Section #2 - Speed Measurements - Page 19
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Line of Sight
Ideally, you should have a clear line of sight to the target vehicle.
•
If there is a momentary break in the beam,
the instrument will
accumulate data and may be able to capture the target vehicle's speed.
The instrument will display an error code if it cannot capture the target
vehicle's speed.
•
If there is an extended break in the beam,
the instrument will display an
error code.
The Cosine Effect
If the target vehicle is moving directly toward or away from you, the speed
measured by the TruSpeed 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.
Measured Speed by Angle: The Cosine Effect
• 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. The table below shows this effect.
Angle
(degrees)
True Speed
50 km/h 70 km/h 90 km/h 110 km/h 130 km/h
Measured Speed (km/h)
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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