Topics: Introduction. Basic traffic signal coordination. Definitions of “coordination, synchronization, and actuation.”
Hello and welcome to the first edition of Talking Traffic. My name is Bill Ruhsam and I am the host of this podcast, as well as the website Talking Traffic dot Oh Arr Gee. Today is August 9, 2007.
Have you ever wondered why you sit at the same traffic signal every day, without fail. What about those cameras that are being mounted everywhere in metropolitan areas. Why is there ALWAYS construction going on during your vacation, no matter where you travel.
This podcast, and its website, will attempt to answer some of these questions in a way that does not require an engineering degree to understand. Because these sorts of things *shouldn’t* require anything but common sense to understand. If they do, then why are we, the transporation professionals installing them on roads and sidewalks where they might hurt people. No, fundamentally, everything I do for a living is very simple and straightforward, where the complexities arise is from the relationships between systems. For example, you might alwasy complain about that traffic signal that you never seem to get through. You are always waiting for it to turn green. You might blame this on shoddy installation or programming, but you might also consider that the signal in question might be carefully coordinated with an upstream or downstream traffic signal to allow traffic on the opposing road to flow. Are you traveling the opposite direction of most other drivers? Are you crossing a major road? I want to help make sense of some of the things you see everyday on the highways and sidewalks of this nation, even if it doesn’t relieve the frustration of sitting at a signal.
Today’s topic is those pesky traffic signals. First off, let me explain a few terms. Coordination, synchronization, and actuation. Coordination means to time
two or more signals in a fashion that allows the best operation of the system as a whole. More on that in a second. Synchronization is a word thrown about by news agencies and people unfamiliar with signal operation and it means to time two or more signals so that they turn green or yellow or red simultaneously. Acutuation refers to the method by which a traffic signal may detect the presense of a vehicle, bicycle or pedestrian.
A coordinated signal system is two or more signals that are optimized to provide the smallest amount of delay to the vehicles moving through the signal system. Delay is measured by the amount of time a vehicle is *not* moving at the speed limit. Any time you must slow down or stop becuase of a signal, you’re being delayed. Think of it this way: On an open road that is one mile long, if you’re traveling 30 miles per hour, it will take you 2 minutes to go from one end to the other. If, instead, you hit a traffic signal in its red phase at the 1/2 mile mark and spend 60 seconds stopped, you’ve now taken 3 minutes to get from one end to the other. You delay is 60 seconds. This is a simplistic measure, but is frequently used by professionals. But back to coordination. Signal engineers attempt to coordinate the signal system to minimize delay. this is easy for a one-way street, you just time the signals so that as a group of vehicles (called a platoon) arrives at each signal, the signal turns green. You may have seen signs in some cities that say “Signals set for 35 MPH” or some other speed. That is what that sign means. If you drive 35 mph, the signal will turn green in front of you.
however, what’s easy for a one way street becomes more difficult for two-way. You could do the exact same thing as the one way street, optimizing the signals for one direction of traffic, but that would mean the opposite direction would hit nearly every single red light. That would make people unhappy, to say the least. Instead, a compromise has to be reached, which means you *won’t* get through the whole signal system without stopping, probably. Now, consider the case of a downtown area where you have multiple roads criss-crossing each other, all with signals. Try and imagine timing that system by hand to minimize the amount of delay that everyone suffers. Trust me, this is a good example of computers doing the dirty work.
of course, getting back to the one-way street example, often times a particular traffic direction is favored at different times of day. During the morning commute, you want to favor the direction of highest traffic volume, in order to move vehicles along as efficiently as possible to their lattes and places of business. this will inevitably cause the opposite direciton, and crossing streets to wait longer than if each direction was treated equally. This might be the reason why you’re always sitting at the same signal every day. There is relatively little time on green allotted to your direction, giving you a low likelihood of hitting it. Or, its possible that the route you are taking is timed in such a fashion that you will always hit the next signal on red. My morning commute takes me in a direction where if I’m not the first, second, or third vehicle at one signal, I know I won’t make the next one before it turns red. There is a 45 second interval for me to get across the intersection, up a hill, across a bridge, and to the next signal before it turns. If I know I’m not going to make it, I go a different direction. Why? Because the second signal, the one I get stopped by after 45 seconds, is a a major Arterial roadway and has a two and a half minute cycle time. I end up spending 160 seconds sitting at that signal waiting for it to turn green so I can make a left onto the arterial. Therefore, if I know I’m not going to make it, I take a differnet route that gets me along to another intersection.
I’ve been spending a lot of time talking about coordination. Let’s talk about “synchronization.” you may read news articles that talk about your local city engineers going out to “synchronize” the signals, or there may be letters to the editor from citizens who, gosh darn it, want those signals on their drive synchronzied so they dont’ hit them every day. “Synchronize” is the wrong word here. to synchronize, as I mentioned before, is to set each signal so that they turn green, yellow, or red simultaneouosly. This is hardly ever a good thing to do. In fact the only time I can think that you’d want to do a “synchronization” is in a busy downtown business district with a grid road netowrk, and even then it’s only a stopgap because you dont’ have enough signal technicians to do the work of coordinating the signal system. I implore you to avoid the word synchronize and use coordinate instead when talking to your local transportation department. they will treat you with a greater level of respect because you know what you’re talking about.
The last word I wanted to talk about with respect to signals is “actuation.” To actuate is to have the signal sense that there is a vehicle or bicycle waiting to get a green light. this is accomplished through magnetic sensors emplaced in the pavement, or through video detection using cameras, or microwave detection using a radar transmitter. You will have seen the loops cut into the pavement near the stop bar at a signal. These are magnetic sensorsa comprising several turns of wire. They sense any large amount of metal above them. You may alos have noticed small cameras like security cameras mounted on the pole or arm holding the signal lights. These are cameras that allow the signal to “see” you while you’re in it’s detection zone.
Why do we actuate signals? It allows us to leave the road that has the majority of traffic on green, and only give a green light to the cross-street when a vehicle approaches and trips the sensor. You might think that, hey, that sounds like a good idea, let’s actuate every signal! But there’s a catch. Yyou can actuate a signal that is all by itself, but you shouldn’t when it’s a part of a coordinated system. If you allow the signal to start changing itself, without respect to its neighbors upstream and downstream, you very quickly have a snarled up morning commute, and a lot of drivers cursing the name of the engineers who installed the signals. In fact, if you live in an area with a lot of traffic, next time you’re driving on a busy arterial, look at the pavement on the main roadway as you’re going past a signal. I bet you wont see any loops cut into the ground, except for the lanes for left turns.
There is a method for using actuated signals inside a coordinated system; it’s called adaptive signal control, but we’ll discuss it on another episode.
So, we’ve learned that signal engineers use coordination (not synchronization) to reduce the delay of vehicles traveling along a roadway. We’ve also learned that they will bias which direction gets the most green time depending on which way the majority of traffic is flowing. And we’ve learned that signals are often equipped with devices that sense the presence of vehicles, in order to assign green time to that direction. While signals are fundamentally very simple, timing them in a fashion that minimizes the time all vehicles spend sitting at them can be a complicated task requiring considerable compromise
To end this episode, I’d like to introduce myself a bit better. As I mentioned earlier, my name is Bill Ruhsam and this is my Podcast. I am a Traffic Engineer working as a consultant in the Atlanta, Georgia area. I am registered as a professional engineer in Georgia and Texas, and I am also a registered traffic operations engineer. Due to the work that I do, I’m more of a jack of all trades than a specialist when it comes to engineering. I have considerable knowledge in striping, signing and traffic impact studies, generalist skills with traffic signals, and I’ve also done my fair share of roadway design. I used to work for the Texas Department of Transportation, so I’m familiar with both the private and public sides of the street.
This podcast is distributed under a creative commons attribution, Non-commerical, no derivatives license which means you can give away, but not change or sell it. Other information including eisode notes can be found at www.talkingtraffic.ORG and you can reach me by email at Bill@talkingtraffic.org. Comments and suggestions are apreciated. have a great day, and always remember to respect your fellow travelers.