Re: How do those lines under the pavement at traffic lights control the lights?

  The metal lines under the road, called "loop detectors," detect the 
presence of cars and motorcycles above them.  Together, the set of 
detectors at various points on a road intersection are used to 
adjust the traffic signal timing -- e.g., they don't make other cars
wait at a red when there are no cars in the other crossing direction.  Loop
detectors must be installed by cutting into the pavement, whether the road
is newly-built or existing.  New intersections, or intersections which have
had a lot of upgrades recently, typically have more loop detectors in more
lanes, for more sophisticated detection of cars and timing of signals.

  Several companies are researching ways to detect cars without cutting
into the pavement, such as video, radar, acoustic, ultrasonic sensors that
can be mounted overhead, or magnetic sensors that can be installed with a
cut in the pavement about the size and shape of a can of soup.  Several
companies even have products on the market, but they are used only at
special intersections and freeways where installation of loops is too
difficult and expensive.  These "non-intrusive" sensors are typically much
more expensive and more difficult to set up and maintain than loop
detectors.

  You can tell where the loop detectors are by looking for cuts in the
pavement.  That way you can make sure that your car is over the lines and
therefore being detected by these sensors.