Our first problem is the device we use to measure speed, the speedometer. It is not the accuracy of the speedometer that is the problem it is how we interpret the speedometer. Speedometers measure speed in terms of miles per hour (mph). But in the event of an emergency you do not have an hour to make decisions. Decisions need to be made in seconds. Also you don’t have a mile to maneuver your vehicle; there are only a couple of feet between success and failure.
We need to rethink our frame of reference when we talk about speed. We need to look at speed in terms of feet per second (fps) not miles per hour. Let’s take a minute to see how the conversion from mph to fps works: If you are traveling 40 miles per hour you are traveling 58.8 feet per second. To convert miles per hour to feet per second, multiply the miles per hour figure by 1.47. The 1.47 comes from dividing how many feet there are in a mile (5280 feet) by 3600, which is how many seconds there are in an hour. Table one is a conversion of mph to fps.
SPEED DISTANCE
30 mph 44.1 ft/sec
40 mph 58.8 ft/sec
50 mph 78.5 ft/sec
60 mph 88.2 ft/sec
70 mph 102.9 ft/sec
Let’s see how using feet per second as a measurement can make a big difference when discussing accidents. You’re driving along at 40 mph (or 58.8 fps). You decide to change the radio station – or worse yet text message someone. You are not paying attention to the road for three seconds. At the same moment, another driver starts to cross an intersection 300 feet (the length of a football field) in front of you. Since your attention was diverted for three seconds and you were traveling at 58.8 fps, you drove a total 177.4 feet without looking where you were going. This puts you 122 feet from the intersection and its conflicting traffic. At this point, you look forward again, see a car blocking the intersection and realize that if you don’t do something quick life is about to become terribly exciting.
Let’s examine this situation a little more closely. You’re 122 feet in front of the conflicting traffic, and closing on them at 58.8 fps. If you can get your foot on the brake in a half of a second, you’re very fast. Traveling at your speed, that half of a second represents about 30 feet. So at the point of applying your brakes, you are about 92 feet from the traffic, still doing 40 mph (58.8 fps). You have just applied your brakes. Can you stop in time? At this point, avoiding a collision would depend more on luck than skill. That scenario is unrealistic because it did not include time to make a decision. Add one second of decision time or 58.8 feet, you would now be about 33 feet from the stopped cars going 58.8 feet per second. It’s going to hurt.
No matter the reason if you divert your attention away from driving for three seconds at 40 mph in an urban environment can be hazardous to your health.
At 60 mph looking away for three seconds will carry you 264 feet.
one sec two sec three sec
20 mph 29.4 Ft 58.8 Ft 88.2 Ft
30 mph 44.1 Ft 88.2 Ft 132.3 Ft
40 mph 58.8 Ft 117.6 Ft 176.4 Ft
50 mph 73.5 Ft 147 Ft 220.5 Ft
60 mph 88.2 Ft 176.4 Ft 264.6 Ft
