph: 175 feet (53 metres) 60 mph: 240 feet (73 metres) 70 mph: 315 feet (96 metres) See also Assured clear distance ahead Brake Cadence braking Skid mark Stopping sight distance Threshold braking Vehicle metrics Vehicular accident reconstruction Notes References Further reading External