Speed Camera Vans.. information please.

[acegeezer]

The question is when these vans are parked up awaiting the unsuspecting motorist what is targeted on the oncoming car.. is it a laser and what does it read to gain the actual speed.. is it the speedo or the engine? .. and further would lower profile tyres affect the readings of rolling speed?
Second part to this is, in my case I have a lower profile tyre all round, now I am aware that this affects the speedo and in my case I believe it is 3mph fast.. based on going through the LED 30 limit beacon that flashes if over 30.. if I drive at 33 it's fine but at 34mph it flashes, simple eh?
Any help on this as I currently drive around at 3 mph above the limit, based on the change in speedo reading as a result of tyre change, only asking because there are several of these crafty vans dotted around lately, I don't want to blunder in and get a penalty

[nightfire10]

Think it bounces off the car mate do would make tyre size irrelevant? That's only my thoughts

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[PeteH]

They have no idea about your car, or anything on it, or any changes you make. They are literally a radar, and they measure the time it takes between the radar pulse leaving the gun, bouncing off your car, and returning to the gun. From that it knows how far away you are. Do that twice with a small interval between and it knows how fast you are going.

It's an absolute and independent speed reading, and it works on anything (cars, bikes, cyclists, large birds etc. )

You could probably fool it by making your car radar "stealthy".....

[fishywick]

If you input your tyre sizes to this calculator it will tell you the percentage that the speedometer is inaccurate.
http://www.autopartuk.com/tyre-calculator/

[qwakers]

or just use a satnav

[GreySilver Beast]

.........or use a Road Angel that warns you of approaching Radar etc

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[Lemon]

.........or use a Road Angel that warns you of approaching Radar etc

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does it warn you of incoming "bogies" and missiles? if so i shall get one and call it "Goose"

[GreySilver Beast]

Nope

Has saved me a few times from being caught though

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[V8 Power]

By the time you see the road side van they will have got you as long as you were being targeted.
I believe any device that disrupts detection is illegal to use for that purpose although it maybe legal to purchase.

I got caught in North Wales on the A5, just as I crested a hill I spotted the van at the bottom approx half a mile away. On the brakes but still got done at 89mph in a 50 area. Shame as it was a lovely straight clear bit of road with great visibility.
Good job I hadn’t got caught a couple of miles back though as had been travelling a little quicker!
Slowed down a lot in general since and it helped my self control selling the C63.

[PaulAV]

I believe some speed detection is done with laser guns, but I think they all use the doppler shift method to determine speed(change in colour/frequency of a beam of light or sound when travelling at speed), I could be wrong though.

[PeteH]

It's probably me being pedantic but....
You can't use Dopler effect with a laser, because the car speed is a tiny percentage of the speed of light. They actually use the same methods that laser "tape" measures use (the type used by estate agents to measure your house). It's either pulsed light, and the pulses are timed, or continuous light, and measure the "phase difference" of the reflected light.
Both methods actually measure the distance, so you need two measurements to get the speed.

[acegeezer]

Both radar and laser systems record the fastest object within the target area and both can be fooled by a number of scenarios, including

- Spoke rotation - when targeting a motorcycle (or car) with large spoked wheel surfaces, the spoke at the top of the wheel is approaching a some 70%-80% faster than the vehicle road speed. As the wheel rotates, the tread in contact with the ground is stationary, while the tread at the top of the wheel is at 200% road speed. The fasted recorded part of the vehicle is therefore the spoke just below the upper rim.

- Reflections - this is where the targeted surface is at an angle such that the beam is reflected away, strikes another surface, such as a road sign or a truck, back to target, then to the device. If the vehicle is approaching such a road sign then the recorded a speed is virtually doubled. If the target vehicle is approaching an oncoming truck (otherside of the road) then the recorded speed can be just less than double the closing speeds.

While these scenarios may seem unlikely, both can be demonstrated with the equipment in general use and have been tested in court.
Operator instructions advise how to avoid them but operators make mistakes.

[Goobie]

I usually see one on a bridge going up the M23 and such, some are fairly high up from road level traveling, with this in mind wouldn’t the speed reading not be accurate? Or do they have to go measure the height of the camera set up vertically to road passing underneath then do some sort of Calibration? As in, if it was road side the beam would just be horizontal, not angled as such.

[warpc0il]

Uncalibrated, the devices high on a bridge would read slightly fast.

Visualise a shallow right-angled triangle with you at the pointy end.
The road surface is the long base of the triangle
The height of the bridge is the short rise opposite you.
The direct line from you to the device is the hypotenuse of the triangle, which is longer than the base according to Pythagoras.
So you move along the hypotenuse slightly faster than along the base.

However, if the bridge was 30m high and you were recorded at a distance of 500m, the difference in length would be less than 0.9m or 0.02% - if I just did the maths right in my head

[Nerdstrike]

Obviously the best way to not get caught by a zero-tolerance policy is to not push the margins of your speedo read error. There's a bit of lenience in enforcement to allow for factors that Dave mentions, but you can't count on that all the time.

does it warn you of incoming "bogies" and missiles? if so i shall get one and call it "Goose"

Only if your bogey has its own active radar. Shorter range missiles are often infrared guided, and flaming exhaust pipes are not helping you!

[PaulAV]

Well they might if your flames are long enough, another reason to shoot flames maybe?

[warpc0il]

Shoot flames = become a target for Sidewinder Missiles

I know VOSA are stepping-up their game but I don't think that's within their budget

[PaulAV]

Bigger flames mean more target behind the car and with no cat there is no glowing hot spot next to the driver. We're safe for now on that score at least.

[warpc0il]

Like an aircraft using flares to provide the missile with an alternative target?
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Got it

[PaulAV]

What model of rex is that, is it the rx9 or rx vision?

[acegeezer]

.........or use a Road Angel that warns you of approaching Radar etc

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Thanks Nigel.. although I have the Sygic Navigation app on my phone which covers all mobile and static radars

[GreySilver Beast]

This is what I use in my car

wordle-headder-image-for-freeweb-road-angel-site5.jpg

[ChrisHolmes]

If you input your tyre sizes to this calculator it will tell you the percentage that the speedometer is inaccurate.
http://www.autopartuk.com/tyre-calculator/

According this my 245/35 18 tyres should produce a sordid error of plus 4.7% yet checked against GPS it is more like 10%. Does this suggest my speedo was 5% fast in the first place?

[PaulAV]

I know my speedo reads over by 2mph with standard wheels and tyres compared to satnav and check your speed camera displays

[ChrisHolmes]

Your speedo should always slightly fast, as it's illegal for them to read slow.

However, useless your travelling along a perfectly straight road, your GPS is going to read slow.

This is because the GPS samples along the route and (must) assume a straight line between sample points, which is likely to be shorter than the actual path travelled.