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How long does it take to stop?

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One of the reasons for driving in rain being more hazardous is reduced traction on
wet roads. All driving maneuvers but especially stopping need to be adjusted for the slippery conditions. This invites a review of the laws of “driving” physics.

A vehicle with brakes and tires in good working condition traveling at 90 kmh [60 mph] covers 27 metres [88 feet] per second. Stopping a vehicle traveling at this speed involves recognizing the need to stop, initiating braking and then braking to a stop.
To perceive that braking is required is estimated to take about three-quarters of a second. This means that the average driver traveling at 90 kmh will have traveled 20 metres before he or she has fully comprehended the need to slow down. It then takes, on average, another three-quarters of a second to initiate braking by moving from the gas pedal to the brake pedal…and thus another 20 metres is covered. In total then, a distance of 40 metres is covered even before the vehicle begins to brake.
Discussing braking invites review of Isaac Newton's First Law of Motion: objects in motion tend to remain in motion. In other words, motor vehicles in motion tend to resist stopping. And the faster they travel, the more they resist. And the more mass they carry, the harder they resist.
At 90 kmh, once braking starts, it takes 42 metres to come to a complete stop. This encompasses approximately 3.1 seconds. So from perceiving a braking situation to stopping, takes 4.6 seconds during which time the car travels over 82 metres, which is almost the length of a football field.

The higher the speed, the more time and distance it takes to stop. For example, at 110 kmh, perception and reaction distance equals 47 metres, and braking distance equals 57 metres, a total of 104 metres (5.2 seconds). At 130 kmh it takes over 128 metres (5.7 seconds) to stop a vehicle, and at 140 kmh more than 155 metres (6.2 seconds), over a tenth of a kilometer. 

These computations are based on dry pavement, using an average braking rate of .870 g. On wet roads, the braking rate drops considerably—from .870 g to .600 g.—and the braking distance grows exponentially. At 90 kmh, braking time increases from 4.6 seconds to 6.1 seconds, and braking distance increases from 82 metres to 101 metres.

It’s easy to see, therefore, why following too closely invites trouble. If a too closely followed car slows down or stops suddenly there is simply not enough time and space for the following car to avoid a collision. To regulate your following distance, a good rule of the thumb to apply is the "3-second rule." When the vehicle ahead of you passes a certain point, such as a sign, count “one-thousand-one, one-thousand-two, one-thousand-three.” This takes about 3 seconds. If you pass the same point before you finish counting, you are following too closely. Driving in rain or snow, add a second or two to this rule.



The writer of this column says that 90 kph is equal to 60 miles per hour. That is incorrect.

100 kph = 62.5 Miles per hour, and 90 kph is equal to 55.92 kph.


Cedric Hughes

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