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 is 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.

TABLE 1:  Conversation from miles per hour to feet per second:

SPEED DISTANCE

20 mph            29.4 ft./sec.

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 on a routine patrol, driving along at 40 mph (or 58.8 fps).  Something causes you to look away from 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.  The problem here is speed, and paying attention to the driving task. Diverting 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. Look at Table two

                         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.

70 mph          1022.9 Ft.        205.8 Ft.          308.7 Ft.

This gives you an idea of not looking where you are going.