# How do speed radars work?



## Cabotinage (Feb 18, 2011)

Hey guys

Just wondering how the speed radars/guns work in NSW? like when a cop is postioned in a bush do they get sides of the road or only the one there facing?

also if a person is caught speeding would they be pulled over?

all hypothetical


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## abnrmal91 (Feb 18, 2011)

If the cop is parked for example between both directions of th F3. They are targeting the direction they are facing, this way they can apprehend the speeder. Pretty much if they see you speeding they will pull you over. However this is mainly highway patrols/motorbikes. Also not all police vehicles are fitted with radars. Fingers cross when you drive past one.


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## jack (Feb 19, 2011)

doppler effect. i believe they use lidar these days, not radar. if you are going fast enough they will pull you over... what one copper deems fast enough will differ to another.


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## Jk888 (Feb 19, 2011)

genrealy if your more than 5km over the limit they will pull you over as jack has said it is the doppler effect which makes radar far less useful these days


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## slim6y (Feb 19, 2011)

I was reading this question as a less technical one and more a question of - how do you know if you've got a ticket or not? 

Is that correct?

You'd like to know did they photograph your rego plate and then you'll get that ticket in the mail in a couple of weeks time... or - do they wave frantically at you to make you pull over so they can issue their speeding infringement... Or... Do they jump in their car and give chase to you at high speeds with lights flashing etc...

Well - not sure I have the answers anymore - considering there's speed detectors in-car now-a-days... You could be driving happily along, minding your own business, breaking that 10% rule... and whammo... pulled over with an accurate speed reading from a cop that was going the other direction....

So my answer is - you'd have to be pulled over.

The general rule is - stay UNDER 10% over the speed limit... So for a 50km/h zone, do no more than 54km/h - but make sure you have an accurate and well calibrated speedometer. If it isn't, your 10% could be out and you'll be in for a ticket.

The way I calibrate is I use GPS and stay at a constant speed. My speedometer is within 1% accurate to the GPS over 5km!!!


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## junglepython2 (Feb 19, 2011)

I was talking to a cop mate last week, if it's police that detect you they have to pull you over to issue you the ticket, they dont send fines in the mail. If it's the contract, mobile speed cameras, that line the streets in Victoria then keep an eye on your letterbox.



slim6y said:


> I was reading this question as a less technical one and more a question of - how do you know if you've got a ticket or not?
> 
> Is that correct?
> 
> ...



Your GPS doesn't take alltitude into account does it? If not you would want to make sure the road is mighty flat to calibrate off that.


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## -Peter (Feb 19, 2011)

Speed cameras in NSW are not just operated by the police. A private company also handles speed infringements and these are mailed to you. When a speed camera is in use it is required that signage stating such be placed well in advance of the site.


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## slim6y (Feb 19, 2011)

junglepython2 said:


> Your GPS doesn't take alltitude into account does it? If not you would want to make sure the road is mighty flat to calibrate off that.



Actually it does - and even corners into account... But my test was on a 5km speedometer test zone... But besides that... Think relatively logically...

It's a triangle... The largest sides are from car point a to satellite to car point b back to satellite - between point a - b is distance moved and time taken... If b is higher than a - then the distance would be marginal compared to the distance between a and the satellite or b and the satellite - it would be an insignificant reading.


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## dadaman (Feb 19, 2011)

I did this for a job.
The radar units in the cars transmits a pulse of electromagnetic (EM) energy, also referred to as RF energy at a moving object. Using the Doppler principle the radar gun ‘listens’ to the change in pitch of the radio waves as they reflect off the moving vehicle and thus determines the speed. If there is 5 cars coming towards the police vehicle, the radar units are equipped with Digital Signal Processing (DSP) and can identify the strongest and fastest target to try and help the police officer identify which vehicle's speed is being displayed on the radar units readout. When police use this, they will turn around and chase you down and issue a ticket on the spot. If you pass a Highway Patrol car and he doesn't turn on you, you won't get a ticket. The police guidlines state they must take into account 10% speedo error and 3% radar error to to give you a TIN. 

In the last 7 years or so Police have turned to Lidar guns (laser guns). These are dead accurate. In this case, they give you 10% speedo error only. Most police will operate these by the side of a main road with a T intersection to pull the offending drives into the side street to make it safe for all. Some will sit in the police car which is positioned at the side of the road and fire the gun out the drives window. The police are taught to aim the beam at the number plates of cars because this is the most reflective part of a car. The police know exactly which car is speeding because when the officer puts the cross hairs onto the plate and pulls the trigger, it gives the car speed in the bottom right of the sight. All the police have to do is release the trigger and the speed will stay on the unit until it is reset. It's always a good idea to ask to have a look at the radar unit if you are getting booked. These police will not send a fine in the post. If your speeding, they will pull you over. If you go past, you can bet your months pay they will chase you down.

The third way is through fixed speed cameras in the new Fords (NSW). These operate a slant radar that is positioned 22 degrees across the road. These will catch 6 lanes of traffic in any direction and capture 7 cars a second. The car has 2 radar heads positioned on the top of the vehicle. One points to the rear and one to the front. These are also equipped with infra-red flashes so during the day or night you won't see a flash so you won't know your done until you get a fine in the post about 2 to 3 weeks later. These are operated by a private company called RedFlex (NSW and VIC)

Like everyone else has said, best rule of thumb is stay under the limit + 10% (100 zone, under 110. In a 40 zone under 44)

Hope this wasn't to much info. I had to go back to the training books to get some of this information


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## Banjo (Feb 19, 2011)

It is the speed cameras you don't see that worry me. I drive everyday for a living and always dread getting a ticket. But I do try my very best to stay at the posted speed limit.


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## Cabotinage (Feb 21, 2011)

thanks dadaman and everyone else in the thread i wasnt sure if they could send them in the mail or not as i get told by people that they do and some they dont


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## Southern_Forest_Drag (Feb 21, 2011)

didnt know you could add 10% though it was only a 3km allowance shall be sitting around 108km/h now in 100 zones.


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## gillsy (Feb 21, 2011)

slim6y said:


> The general rule is - stay UNDER 10% over the speed limit... So for a 50km/h zone, do no more than 54km/h - but make sure you have an accurate and well calibrated speedometer. If it isn't, your 10% could be out and you'll be in for a ticket.
> 
> The way I calibrate is I use GPS and stay at a constant speed. My speedometer is within 1% accurate to the GPS over 5km!!!


 
Not in Victoria, they have 3km variance. No if's not buts.


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## MatE (Feb 21, 2011)

I just wish people would do the speed limit,we now have 25km's of 80km/hr zone all the way to town.And they have taken out all the overtaking zones.So if one moron is doing 60km/hr the traffic banks up for miles.Cant wait till the new highway is done.Also i a side point not all fixed cameras work both sides,but im not telling which ones lol.


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## slim6y (Feb 21, 2011)

gillsy said:


> Not in Victoria, they have 3km variance. No if's not buts.


 
Sux to be in Vic then!!!

Also - I couldn't tell if I was doing 50 or 53 on my speedometer... And I doubt anyone here (even with the greatest of knowledge) could overcome the uncertainty of telling 3km/hr when the error is +/- 5km/hr (half of the measurement gap - so in my case 10km/hr per gap). 

Though I very rarely test that theory - by - not speeding!!!

I use cruise control on the open roads, or 80km/hr zones and I don't stress if people are following, so long I do the speed limit (at least).


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## Southern_Forest_Drag (Feb 21, 2011)

gillsy said:


> Not in Victoria, they have 3km variance. No if's not buts.


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## Fuscus (Feb 21, 2011)

I've been using the Illuminated speed signs to check my speedo. They light up only if I do 102km.
Also the new ones are point to point. They photo you at one point then if you reach point 2 under a certain time then you are fined. Currently this is only applied to trucks in NSW but ....


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## Virides (Feb 21, 2011)

I went on a drive to Melbourne once and went down near Canberra and on the roads there I noticed these massive white lines in intervals. It wasn't until I got back to Brisbane at my parent's house when my Dad said that it used to be for aerial speed cameras. They would fly a plane down the highway and catch speeders in mass. But it was way too expensive to keep going and wasn't entirely accurate and didn't catch many people.


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