Braking distance is an important factor in driving. Reaction times for a racing driver will be around 0.25 seconds but perhaps for an older person whose mind and legs are not as agile is one second.
The best braking deceleration is generally considered as 1g. 1g is the retardation due to gravity if you throw a stone into the air. More fierce braking (1.0g – 1.2g) than this is possible for some cars as tires key into road surface than rub across.
Thinking distance (reaction distance) plus braking distance is the recommended stopping distance. The Highway Code assumes that the reaction time of an average driver is 0.7 seconds to think and then the driver can operate the brake pedal. The reaction time for a driver at 30 mph is 30 feet, 40 feet at 40 mph etc. A car at 30 mph stops in 30 feet while braking at 1g. In order for a gradual build-up to maximum braking in practice, the Highway Code distances are calculated at 0.7g (43 feet from 30 mph). With 0.25 second reaction time and good road car brakes, a racing car driver might stop from 30 mph in 11 + 25 = 37 feet, while with 1 second reaction time and normal car braked, an older person would need 44 + 43 = 87 feet.
Increase of braking distance is as speed x speed – so 43 feet from 30 mph is 172 feet from 60 mph. So it is always recommended to keep your safe distance from the car in your front. While traveling on fast roads, the safest is to leave at least two seconds between the car in front and your own car passing the same spot. It needs to be more if the road is slippery.
