CAR speed detector

YES I do understand that RON
I am thinking some thing that is available to the public

Thank you do they work at night

I know nothing about them that is not shown on their respective Ebay site.

If it's radar it shouldn't matter if it's day or night.

Does your GPS, mobile phone and radio still work at night?!

Umm ,err , 'their speed plus yours' ? Actually their speed of approach MINUS yours , lets say 'they' are doing 180 and you are doing 100 in effect 'they' are 80 faster than you not 280 or even 180 and 'if you had a head-on the impact would then be 180 + 100......

If the speed detection device is mounted on your vehicle, and measures an approaching vehicle speed then yes it will show sum of the two speeds, or ‘closing speed’.
It will record the closing speed.

Scientifically speaking, if you were to measure the speed of a car coming towards you then it will show the sum of your speed plus their speed. That is why radar systems in moving police cars subtract or add the police car speed to get the real speed of the other car depending on which way the cars are moving. However, if you were to have a head on crash where you were both doing 80kph, it is only the equivalent of you hitting a brick wall at 80kph, it's NOT a 160kph crash. There's physics equations to prove it on Google. Kinetic energy is the key.

Correct, closing speed is not the same as impact speed, which wasn’t the original question.

Thanks Michael. Very interesting. Have never bothered looking at that, just accepted the old furphies from mis-informed but well-intentioned road safety advice back in the "olden days".

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Michael, I believed that two opposing vehicles, each travelling at 80kph, and colliding head-on would represent a vehicle travelling at 160kph and colliding head-on with a stationary vehicle.
Introducing a "brick wall" changes the hypothesis of 'two cars' colliding.

I wondered if I was missing something here and searched Google as you suggested but was unable to find anything that supported your notion. Could you give me a link to it?

Google 'car crash physics energy head on brick wall' Allan. Plenty should pop up. Forces and energy are discussed, along with the formulae involved.

Real world intricacies aside, hitting a wall at 160kph is "exactly" DOUBLE the force/energy of two vehicles colliding head on at 80kph.

It is hard to imagine how it isn't a 160kph crash as it seems to defy logic. However, if you really think about it, if a car travelling at 80kph hits a stationary car, then the stationary car will be thrown backwards absorbing a lot of the moving car's energy. Hitting a stationary car at 80kph is not the same as hitting a solid brick wall at 80kph. The brick wall is a true 80kph crash because the wall won't give. When 2 cars hit at 80kph each, they both in effect act as a brick wall to each other, neither will be thrown back the same way as they would if either were stationary. That's the explanation without the formulae.

Further to that, the forces at play are different. In the 2 car scenario, there are 2 opposing forces, in the single 160kph crash into a wall there is only 1 force in one direction. So there is 160kph worth of energy absorbed by one car in the single car scenario, and 160kph worth of energy absorbed by 2 cars in the other scenario. Thats 80kph worth of energy each, the same as if they hit a brick wall doing 80kph.

It probably won't be like hitting a brick wall - brick walls generally aren't as solid or immovable as many would think... (especially if you drive a car at them at speeds like 80km/h)!



Perhaps more like hitting a tree, retaining wall, embassy fence or other immovable object...

You are correct. For simplicity's sake we assume 2 cars of equal mass and an immovable wall. The wall has no kinetic energy as it is stationary.

I know! :-) It was a little light-hearted diversion...

Though discussions about crashing cars into immovable objects or other cars is not really a light-hearted matter.

Just thinking guys do they measure cars moving away and towards you

If the road safety type, they usually measure as you approach, same as those green led ones they put in suburban back streets to ‘remind’ traffic of speed they’re doing.

I saw a couple of red / green led road safety ones on the way into a private dam access road, they just lit up red with your speed as approaching if over 40 (very easy on this particular road) and turn green led when you hit 40 or under.
I am sure they can be adjusted digitally to any speed you like.

I purposely looked in my rear view to see if they also lit up as I travelled out the other way when exiting.
They didn’t.

He wants to be a freelance cop perhaps?!

Just wants to put the wind up the mobility scooter set at the local mall.

Regards

Chris, the mobile traffic enforcement ones that WA HP used to use (mounted on the driver's side-B pillar area) used to do both directions.

Normal practice also is for vehicle-mounted enforcement units to read the user's road speed via a separate radar unit mounted under the front of the vehicle, and the head unit adds/subtracts that from the target speed. (Direction of target is simply resolved in Doppler radar).