Set up your observation in an area of a known distance. If you don't know the distance, you may have to measure it. For instance, if you are watching cars travel down a section of highway, measure of a distance of 1,320 feet, which is a fourth of a mile.
Watch an object move through the marked area and count from the time the object enters the area until the object leaves the area. Count with a smooth unhurried rhythm such as "one-one thousand, two-one thousand . . ."
Divide the distance from Step 1 by the time from Step 2. For example, if you watched a car travel 0.25 miles in a time of 30 seconds, divide 0.25 by 30 to get 0.0083. This means that the car is traveling an estimated speed of 0.0083 miles per second.
Convert your answer to miles per hour. In this case, you have to convert seconds to hours. There are 60 seconds in a minute and 60 minutes in an hour. Multiply 60 times 60 to get a conversion factor of 3,600. Then multiply your answer from Step 3 (0.0083) times 3,600 to get an estimated speed of 29.88 miles per hour. You may want to round this figure to an even number, such as 30 miles per hour.