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Reaction Time: A Crucial Component to Braking

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Reaction Time: A Crucial Component to Braking

On the Scene E-Newsletter, Edition 111, March 18, 2010

I20100314_2n many cases, the time it takes for someone to react to a change in their environment is integral to assigning liability. In an auto accident for example, there is more to consider than the simple braking distance of a vehicle. Before the car can begin braking, the driver must notice a situation, recognize it as a danger, derive available courses of action, choose one and then begin to implement it. This all happens very quickly, but it still takes time. This Reaction Time must be added to the device response time to come up with the total stopping time or the total stopping distance.

Consider this simple example: Dudley is driving his car on a dry, level road at 55 mph (81 feet/sec) during a pleasant day in Canada. As he drives around a curve, he suddenly spots a moose that is crossing the road in front of him. What is the shortest stopping distance that Dudley can reasonably expect?

Suppose Dudley ‘s reaction time is 1.5 seconds. During this time, Dudley looks up, his eyes widen. He quickly realizes he can swerve left, right or try to stop but ultimately decides to slam on his brakes.

This means that Dudley ‘s car will travel for 1.5 seconds at 81 ft/second or approximately 122 feet before the brakes are even applied. Assuming that the brakes engage immediately upon the foot touching the pedal, the physical stopping distance is (D = S² / (30 x f) where ‘S'= mph and ‘f'= the coefficient of friction) approximately 134 feet.

So, the Total Stopping Distance = 122 ft + 134 ft = 256 ft. Almost half the distance is taken up by Dudley ‘s reaction time!

The 1.5 seconds reaction time used in this example is somewhat conservative. It is used here to make a further point. Although 1.5 is an often used “canned” value, it is also often misused as no two situations are ever the exact same. Response speed depends on many factors, such as the level of alertness (was Dudley texting or playing with the radio?), visibility (was it at night or foggy?) and the nature of the danger (would Dudley notice a small grey squirrel as quickly as a large moose?). So there can be no single, reaction time value that applies universally. Each type of reaction time has its own peculiarities that must be identified and examined. As experts in vehicular accident reconstruction, CED engineers are trained and experienced in identifying factors that affect reaction time as well as applying those factors in the overall accident analysis.

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