Aliursis

At a lot of stop lights there are metal strips making a square shape in the street placed in just the right spot for an approaching vechical to the intersection. They're not at every intersection but most around where I live. I've been told these are sensors for the stop light to indicate it should change because a car is waiting. Is this true? If they are sensors how do they work? Do they sence the weight or some kind of electrical signal?

Squirrel

They are a combination of sensors and scales, and they are at all intersections, the ones that show the metal strips are ones that have been replaced. Usually they are installed when the street is repaired, or installed depending on how new the road is. Anyways the street has two kinds of detectors, weight and metal. The older style is the weight sensor, basically the detected if there was any cars waiting, if the sensor detected a certain amount of weight. Once three or four of these sensors were tripped, the countdown for the light change would start. These are still used, but mostly in areas that have almost nothing but car and truck traffic.
The other type of sensor is a magnetic sensor. These sensors trip once a metallic presence is detected. they are mostly found in cities and major traffic areas, more specifically areas with a lot of motorcycles. This type of sensor was a direct result of new automobile technology. Simply stated, as cars grew lighter, they wouldn't set off the weight sensors, but they would set off the metal detectors, same with motorcycles. They do have new sensors that are coming out that will visually determine how many cars are waiting via a little camera and a computer. But those won't be to popular i think due to cost, but still one never knows.

Despite these types of sensors, most the time traffic lights are set on a timer, they change in 1-5 minute intervals depending on whether it is communte time or not.