LESSON 1/TASK 1
Mirror Boxes. Mirror boxes are a locally manufactured device. The boxes contain a mirror that
"flashes" as a vehicle passes. The MP activates a stopwatch at the first box and deactivates it at the
second "flash." The boxes must be set a measured distance apart. The speed can then be calculated
based on the distance between the two boxes and the time between the two "flashes." They are fairly
accurate devices, depending on the training and experience of the operator.
Pace Method. This is as much a method as it is an item of equipment. The pace method "equipment"
is the calibrated speedometer of a vehicle. It is the least accurate manner of determining speed. It is
also the most time consuming. Vehicles must be followed one-by-one. In areas of high traffic, this can
become a problem, although more than one pace vehicle may be used. That, however, uses a great
deal of manpower and equipment. The pace vehicle's speedometer must be calibrated. Additionally,
the car must be an unmarked, relatively non-descript vehicle so that the motorists being observed
continue their normal driving habits. The MP assigned to the pace vehicle must be well trained in this
method for the data gathered to be accurate.
Study Data
Once the data on a speed study has been collected, it must be analyzed to be of value. The most
important facts to be determined are the maximum and minimum speed under normal conditions. The
maximum safe speed is considered to be the 85th percentile speed and the minimum safe speed is the
15th percentile speed. In other words, 85% of the drivers will operate their vehicles at or below the
maximum safe speed. To determine these speeds, follow these steps:
o Determine the cumulative total. This is the total number of vehicles checked. Use figure 3 as
an example. The cumulative total is 171.
o Multiply the cumulative total by .15. This procedure determines what number is 15% of the
cumulative total. The results should be rounded off to the nearest whole number. In the case of
figure 3, this would be 25.65 or 26. The 85th percentile is 26. (100%, less 15%, equals 85%.)
o You also need to determine the number that is the same as 85% of the cumulative total. The
process is the same, except .85 is substituted for .15. 85% of 171 is 145.35 or 145.
18
Previous Page
