Section #2 - Speed Measurements
Page 19
The cosine effect decreases as the range to the target vehicle increases.
• At the maximum range of the instrument, the vehicle is so far away that
the angle between it and the instrument is very small indeed. The
instrument's perception of the target's speed is identical to its true
speed.
• As the vehicle approaches, the angle increases until it becomes large
enough to affect the measurement.
The table below shows acceptable parameters for minimizing the cosine effect.
The chart indicates the percentage of true speed measured, given the distance
from the roadway and the distance from the target vehicle. To find a target's
measured speed, multiply the true speed by the number in the chart.
The diagonal created by the boldface numbers indicates the boundary between
acceptable and unacceptable parameters.
•Numbers
above
the diagonal are acceptable margins of error.
•Numbers
below
the diagonal are unacceptable margins of error.
Remember that the cosine effect is always in the motorist's favor.
L
To minimize the cosine effect, keep the angle small. Set up the
instrument as close to the road as possible without creating safety
risks, and target down the road at ranges sufficient to keep the angular
difference small.
Distance
off the
roadway
(feet)
Range to Target Vehicle
100 ft 250 ft 500 ft 1000 ft 2000 ft
fraction of the True Speed that will be measured
10
.9950
.9992 .9998 .9999 1.0000
25 .9682
.9950
.9987 .9997 .9999
50 .8660 .9798
.9950
.9987 .9997
100 .0000 .9165 .9798
.9950
.9987
200 .0000 .6000 .9165 .9798
.9950
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As a general rule, do not exceed 10 feet off the road for every 100 feet
shooting down range to the targets. If target vehicles will be 500 feet
down the road, set up no more than 50 feet off the road.
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