Saturday, April 25, 2015

Loop Detection

I remember in college my friend, Steve, lived in a dormitory on the other side of campus.  To get to his dorm room, we had to cross a major street.  There was a pedestrian crossing but it was not actuated.  It simply provided a pedestrian phase in synch with traffic progression along the street.  In other words, it provided a pedestrian crossing every 120 seconds or so.

But as engineering students, we didn't know about traffic engineering yet.  Someone had told my friend that the "traffic" pullbox was really a pressure sensor.  If you wanted to cross the intersection, you simply needed to jump up and down on the box and soon you would get a green phase.

It worked!

 If we jumped up and down on the pullbox, we would get a green phase... about every 2 minutes or so.  Remarkable.  The sad thing is that I believed this until I became a traffic engineer.  I look back and wonder how dumb I must have been.

There are many ways to actuate a signal now, but back then, almost everything was done by loop detection.  A loop detector is a wire coil embedded into the street.  A small current is constantly being passed through the loop creating an inductance field.   When a vehicle passes over the loop detector, it creates a small disturbance in the field which is amplified through a loop amplifier and the signal is then sent to the controller.

The loop detectors are quite noticable at most intersections - as it is usually placed into the intersection by sawcutting the pavement.  This leaves a terrific scar on the roadway.  People know this scar has to do with vehicle activation, but they aren't really sure as to how it works.

Over the years, I have heard some pretty amazing theories.  I have heard that there are magnets in the roadway.  I have heard that they detect weight.  I have heard that they detect temperature, sound, and exhaust.  But nope.  They simply detect the change in the field created by the current in the loop.

I bring this up because of a recent article about loop detectors and bicyclists in Chicago.  The article states "Motorcycles and bicycles often aren’t big enough to trigger magnetic sensors that switch traffic lights from red to green, WBBM Newsradio’s Alex Degman reports."

Actually, they aren't magnetic sensors in the roadway.  The inductance field is a magnetic field, but theren't aren't any magnets in the roadway at all.  But, if you jump up and down really hard...