LESSON 1/TASK 1
o Look at the next to last column of figure 3. It is titled "Cum total." To determine the 85th
percentile, follow the column down until you reach the number that is the same as 26, or closest
to it. In this case it is the number 27. Look across to the appropriate speed column to
determine the speed. The 85th percentile speed is 21.4 (or 42.8, depending on which distance
column was used). The 15th percentile would be 145. That speed is 12 or 24 mph.
Uses
The speed study shows what drivers feel is the maximum and minimum safe driving speeds. A range,
based on the 85th and 15th percentiles, is much more helpful and accurate than a simple average.
Averages can be affected by one or two or a few vehicles going excessively fast or excessively slow.
The use of the percentile range corrects for these extremes.
From this range, speed limits can be adjusted. Speed limits that are too fast would be inherently
dangerous. Excessively slow speed limits encourage drivers to ignore them. This in turn creates bad
driver habits that may cause them to ignore speed limits generally.
The information determined from a speed study helps determine where traffic control devices are
required to encourage safe approach speeds. Such locations might include dangerous intersections or
curves, among others.
The timing of signals can also be adjusted based, at least in part, on the data collected in a speed
study. The timing of the amber light will depend on the speed of approaching traffic. Speed limits are
never set based on the timing of the signals.
A traffic speed map can be prepared showing areas where slow speeds cause congestion and fast
speeds create danger areas. Such a map is useful in selecting alternate routes and through streets.
Speed studies can also be used to determine the validity of complaints about speeding.
It is important to remember that speed studies are only as valid as the data on which they are based.
They must be conducted properly. MP selected to conduct speed studies must understand their
purpose as well as how to conduct them.
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