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Rain fog and snow can impact the reliability of police laser and radar

Nice Weather and Speeding Tickets

It rains almost every day, month after month, in the Vancouver area. So when the clouds clear people like to get out and many of us like to drive. And when the roads are dry it can be particularly enjoyable to drive a little faster. One problem is that the traffic cops seem to always appear just when the sun comes out. When it’s rainy most police officers would prefer to enjoy their donuts in the warm dry coffee room back at the detachment. Nobody wants to stand out in the rain. As a result, nice weather and speeding tickets have a direct correlation.

The police hand out a lot more speeding tickets when it’s not raining, but it’s not just because the donuts stay dry and people drive faster when the weather is nice. You may not be aware that police laser and radar devices work differently in different weather conditions. The operators manuals typically explain that range in particular as well as accuracy may be adversely affected by fog, rain and snow. What does this mean?

Firstly it means that if the weather is nice, it increases the likelihood that the speed reading obtained is accurate. Although every device used by the police to detect vehicle speed can provide an inaccurate reading, when the weather is clear the reading is more likely to be correct.

In snow and rain, the signal needs to travel twice through the interfering weather before returning to the receiver.

Fog, rain and snow are all known to interfere with both radar and laser speed measurement. The problem is easy to understand. With radar speed devices a radar signal is transmitted from the device toward the target. The signal bounces back with a different frequency. The unit interpolates the speed based on the change in the frequency of the signal. In snow and rain, the signal needs to travel twice through the interfering weather before returning to the receiver. Invariably the strength of the signal of the target is reduced depending on the distance and the severity of the weather. In many circumstances, fog, snow and rain can make it impossible to positively conclude that the no other vehicles are in the line of sight of the radar signal.

With lasers a narrow beam of light needs to pass twice through the interfering weather before returning to the device. Although the designers of the devices have incorporated features to deal with poor weather, they advise users that accuracy is impacted by heavy fog, rain or snow.

What constitutes heavy rain? How does a police officer conclude that the rain at any given moment is light enough to ensure that the signal is accurate?

This is the part that is never covered in the operators manuals or the RCMP manuals that are used by most of the police forces in BC. From what we see, if it was a rainy day the police officer who used the device will rarely have any recollection of the weather conditions at the particular moment when they used the device. And when it comes time to testify in court this can be very important in the defence of a speeding ticket.

But of course, if it’s sunny out when you are nabbed for speeding these defences won’t help you. The police know this. Which is one of the reasons why nice weather and speeding tickets go hand in hand.