Section #2 – Speed Measurements 25
Electronic Copy of LTI's UltraLyte LR B User's Manual - 2
nd
Edition © 2002
The Cosine Effect
If the target vehicle is moving directly toward or away from you, the
speed measured by the UltraLyte LR B is identical to the vehicle’s true
speed. However, the instrument is usually set up on the side of the road
for safety. This results in an angle between the instrument’s position
and the target vehicle's direction of travel. When the angle is
significant, the measured speed is less than the target's true speed. The
phenomenon is known as the
cosine effect
. Cosine is the trigonometric
function that relates to this phenomenon.
The difference between the measured speed and the true speed
depends upon the angle between the instrument’s ideal position— the
position where targets would be moving in direct line with the
instrument—and its actual position.
•
The larger the angle, the lower the measured speed.
• The effect always works to the motorist's advantage.
• Loosely speaking, the cosine effect is not significant as
long as the angle remains small. Table 6 shows this
effect.
Table 6. Measured Speed by Angle: The Cosine Effect
True Speed
30 m
h 40 m
h 50 m
h 60 m
h 70 m
h
Angle
(degrees)
Measured Speed (mph)
0 30.00 40.00 50.00 60.00 70.00
1 29.99 39.99 49.99 59.99 69.99
3 29.96 39.94 49.93 59.92 69.90
5 29.89 39.85 49.81 59.77 69.73
10 29.54 39.39 49.24 59.09 68.94
15 28.98 38.64 48.30 57.94 67.61
20 28.19 37.59 46.99 56.38 65.78
45 21.21 28.28 35.36 42.43 49.50
90 00.00 00.00 00.00 00.00 00.00
50 kmh 70 kmh 90 kmh 110 kmh 130 kmh (degrees)
Measured Speed (KmH)
0 50.00 70.00 90.00 110.00 130.00
1 49.99 69.99 89.99 109.98 129.98
3 49.93 69.90 89.88 109.85 129.82
5 49.81 69.73 89.66 109.58 129.50
10 49.24 68.94 88.63 108.33 128.02
15 49.30 67.62 86.93 106.25 125.57
20 46.98 65.78 84.57 103.37 122.16
45 35.36 49.50 63.64 77.78 91.92
90 00.00 00.00 00.00 00.00 00.00
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