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Mudassir Ali

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Drivers typically overestimate their ability to suddenly stop their car in an emergency. This is exemplified by the routine tailgating on American freeways. For example, a typical car and driver would take a little more than the length of a football field, 100 yards, to stop a car going 60 miles an hour. And this is on a dry surface.

“Highway traffic and safety engineers have some general guidelines they have developed over the years and hold now as standards. As an example, if a street surface is dry, the average driver can safely decelerate an automobile or light truck with reasonably good tires at the rate of about 15 feet per second (fps). That is, a driver can slow down at this rate without anticipated probability that control of the vehicle will be lost in the process.”

“The measure of velocity is distance divided by time (fps), stated as feet per second. The measure of acceleration (or deceleration in this case) is feet per second per second. That assumes a reasonably good co-efficient of friction of about .75; better is .8 or higher while conditions or tire quality might yield a worse factor of .7 or lower.”

“No matter the velocity, that velocity is reduced 15 fps every second. If the initial velocity is 60 mph, 88 fps, after 1 second elapsed, the vehicle velocity would be 73 fps, after 2 seconds it would be 58 fps decreasing progressively thereafter. For the true mathematical perfectionist (one who carries PI to 1000 decimal places), it would have been technically correct to indicated the formula is ‘fpsps’ rather than ‘fps’, but far less understandable to most drivers. Since at speeds of 200 mph or less, the difference from one method to the other is in thousanths of seconds, our calculations in these examples are based on the simple fps calculations.”

“Given the previous set of conditions, it would mean that a driver could stop the described vehicle in a total of 6.87 seconds (including a 1 second delay for driver reaction) and your total stopping distance would be 302.28 feet, slightly more than a football field in length!”

“Virtually all current production vehicles’ published road braking performance tests indicate stopping distances from 60 mph that are typically 120 to 140 feet, slightly less than half of the projected safety distances. While the figures are probably achievable, they are not realistic and certainly not average; they tend to be misleading and to those that actually read them, they create a false sense of security.”