Police use of radar to catch Speeders

[jabjab]

I was coming back from Lethbridge a few days ago and my radar detector went off but there was nothing in sight and it maybe took a full minute of driving to realize there was a police officer pulled over on the side of the road , I wasn't speeding (110 in hour maybe)

If my radar detector can pick up the signal from so far away then i'm assuming a cops use of radar is similar but if they can pick up something that is moving faster without seeing the car how do they know which car it is if there are multiple cars on that highway?

[A790]

1) https://www.nolo.com/legal-encyclopedia/free-books/beat-ticket-book/chapter6-1.html

2) https://www.quora.com/How-accurate-is-a-police-radar-when-reading-a-cluster-of-cars

[jacky4566]

In a nutshell, The receiver in the cops device is very directional and focused while your radar detector is onmi-directional.

The radar guns blast out X, K and Ka band signals and looks for a directed Doppler response withing a certain angle and of time to determine speed through the Doppler effect.

But the emitter is also hitting things that can reflect in a different direction. Windshields, Semi trucks etc, and your detector is getting this reflection.

All your radar detector does is look for specific frequencies and patterns regardless of the direction. While the cop needs to see a directional response to work correctly.

[revelations]

Having been on the back end of the radar units in action - with multiple vehicles approaching (multiple lanes), the unit just spits out the highest number - its up to the member to decide who was going the fastest.

The radar units also emit sound in the cruiser so that normal traffic (eg. 50 kph) sounds like a steady tone (eg. 300hz) while someone going 70 kph would sound at 400hz, which would immediately alert the member.

[jabjab]

So really if the cops radar goes off with a high speed of lets say 130km/h and the cars aren't in vision yet, he will most likely look for a the sportier car to be the culprit then vs a mini van for example?

[Misterman]

So really if the cops radar goes off with a high speed of lets say 130km/h and the cars aren't in vision yet, he will most likely look for a the sportier car to be the culprit then vs a mini van for example?

That's unlikely to happen, but if for some reason the operator received a speed readout before a vehicle was in sight, the operator would probably wait for vehicles to come in sight and then fix a target.

[revelations]

So really if the cops radar goes off with a high speed of lets say 130km/h and the cars aren't in vision yet, he will most likely look for a the sportier car to be the culprit then vs a mini van for example?

No, thats generally not the case. However there have been tickets thrown out because the member had the wrong vehicle written down or that, under cross in traffic court, the member could not determine which vehicle was speeding, in a multi lane situation.

With single lane roads its usually pretty simple. However, and this is rare, if you have a person in a small car doing the speed limit and a large truck is approaching behind them at 20 over, the radar will pickup the stronger signal from the truck. Most members are trained for all this though and mistakes are pretty rare.

What IS the case, with stationary LIDAR traps, is that members will tend to laser the vehicles in the fast lane - or if two vehicles side by side, they are more likely to scan a bike over a minivan - because of course, bikes always speed!

[jabjab]

I'm just trying to make sense of a ticket I received a few months ago where I was not going anywhere near what the RCMP officer said I was. With a busy highway I was the only sports car out of 5 or 6 other vehicles. With a far stretch of not seeing on coming traffic (partly due to a section of hills and dips) I don't see how he can say it's me. I admitted to speeding but not the speed he said.

Court is outside of Calgary and they refused to negotiate anything so I set up a trial date.