Cars go whizzing past us on the freeway and a fun thing to do is to measure how fast they are going. There is an easy way to measure precisely, at least in theory, the speed of the passing car. Start counting just as their rear bumper passes your front bumper: One thousand one, One thousand two – whatever your favorite way of counting seconds is – and as you count look down at you speedometer. If you are going 60, count for six seconds and at that instant note exactly where the passing car’s rear bumper is at and continue at your speed until your bumper passes over that exact spot. If it takes exactly eight seconds the passing car is going eighty miles per hour.

This system is mathematically accurate, and the only errors are the inexact qualities of your speedometer, your consistency of counting and your ability to see the exact spot where the passing car actually is at the time of your count. Each of these are variable to five percent or so, but the accuracy of your measurement should be within a mile or two of their actual speed. If someone passed you going really fast it helps to count faster, because they will be a long ways away when you finish your normally slow one second count.

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