How do stoplights detect a car?

Most of them are just set on timers but if you look closley they have sensors on the top of them that sence when the road is clear and it is safe for the lights to change and so on im not to sure what kind of sensors they have maby light or movment

Hi there... I had always thought it was the rubber tubing that runs across the road, before you get to the traffic lights, so anyway, I got busy and searched it and came up with this site. Check it out, this will answer your question. Have a great day... http://auto.howstuffworks.com/question234.htm

In my country its not the stoplights that detect if a car has arrived at an intersection, there are sensors in the ground under the road that react when a car stops over them. The weight lets the system know to change the lights. Also it cant be set off by a person jumping because a car is much heavier and the sensor is set for this weight. If the lights themselves were set to change when there is motion then they would be changing every few seconds because of all the movement, including animals or wind moving a tree etc.

most stoplights are timed nut some have weight sensors under the road when a car is on it it triggers the light

In the UK, virtually all traffic signals monitor the traffic for demand presence. Also, traffic lights here don't switch off at night, they just rest on the previously requested phase, until another vehicle arrives at the approach which is on red, when it will change to serve that car and remain still again in that phase.