Red Light Camera Sensitivity

Great question. I did a Google search on "traffic light sensor" and found a couple of interesting articles in How Stuff Works that described the common triggers for traffic lights and red light cameras.

From what I gathered from the articles, if the trigger is an inductive loop and if your cycle does not trip the traffic signal, it may not trip the red light camera.

I am an avid bicyclist in Tempe, AZ and there are two intersections I travel that I only need to stop the bike in a certain loop and it triggers the light to change. No need to ride to the curb to hit the buttons any more, just position the bicycle in the marked zone and the light changes. I hope me and the cycle do not weigh 600lbs, so I am certain they are using induction here.

If you look at the road, there should be a circle or a square where you'd think there is a pad that measures weight. It's not. There is a continuous piece of wire laid in those small "trenches" which when something metallic (i.e. the bottom of a car) passes through this field, it sends a signal to the processing board to change the lights.

It's all done with magnetic fields, and not weight.

There are devices made for bikes to help trigger traffic lights. Here's an example, I know nothing about this one, it's just the first one that came up in the search:
http://www.thelightknight.com/traffic-light-changer.php

Interesting; looks like a $14 metal plate with a free zip tie!

Around here there are smaller inductive loops in some bike lanes, and handlebar-level push-buttons (just like for pedestrian crossings) in others.

Presumably these are two different sensors though.
One really insensitive one, to get you so fustrated you ride through the redlight and another really sensitive one, to make sure they capture you on film doing it

Who cares? No license plate

One of these nights I just might test the theory. Maybe pull up, cover up, and drive through! Someone told me once that in CA it's like the toll booths where if it does not change, you lean on your horn then go through. I need to get a couple of those magnetic american express advertisements that you get in the mail and when I see the opp just drop them on the plate and go through. Some places have the bike line and others I have to angle the bike to get more of it in the zone.

DarkCanuck is absolutely correct. These sensors are known simply as "loops" and are just lengths of wire set into shallow saw cuts in the road and connected to the intersection controller. They tend to be fairly imprecise and usually require a fairly large hunk of metal to "trip". Not only is this a problem for bicycles but can also be an issue for motorcycles.

The current movement is toward video vehicle detection that uses fixed cameras looking into oncoming traffic to sense the presence or absence of vehicles in the traffic lane(s). These cameras are usually small, cylindrical units mounted fairly high up on cantilevered arms over the intersection and are often mistakenly assumed to be red-light cameras. Unfortunately, this technology also has its limitations and can be adversely impacted by lighting conditions (bright sun, heavy shadows, etc.) and weather (rain, fog, snow). There are a number of vendors that provide this technology and I suspect that the machine vision algorithms from each differ on how well (if at all) they would detect a bicycle.

If you were very naughty, you would get an IR emitter like those used by emergency vehicles for traffic signal pre-emption. Push the button, the light turns green and you just pedal through. Just don't get caught since I'd guess that the police would take a dim view of a private citizen using one of these.

http://en.wikipedia.org/wiki/Traffic_signal_preemption

I wonder if the engineers designing these law enforcement device also design things to beat them.

Look into buying some neodymium magnets.

Do it at night so you can see the flash. cover your plates you should not get ticketed.

I have absolutely no reason to think this fact except that in NYC there is no way sensors in the road way would work to trigger the camera. I think we do it here by having an infrared or sonic beam going into the intersection for a few seconds after the light turns red. I have witnessed cars go through the intersection 10 or 15 seconds after the light changes and no flash from the camera.

There is a loop under the pavement that detect de metal on your vehicle. So if you dont go over it, it wont work.

I have witnessed 3 or 4 camera installations in NYC and I have not seen any pavement disturbed in any way during an installation. Also if there was a loop under the pavement the magnetic field would trigger the camera at any time the light is red and a car goes through the intersection against the red light. That just does not happen at any intersection where I have witnessed cars going through the red after the signal has been red for more than 10 - 15 seconds.

The comments were in regards to intersection control not red-light cameras.

While a red-light camera system must interface with the intersection controller in order to know when the light is red, there are likely sensors other than "loops" in play to detect vehicles in the intersection. My guess is these are video (machine vision) or radar/microwave based and are completely independent of the those used for intersection control.

Isn't that the point of the red-light camera? There are usually clearance intervals built into intersection controllers (and possibly red-light camera systems), but if the camera is not triggered by a car in the intersection 10-15 seconds after the light turns red then something's wrong.

In my home town there is one spot the light's are controled by camera or some kind of infra red device and there is no loop in the pavement.