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Author Topic: How do traffic lights detect the approach of a car? Do they use magnetism?  (Read 11230 times)

Offline Seany

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Is it true that traffic lights work by realising a bigger magnetic field when a car approaches??

Because apparently there is a coil underneath the ground..  And when a car pauses over it, it works as the core, and produces a bigger magnetic field.. Thus changes the traffic light signal!!

Is this how it works??

So then, would you need to dig up the whole ground to put some sort of coils underground?
Surely this would be an expensive procedure?
« Last Edit: 07/04/2008 22:41:51 by chris »


 

lyner

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Not a problem if you do it when the road is due to be resurfaced. It used to be done with a rubber strip which was squashed by a car running over it. Hopelessly expensive to maintain compared with a buried loop. Fit once and forget it.
BTW, some systems use radar.
The loop system works on the inductance of the loop changing in the presence of all that extra steel.
 

Offline Seany

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Tank yoo!!

The rubber strip sounds a bit clumbsy!!
 

Offline turnipsock

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The Tescos in Stirling has loops under the road. You can see where they are and if you stop short of the loops you can watch the chaos theory in it's full glory.



Within a few minutes, the traffic backs onto the roundabout...chaos.
 

another_someone

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I thought the preference these days was to use IR detectors (not unlike the things used for burglar alarms, but with a narrower field of view).  Are those not what those little oblong boxes pointing down from above traffic lights these days are (not all of them use them - some are just on crude timing systems).
 

Offline turnipsock

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We are the only country that still has a red amber phase in traffic lights.

Some people seem to think flashing their lights at the detectors make the lights change, could that be true?
 

lyner

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We are the only country that still has a red amber phase in traffic lights.

I can't understand it. That phase is so useful in getting people prepared to move off. Some drivers take a long while to find their pedals and gears - especially if they are are in conversation.
 

Offline techmind

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Is it true that traffic lights work by realising a bigger magnetic field when a car approaches??

Because apparently there is a coil underneath the ground..  And when a car pauses over it, it works as the core, and produces a bigger magnetic field.. Thus changes the traffic light signal!!

Is this how it works??

So then, would you need to dig up the whole ground to put some sort of coils underground?
Surely this would be an expensive procedure?

Many traffic light systems use the buried-coil induction loop system. They cut a number of crossing narrow slots in the road surface using a circular-saw, and lay a wire in the slots to make a loop of just two or three turns. The loop is typically something like 2m x 0.5m, but different shapes (including diamonds and rhombusses) are used. The loops operate at a frequency around 60-70kHz (UK law mandates a certain frequency range for this purpose).
At the c.70kHz frequency at which they operate, ferromagnetic effects in commom iron are not very significant, much more so the "shorted turn" effect which lowers the inductance of the loop. They are therefore sensitive to sufficiently large pieces of other metals such as aluminium and copper too.

They can just-about detect a bicycle wheel if you accurately align it with the buried-strip in the road (ride right over the buried wire longways) - or a bicycle frame laid flat over the loop as a whole. The same system is often used for carpark barriers, and you should be able to raise these by laying a couple of square-feet of aluminium foil over the corner/edge of the buried loop! We have a loop-activated barrier at work, so lots of scope for experimentation ;-)

Some traffic lights use a "Doppler Radar" module. These use radio waves at 10GHz to 30GHz and detect the radio waves reflected back off the vehicle. Because they "beat" the recieved signal against a portion of that transmitted, they only detect motion (and by using quadrature receivers) can distinguish between "towards" and "away" motion. These detectors are essentially the same as can be used to measure traffic speed.

Loop detectors can easily sense different lanes of traffic uniquely, which you can't really do with a radar device.

Here's an image of a buried loop I just found on Google:
http://www.geocities.com/SiliconValley/8443/Images_sp-200-sensor.jpg

Picture on this page appears to show cutting the slot for the loop:
http://www.uscotraffic.co.uk/services.htm

How about this: loopcutting.co.uk !!!
http://www.loopcutting.co.uk/content.aspx?sid=705e492a-c453-42e8-b5e9-0879fef1a8d7&pid=a1bbafdd-7dfe-4de6-9769-7cbf71b017bd


We also had a fairly recent thread on Induction that you might find relevant: http://www.thenakedscientists.com/forum/index.php?topic=13637.0
« Last Edit: 08/04/2008 14:29:38 by techmind »
 

Offline techmind

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The rubber strip sounds a bit clumbsy!!

They use the squashy rubber strip for temporary traffic-flow surveys. They lay the strip on the surface of the road, and chain the counting-box to a lamp-post and leave it there for a few weeks. When they're done they just collect the counter-box and rip up the rubber pipe (or leave it to decay).
 

Offline Seany

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Is it true that traffic lights work by realising a bigger magnetic field when a car approaches??

Because apparently there is a coil underneath the ground..  And when a car pauses over it, it works as the core, and produces a bigger magnetic field.. Thus changes the traffic light signal!!

Is this how it works??

So then, would you need to dig up the whole ground to put some sort of coils underground?
Surely this would be an expensive procedure?

Many traffic light systems use the buried-coil induction loop system. They cut a number of crossing narrow slots in the road surface using a circular-saw, and lay a wire in the slots to make a loop of just two or three turns. The loop is typically something like 2m x 0.5m, but different shapes (including diamonds and rhombusses) are used. The loops operate at a frequency around 60-70kHz (UK law mandates a certain frequency range for this purpose).
At the c.70kHz frequency at which they operate, ferromagnetic effects in commom iron are not very significant, much more so the "shorted turn" effect which lowers the inductance of the loop. They are therefore sensitive to sufficiently large pieces of other metals such as aluminium and copper too.

They can just-about detect a bicycle wheel if you accurately align it with the buried-strip in the road (ride right over the buried wire longways) - or a bicycle frame laid flat over the loop as a whole. The same system is often used for carpark barriers, and you should be able to raise these by laying a couple of square-feet of aluminium foil over the corner/edge of the buried loop! We have a loop-activated barrier at work, so lots of scope for experimentation ;-)

Some traffic lights use a "Doppler Radar" module. These use radio waves at 10GHz to 30GHz and detect the radio waves reflected back off the vehicle. Because they "beat" the recieved signal against a portion of that transmitted, they only detect motion (and by using quadrature receivers) can distinguish between "towards" and "away" motion. These detectors are essentially the same as can be used to measure traffic speed.

Loop detectors can easily sense different lanes of traffic uniquely, which you can't really do with a radar device.

Here's an image of a buried loop I just found on Google:
http://www.geocities.com/SiliconValley/8443/Images_sp-200-sensor.jpg

Picture on this page appears to show cutting the slot for the loop:
http://www.uscotraffic.co.uk/services.htm

How about this: loopcutting.co.uk !!!
http://www.loopcutting.co.uk/content.aspx?sid=705e492a-c453-42e8-b5e9-0879fef1a8d7&pid=a1bbafdd-7dfe-4de6-9769-7cbf71b017bd


We also had a fairly recent thread on Induction that you might find relevant: http://www.thenakedscientists.com/forum/index.php?topic=13637.0

Thank you very much for this! :)
 

another_someone

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OK, having done a bit more background research, it seems I was confusing two different concepts with the IR detectors.  The IR detectors are used to receive signals from emergency vehicles to allow them to force the lights in their favour (this would be consistent with the suggestion that some people might believe that flashing one's headlights at the sensors might force the lights in their favour - I suspect you'd need to time these quite precisely to achieve that, but it appears that the boxes that the emergency vehicles use to force the traffic lights are in fact available over the internet, and causing some concern to various authorities).
 

another_someone

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We are the only country that still has a red amber phase in traffic lights.

I can't understand it. That phase is so useful in getting people prepared to move off. Some drivers take a long while to find their pedals and gears - especially if they are are in conversation.

While we still do have red+amber, it does seem an increasing trend to have traffic lights shielded so that cars at a junction cannot see when the lights in the opposite direction change to red (which is usually the point where I start to prepare to move off - e.g. release handbrake).  It does seem the intent of the authorities is that drivers do what they are told, and when they are told, and should not seek to anticipate what will happen next (i.e. dumb obedience - with the emphasis on dumb).
 

Offline turnipsock

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We are the only country that still has a red amber phase in traffic lights.

I can't understand it. That phase is so useful in getting people prepared to move off. Some drivers take a long while to find their pedals and gears - especially if they are are in conversation.

While we still do have red+amber, it does seem an increasing trend to have traffic lights shielded so that cars at a junction cannot see when the lights in the opposite direction change to red (which is usually the point where I start to prepare to move off - e.g. release handbrake).  It does seem the intent of the authorities is that drivers do what they are told, and when they are told, and should not seek to anticipate what will happen next (i.e. dumb obedience - with the emphasis on dumb).

One thing I have noticed, in my travels, is that it is only in the UK that drivers go by the lights, all they see is the lights. it doesn't matter if they hear siriens or see blue flashing lights...if that light turns green, they are off. When you watch drivers in Spain, for instance, the light goes to green...then the driver looks both ways before moving off!
 

Offline Seany

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Yup.. .England is just the lights..
I guess if one of the traffic lights go wrong..

*Crash*
 

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