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Is there a “margin of error” for police speed guns?

By: Ticket Lawyer on March 28, 2018


Let’s get one thing straight from the outset: If you are doing even 1 km/h above the speed limit, you are speeding. So if you were in a 60 km/h zone and a police officer clocks you with a speed gun doing 61 km/h, they are within their rights to issue a driving ticket and even a penalty.

Anecdotal evidence, however, suggests the police are unlikely to stop a car unless they believe it is traveling at more than 20 km/h above the legal limit for that stretch of road. That is not to say they will not stop speeders who are doing 1 km/h to 19 km/h above the designated speed, but they simply would not have enough time if they stopped everyone going slightly over.

But say you were issued with a speeding ticket and you decide to challenge it, could you argue that the device used to measure your speed has an inherent margin of error? And if that window is wide enough, is it possible the motorist could have been recorded as going too fast even though they were travelling at an acceptable speed?

How police speed guns work

Under the Motor Vehicle Act, an electronic or paper record from a speed monitoring device can be submitted as evidence someone was speeding. One of the devices they use is a laser speed detection system.

Police measure the speed at which a vehicle is travelling by pointing the instrument at the licence plate area of the target and squeezing the trigger. Laser laser signals bounce off the car and are picked up by the machine to calculate how quickly an object is travelling.

All devices have to undergo checks to ensure they are making accurate measurements before and after an officer’s shift. One of them is called a “fixed distance, zero velocity check” which involves pointing the laser device at a static object 50m away. If the speed reading comes back as zero, it is calibrated correctly.

Another check is called the “self test”, which as the name suggests involves the machine running a diagnostic on itself. Failure to do these checks can result in decisions being overturned, such as in this case in Ontario, where a judge rules that an officer’s failure to so a self test created a reasonable doubt over whether the device was properly working.

The manufacturer also instructs users to carry out a “display test” to ensure the monitor is functioning as it should. As well as being in good working order, the device must also be used in the correct manner by a properly trained police officer.

“You have to have a human element when giving out traffic tickets.” – Grant Gottgetreu

What is the margin of error for police visual estimates?

If all of the above requirements are met, the readings captured by a laser speed device are considered to be valid evidence. In some instances, however, even one or two km/h could make a difference to a court’s decision — for instance, driving at “excessive speed”. So if a person is driving above the speed limit, they are likely to be pulled over by police and punished for speeding. If they are found to have been travelling at more than 40 km/h above the limit for that road they could also be convicted of “excessive speed” which means the driver faces a heftier fine, penalty points on their driving record and the immediate impounding of their vehicle.

Evidence in court will often consist of both the police officer’s visual estimate of the vehicle’s speed and the reading from a speed monitoring device. An officer’s visual estimate of speed will be less accurate than a laser device and as such the margin of error will be greater. Depending on what an officer says in court, the margin of error for a visual estimate could be anything from +/- 5  km/h to +/- 10 km/h. For more about visual estimates click here.

An attempt was made to appeal against an excessive speed conviction based partly on the margin of error of visual estimates. In this case from last year, the driver was caught speeding in an 80 km/h zone. The evidence against him consisted of the police officer’s visual estimate which was 130 km/h and the laser device’s recording of 128 km/h, putting the motorist in the excessive speed category. In court the police officer stated he was able to estimate speed with a margin of error of between 5 and 10 km/h. By applying the margin of error to the visual estimate, the vehicle could have been traveling at 120 km/h. To convict the driver of excessive speed the judge had to be satisfied he was traveling at more than 120 km/h.

Ultimately, the conviction for excessive speed was upheld because of the laser device evidence, despite the driver’s attempt to question its accuracy.

According to former cop Grant Gottgetreu, “You have to have a human element when giving out traffic tickets.” He said having a visual estimation is mandatory because the speed monitoring device is meant to reliably corroborate their suspicion.

What is the margin of error for police speed guns?

This raises the question, what is the margin of error for speed monitoring devices? According to the machine’s handbook, their speed accuracy can be +/- 2 km/h. This means it is less likely a speeding ticket could be overturned based on the machine getting it wrong, however, it might have other implications, such as when it comes to calculating a fine. A prosecutor only needs to establish that a driver was speeding and the number of kilometres per hour over the limit determines the issue of penalty.

In this case in Ontario, a driver who was convicted of travelling at 75 km/h on a 50 km/h road was able to successfully reduce his fine after he lodged an appeal. The judge found at a previous hearing a justice of the peace failed to take into account a police officer’s testimony about the accuracy of a laser device when calculating the fine. Although the driver was still proven to be speeding, it meant his sentence was varied and instead of his fine being based on travelling at 75 km/h in a 50 km/h zone, the reduced fee was calculated at a lower rate of 73 km/h.

Does distance affect the accuracy of police speed guns?

Another factor in the accuracy of speed guns is the distance of the vehicle. Although we couldn’t find any examples of the device’s speed accuracy being reduced over a longer distance, it can be more difficult to prove a particular vehicle was the one speeding the farther away it is.

The maximum range of police speed guns is 1,000 m, however, a judge in a Newfoundland and Labrador speeding case cited a book, The Law on Speeding and Speed Detection Devices by Shakoor Manraj QC and Paul. D. Haines, which states: “Do not target a vehicle beyond 1000 feet (300 m), since the court, in many jurisdictions, has the right to ask for strong supporting evidence and expert testimony.”

The authors go on to say: “There is a possibility past 1000 feet that part of the lasers being could strike and adjacent vehicle targeted by the laser gun.” The judge acquitted a driver of speeding in this instance because the speed gun took a reading from 372 m away.

The maximum range of a speed gun depends upon how reflective the target is. For instance, its range while aiming at the back of a vehicle can go up to 1,200 m but for the front of that same target, it is only 600 m. This is because the tail lights are usually more reflective than anything on the front. Colour, surface finish and shape of the target all affect range. Pointing the device at the windshield and not the licence plate also reduces the effective distance.

Could photo radar make a comeback?

Photo radar devices have not been used in BC since 2001, but as we previously reported, there has been some talk recently about having them reintroduced. The public turned against photo radar for a number of reasons but it appears the question of how accurate the devices has been raised. Say you were driving behind a car that was going too fast and a speed monitoring device picked this up, sending a signal to the camera to take a picture. How long is this “lag time” between when the sensor is alerted and the camera is activated? Is it possible that there is a delay that could lead to the wrong car being photographed?

This line of thinking proved to be unsuccessful. In this appeal case, a car was recorded doing 65  km/h in a 50 km/h zone. The driver, who had been convicted of speeding based on photo evidence, appealed against the decision. He argued there is a lag time between when the device measures the speed of a vehicle and when the photo is taken. Because of this, he argued, it did not fulfill the Motor Vehicle Act’s definition of a speed monitoring device, which must accurately and simultaneously” measure and record the speed of moving vehicles.

The driver submitted that lag time created a reasonable doubt as to whether his car had triggered the camera and he suggested that he was in a vehicle shown to have entered the radar beam three seconds later travelling at a lawful speed of 50 km/h. The appeal was eventually dismissed by the Court of Appeal for British Columbia after evidence given by a representative of the manufacturer of the photo radar system stated that the lag time would have been no more than microseconds. The judges therefore decided the delay was not enough to counteract the presumption of accuracy of the device to the point of raising a reasonable doubt.

The fact of the matter is that every case is different and it can be difficult to argue which is why you should hire a lawyer if you feel you have a case. At BC Driving lawyers, we are experts in driving law, and we are very familiar with the limitations of evidence from speed monitoring devices. Give us a call on 604-608-1200.