Traffic Light Tutorial

Judging by the way people behave when they get to a traffic light, it may be time for a short tutorial. 

First, there are two basic types of traffic light. There is the timed light and the “On Demand.” Of course, there are numerous derivatives. 

The timed light just has a set time it goes by for each direction. These are usually at major intersections that have a lot of traffic both ways. In Austin, one such is at Lamar and Rundberg. So, the green light may last for 30 seconds each way, or may be 30 for the bigger of the two, and then 20 for the other. Have you ever seen a black hose stretched across the road? It is recording traffic and one of the things it's used for is to decide light timing (among other reasons). 

The On Demand light is a little trickier. It is based on if/when you get to the light. First though some terms. 

Most intersections have a thick white line painted across each of the lanes. I was taught that this line is called the “Threshold.” Recently I’ve learned that it is sometimes called a “Stop line.” I prefer Threshold. It demarcates where you are supposed to stop. Always get as close to it as possible, but don’t go over. I’ll get to that next. 

The Treadle

Buried under each lane at the intersection is what is called the “treadle.” Sometimes you can tell where it is because there are very thin cut lines in the pavement showing where it is buried. It is buried from the threshold to about 10 feet back. So,when you run over the threshold it lets the light know you are there. The On Demand light will soon change when you run over that treadle. 

This is why you must pull up to the threshold. If you don't and it's an On Demand light, you may be there for some time. I've seen this many times. A guy comes to the intersection at a left turn light, but is too far away from the threshold. Typically, the person gets mad because the light won’t change and so runs the light. 

Displaced threshold

Before I mention derivatives of the two lights, let me tell you why you see offset thresholds. This is a threshold that is typically in the left lane and it's set further back from the lane that goes straight. It is painted like this to accommodate turning traffic—especially buses and trucks. If you stop past the threshold it will make it very difficult for those vehicles to turn. It could also lead to your vehicle getting hit. So stop at the threshold/stop line.

A very common derivative of the above traffic lights is a “timed & On Demand” light. It is basically an On Demand light, but it will stay green on a timed sequence. So if you roll up to it, the light will change but stay green for only a short time. Sometimes the time gets extended if multiple cars run across the treadle. In other words, the light is informed that there is more traffic and so it should stay on longer.

We see this light most often at a major street that has a side street that intersects. The light will stay green for the main street and only change if someone pulls up across the treadle on the side street. Often it can take a while for the side street to change even if you have crossed the treadle because the major traffic flow has priority. 

Within the past few years, cities have begun to use a traffic light system that uses a camera to decide if it’s time to change. My experience with these is that it seems less reliable. I’m not sure if it’s because the camera is aimed wrong, has too narrow of a target field or if the software running it is sketchy. Or maybe I’m just going to the wrong intersections. Anyway, this seems to be the future.

So the bottom line of all of this is to be sure and pull up to the threshold/stop line. Then, cross your fingers that it’s not too long of a wait for the light to change.

Ahhh, James Thurber

For this tutorial, I didn't do extensive research, I just knew a lot of it because of my upbringing. My Dad was the City Engineer of a number of cities and so he would often impart knowledge about such things as we drove along. In other words, when other kids riding along with their Dad would get to stop someplace and get an ice cream cone, we would stop at a sewage treatment plant and go look around. Of course, this was years ago so I'm sure that some of the tutorial information has been changed or improved, but for the most part, from current observation, most of it is still present at today's traffic lights. We at The Thurber Brigade hope you enjoyed this little side step and rest assured, we'll get back to blogs about things like relationships, male/female behavior, politics, etc. real soon. Maybe.