Speed studies are used to accomplish the following:
One or two MPs are required to conduct this study at each location. They should not be in uniform and MP patrols should avoid the area.
To be reflective of normal driving behavior, the study should be made in good weather and without Military Police presence. MP observers should be inconspicuously located while collecting data.
The study is divided into three time periods: 0900-1200, 1500-1800 and 2000-2200. Each location should be studied at least one (1) hour with at least 50 vehicles passing the location.
Speeds are checked on a controlled basis, such as every second or every third vehicle.
The study should not be conducted at a location that has recently been subjected to a speed enforcement program.
Speed studies are usually made at the following types of locations:
Roadways which have recently had changes in the speed limit or where existing speed limits need to be evaluated.
Locations where traffic control devices are proposed.
All major thoroughfares.
All high accident frequency locations.
Locations which normally require a reduction in speed are not studied unless specifically desired. They do not provide information on normal driving speeds.
The sample field sheet shown below will assist in compiling data. One sheet should be used for each direction of travel.
There are several types of equipment or methods that may be used in conducting speed tests. These include:
Radar--If available, radar equipment provides highly accurate readings. It can be used day or night, but must not be seen by motorists. If it is, they tend to slow down, and a normal speed is not obtained.
Electric Timers--These are also highly accurate. They are electric stopwatches which are actuated and stopped by vehicles passing over air tubes stretched over the roadway.
Mirror Boxes--Mirror boxes and stopwatches used over a measured course will give fairly accurate readings of speed.
Pace Method--This is the most unreliable and inaccurate method. If employed, the pace cars must be calibrated and unmarked.
In analyzing data received in a speed study, maximum and minimum speeds under normal conditions are the important facts. Average speed is not useful.
Maximum safe speed is considered to be the 85th percentile speed and minimum safe speed is the 15th percentile speed.
Follow these steps to find 85th and 15th percentile speeds:
1. Find the total number of vehicles checked. This is the cumulative total.
2. Find 15 percent of this total number (multiply the number by .15). Round off the results to the nearest whole number.
3. Subtract this 15 percent from the cumulative total. The result is the 85th percentile.
4. Find the total number of vehicles traveling in each speed classification. (For example, 1 vehicle was traveling 66.6 mph, 2 vehicles at 60.0 mph, 2 at 54.5 mph, etc.) In the cumulative total column, make a running total.
5. Starting at the top of the cumulative total column (the highest speed recorded), add the numbers downward until the number most equal to 15 percent of the total is found. This is the 85 percent percentile speed.
6. To find the 15 percentile speed, subtract the 15 percent from the cumulative total.
7. The 15th percentile will be the 85th percentile speed. The 85th percentile will be the 15th percentile speed.
A speed study shows what drivers feel is the maximum and minimum safe driving speeds.
From this information, speed limits can be readjusted. (Excessively low speed limits encourage drivers to ignore them.)
A study helps determine where traffic control devices are needed to encourage safe approach speeds to intersections or other dangerous locations.
A study assists in timing traffic signals. The length of a proper amber time depends on the speed of approaching traffic. Speed limits are never set to match the timing of 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 when selecting alternate routes or through streets.
Speed studies can determine if complaints about speeding are justified.
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