Calculating Stopping Distance
* 1 Multiply vehicle velocity (miles per hour) by 5280 feet, and divide by 3600 seconds to convert from miles per hour to feet per second.
Example: (60 miles per hour) x (5280 feet) / (3600 seconds) = 88 feet per second
* 2 All drivers require time to perceive a hazard and react by beginning to brake. Average perception time is 0.75 second, and average reaction time is also 0.75 second, for a total of 1.5 seconds. Multiply velocity in feet per second by 1.5 seconds to find the distance traveled while the driver reacts and moves to brake.
Example: (88 feet per second) x (1.5 seconds) = 132 feet
* 3 Divide the initial velocity in feet per second by a reasonable acceleration (braking causes negative acceleration) to determine how long it will take the vehicle to stop. According to the Transportation Research Board of the National Academies, acceleration of -20 feet per second is a good estimate.
Example: (88 feet per second) / (- 20 feet per second) = 4.4 seconds
* 4 Calculate the distance traveled while braking by multiplying the total time spent braking by the average velocity (initial feet per second divided by 2). This is the braking distance.
Example: (4.4 seconds) x (88 feet per second / 2) = 193.6 feet
Be aware that higher velocity means a longer braking distance, but the relationship is not linear. A change in velocity from 30 mph to 60 mph increases braking distance four times.
* 5 Add together the perception and reaction distance from Step 2 and the braking distance from Step 4 to find the total stopping distance.
Example: (132 feet) + (193.6 feet) = 325.6 feet
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