Shining light on speed

Measuring vehicle speeds with a laser

Have you ever wondered about the instruments the police use to measure vehicle speeds on our highways?

My favourite tool was the laser because it let me accurately measure the speeds of individual vehicles even when they were in a group on a busy highway.

Although the laser had to be used from a stationary position, either hand held or on a tripod, I was willing to trade my moving radar for it when I worked on busy, multi-lane highways.

When I pulled the trigger on the laser, it sent a train of infrared laser pulses toward the vehicle I had aimed it at. These pulses reflected off of the vehicle back to the laser device.

It had to see 80 per cent of these pulses returned in recognizable form or it wouldn't display a reading.

The time for a single pulse to return allowed the laser to calculate how far away the vehicle being measured was.

The speed of light in air is a constant, and the time base in the laser knew when the pulse was sent and how long it took to return.

The train of pulses allowed a series of distance measurements to be made and the change in those distances calculated. Of course, the change in distance over time is the speed of the vehicle being measured.

All of this was accomplished in a fraction of a second and a speed displayed on the readout. The only decision required at this point was whether I wanted to deal with the vehicle I had measured or continue to measure more vehicles in the hope of hooking a faster fish.

The beauty of the laser is the accuracy with which it can be aimed. My radar was much like using a flashlight — you were lighting up the whole world  50 metres away and it was up to you to identify what you were seeing in the beam.

In contrast, the laser emits a very tight beam. So tight in fact that it would cover a spot about the diameter of an orange at a distance of 250 metres. If you worked at a slight angle to the highway, it was possible to measure all the vehicles individually.

Whoops, at an angle to the highway? Won’t that affect the speed that the laser measures? Yes, both the laser and radar are subject to what is known as “cosine error.”

Simply put, the speed varies according to the cosine of the angle away from straight toward the unit. Fortunately for violators, the cosine error reduces the measured speed giving them a small break.

Some lasers can measure the distance between vehicles. One measured the first vehicle and then immediately measured the vehicle following directly behind it.

The laser would display both vehicle’s speeds and the distance between the two measuring points used. It was a very accurate way to issue following-too-closely violations.

The laser was simple to test as well as to use. When it was turned on, it did a self test just like your computer does.

If I did not see the readings designated by the manufacturer, it was not working properly and needed to be repaired before I could use it.

Next, I needed to measure three set distances and receive zero speed. If these measured correctly, I knew that the time base in the laser was accurate. Again, if not, the laser was not suitable to measure speed and needed to be repaired.

Finally, I needed to test the aim point of the scope. A telephone pole about 100 metres distant with only sky behind was ideal.

In this test mode the laser emitted a tone based on the distance of the reflecting object.

I would pan the aim dot across the pole and cross arm, horizontally and vertically. If the tone changed at exactly the point where the dot in the scope crossed the edge of the pole, it was aimed correctly.

If it wasn’t, I was able to adjust it and test again.

Now, I only needed a safe spot to operate that was not obstructed and the ability to hold the device very steady if it was not mounted on a tripod.

The laser was smart enough that if I was not steady or a waving bunch of grass or branches was in the way I would receive an error message instead of a speed reading.

Laser detectors find it very difficult to “see” the laser, especially when it was aimed low to catch the front licence plate.

Typically, they are not nearly as useful for early warning as a radar detector is, simply because of the beam width.

Article URL: http://www.drivesmartbc.ca/police/measuring-vehicle-speeds-laser


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About the Author

Tim Schewe is a retired constable with many years of traffic law enforcement experience. He has been writing his column for most of the 20 years of his service in the RCMP.

The column was 'The Beat Goes On' in Fort St. John, 'Traffic Tips' in the South Okanagan and now 'Behind the Wheel' on Vancouver Island and here on Castanet.net.

Schewe retired from the force in January of 2006, but the column has become a habit, and continues.

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