Turn lanes and traffic lights

Congratulations to Bill Ruhsam for the first episode of Talking Traffic, a podcast explaining issues in traffic engineering to the vehicle-driving public. Full disclosure, of course: Bill is a good friend of mine from college. That doesn’t mean, however, that the podcast isn’t anything but top notch. Go have a listen!

In the first episode of Talking Traffic (listen here!) Bill elucidates for us the difference between coordinated, synchronized, and actuated traffic signals. Actuated signals use metal loops embedded in the pavement as a switch to inform the traffic signal’s computer that a vehicle is present. But how do these loops work?

For the longest time, the naïve little physicist in me thought that somehow these devices were actually huge scales that weigh your vehicle — any significant weight would be enough to trip the signal and get you that green light. Silly me!

Using massive scales would be impractical. Have you ever seen a road construction worker using a jackhammer to chip away at pavement? The area comprised by the metal loops in the road is probably about eight feet by eight feet, so a jackhammer operator would have to be working on the road for weeks just to clear enough pavement to put in a device. Such a scale would be expensive, hard to maintain and install, and barely be worth the effort.

Only after I started studying physics did I realize how those loops could really work. Using the natural phenomena of electricity and magnetism, metal loops can determine the presence of objects near them without any contact at all. This is called electromagnetic induction.

It may not be obvious that electricity and magnetism are related. When electric charges move, they produce an electric current. When an electric current travels in a long straight wire, it produces a magnetic field in concentric circles around the wire. If you coil up the wire, the fields add up in such a way that it’s as if there’s a large magnetic field that points in the same direction as the object you’re coiling the wire around. The picture on this page supplied by the Collaboration for Nondestructive Testing says in an elegant picture that which I have trouble saying in words.

Perhaps in science class you made an elementary electromagnet by wrapping wire around a nail, and then connecting the wire to a battery? You didn’t? Try it now! You can pick up paper clips! The more turns of wire (or the higher the voltage of the battery), the more paper clips you can pick up. This is the exact same phenomenon.

Let’s extend this to the traffic loop. If we provide a small current, a magnetic field will be produced pointing upwards out of the surface of the road (or inwards, down to the ground, depending on the direction) of the current in the wire. Energy is stored in that field, which (essentially) serves to resist any change to its state. The quantity that defines the amount of resistance to change is called the inductance.

It’s possible to measure the inductance of a loop. Cars are made of conducting materials. When a car enters the area over the loop, the conducting material enters a magnetic field. Small loops of current are created. These “eddy currents” serve to make their own field which tries to resist the changing field. That is, an opposing magnetic field is produced.

The opposing magnetic field manifests itself by changing the inductance of the loop. A computer, probably in one of those unmarked boxes at the corners of intersections, constantly polls the loop for its inductance value. Once an inductance outside the normal range is measured, the computer handles the traffic signals accordingly.

The concept of electromagnetic induction is very tricky to grasp. In fact, induction is the key concept in electrical transformers, power generators, magnetic-levitation trains, and some braking mechanisms. Without a firm grasp of induction, much of our modern electrical infrastructure wouldn’t exist.

Thanks for driving by.

6 Comments

• 

  By Gillian, August 6, 2007 @ 10:45 pm

  Which is, of course, why one motocycle won’t set it off. Two arriving over the loop together might be enough. Alas, if one arrives and then the other joins him, the two little inductions are not enough to set off the meter and change the light for them.

  Alas, too, the physics says that if the motorcycle already has a green light and rides fast across the loop, it might induce enough current to trip the switch. But that isn’t usually when the bike wants the switch tripped anyway!

  Oh yeah - and the chances are that a hog (big fat Harley) will be just enough to trip things, but there’s no way my little C90 with the plastic fairing ever could :-(

  (Yes, I have parked the bike and ducked off to the pedestrian light and pressed and and got back on the bike, just to make the light change).

• 

  By Bill Ruhsam, August 6, 2007 @ 11:09 pm

  Thanks for the shout out, Jim.

  The detection loops at signals are typically 6′x6′ or 6′x40′. The 6′x6′ loops are used as pulse loops and are set back several hundred feet from the stop bar, depending on the prevailing speed. The 6′x40′ loop is positioned at the stop bar and is called a presence loop. These are typical values and do change depending on location and design.

• 

  By Harliquinn, August 7, 2007 @ 8:56 am

  So, in order to get my bike to trip of on the detection loops, I’ld need to be a) a lot larger and not made out of fiberglass or b) create a large EM field of my own that would trip the loops for me?

• 

  By B.Ruhsam, August 7, 2007 @ 1:06 pm

  Some presence loops are designed with an extra three turns of wire in the first 6′ of the loop. This is intended to pick up motorcycles and bicycles, i.e. vehicles with significantly less metal.

  Of course, I’ve also read motorcycle advocacy forums that recommend you get off the bike and hit the ped pushbutton. That, of course, only works when there *is* a ped pushbutton.

• 

  By Jim, August 7, 2007 @ 2:33 pm

  @B.Ruhsam:

  Mostly around here the pedestrian pushbuttons trigger a pedestrian-only cycle in the light, where all traffic lights go red and all crosswalks simultaneously allow crossing. That only helps the bicyclist if they walk across the intersection and doesn’t help the motorcyclists at all.

  As more places use video cameras instead of loops as sensors, perhaps this problem will go away.

Other Links to this Post

1. Traffic Tidbits: 8 August 2007 : Talking Traffic — August 8, 2007 @ 10:40 am

RSS feed for comments on this post. TrackBack URI

Leave a comment