How to Measure the Stopping Distance in Defensive Driving
Stopping distance is a combination of perception distance, reaction distance and braking distance, according to the California Department of Motor Vehicles. Understanding how stopping distance changes as a result of speed and road conditions is an important part of good defensive driving. Although precise calculation of stopping distance uses physics and requires exact values for vehicle velocity, acceleration, coefficient of friction, and perception and reaction times, an accurate estimate can also be found using simple math.
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Instructions
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Calculating Stopping Distance
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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
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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
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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
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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.
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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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Tips & Warnings
For an easy and close approximation of miles per hour to feet per second, a conversion factor of 1.5--30 miles per hour would convert to 45 feet per second. This approximate feet per second is slightly higher than the actual value, making the estimate of stopping distance slightly more conservative, a good outcome for defensive driving.
Calculated stopping distances never guarantee your or your car's performance. If your perception and reaction time is slower than 1.5 seconds or your brakes and tires perform poorly, it will take longer to stop. It is your responsibility to be alert and know your vehicle's performance so that you follow at an adequate distance to come to a safe stop.
The above calculations assume typical dry road conditions. However, if roads are wet or icy, stopping distance will increase. SmartMotorist.com recommends drivers follow the three-second rule---it should take 3 seconds after the vehicle in front of you passes a stationary point for your vehicle to reach that same point. In traffic or at night, this should be increased to 6 seconds; and in heavy rain, or especially in snow or ice, to 9 seconds. Defensive driving means erring on the side of caution.
