Measuring Vehicle Speeds With Laser

Laser GunHave you ever wondered about the instruments that the police use to measure vehicle speeds on our highways? My favourite tool was the laser because it gave me the ability to 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% of these pulses returned in recognizable form or it would refuse to 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 to traffic enforcement is the accuracy with which it can be aimed. My radar was much like using a flashlight. 50 meters away, you were lighting up the whole world 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 meters. 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.

One interesting feature of some lasers is the ability for it to 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 3 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 meters 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 license plate. Typically, they are not nearly as useful for early warning as a radar detector is, simply because of the beam width.

This brings to mind a co-worker of mine and a violator with a shiny, new, very expensive laser detector. The co-worker had pulled over the violator and the detector had not so much as emitted a single peep. Its owner was not impressed with the situation and said so.

After signing his ticket, the violator asked my co-worker to point the laser at his detector. He complied while the violator aimed the detector, turned it on and off, and finally banged on it a few times. There was nothing but dead silence from the detector. The violator left the site in a foul mood muttering something about getting a refund for the piece of junk that hadn’t saved him from a ticket.

As the vehicle pulled away, my co-worker turned to the other officer he was working with and said: “He didn’t ask me to pull the trigger....” Maybe you have to be a traffic cop to appreciate the humour.



This is an old article but still a good read. Glad to see you've revived it. I do like the joke at the end.

Our Policy Is To ALWAYS Blame The Computer


I didn't think that anyone would remember.


I recognise that as a police officer, you had certain latitudes but they were heavily regulated.  My problem with speed enforcement is that it, because it is easily measured, has become the end all about traffic.  Speed in itself is not the problem. What is the problem is the inability to control a vehicle, no matter at what speed.

Indeed, speed limits are set by highway engineers for optimum conditions and consideration of less than perfect drivers. Let's remember here that 95% of drivers consider themselves as "above average".  It is quite amazing how many do not understand that stopping distance and centrifigal force are affected by the square of the speed but only a direct relationship to weight.

Also, are we to forget about all those other factors?  Like weather, wet, dry, sunshine, dark, mist and snow?  How about tire wear, vehicle condition, alignment and so on? And driver fatigue, distraction, impairment (another "measurable") or plain lack of ability?

Speed in a School Zone and high traffic areas is foolhardy but setting radar/laser traps at the foot of long hills on an open highway smacks of entrapment.  Prior to the speed limit increases (110 - 120 KPH) many a trucker got busted for being 10-15 KPH over on the roll out .... on an open highway with no other traffic.

Years ago, I did some rally driving.  Not the stuff you see today on closed road sections, but sometimes pretty hairy.  But I never hit anything nor did I go off into the weeds.  In 1962 I bought a PV544 B-18 Volvo to discover that the orininal tires were (not good) so decided to learn how to drive it hard on dry pavement.  We lived above the Upper Levels Highway in West Van back then and coming home about 2 AM I was in a left hand bend with the inside front wheel about 18 inches up .... all committed to the corner.  Coming around the corner towards me was Johnny Bates, a West Van cop in his patrol car.  What do do?  Couldn't lift off ... so I waved.  Around the next bend, I pulled over because I just knew Johnny was coming!  He was ... and darn near hit me getting stopped.  He was a tad upset (Read me the riot act) but when he calmed down he started asking how I did that.  Next, I've got Johnny in the passenger seat going through the same corner with the front wheel up!  Yes, Johnny was a good guy .... always wondered where he ended up.

But I digress ..... I think the point is to recognise that "speed" is only one part of the equasion.  Other factors are just as, if not more important.  News stories quoting, "Speed was a factor" show ignorance.  Of course speed was a factor because if it wasn't moving (speed) it wouldn't have hit anything.  But "Excessive speed" I can live with.