They also have another use. Often, they'll be at stoplights. They sense the presence of a large metallic object (read, a car) over them, so that the light knows when to change. For example, coming down the hill from my house, my smaller (2-lane) road leads into a larger (4-lane, plus turn lane) road. My road, where it meets the larger road, has a sensor, and the turn lane that turns left onto my road has one. So, if no cars are waiting, the main road will stay green, but if one is, it'll activate the timer to change the signal. They seem to be able to sense how many cars are lined up, too. So, if only one or two cars are waiting to turn left onto my street, the light for them will go red, and the light for any cars waiting to turn off my street will turn green first, but if more are waiting, the cars turning onto my street will get a short green arrow, then the cars on my street will get a green light, then the remaining cars waiting to turn onto my street will get another, longer, green arrow at the same time that the cars facing in the same direction get a light to go straight. After they clear, the opposite direction (cars that, if they were turning, would be going right) gets a green light, and the main street returns to two-way traffic.
As for other ones, they actually aren't commonly used for speeds as much as to count the number of cars passing over them in a certain period of time, in order to collect traffic pattern data.
