Green Line, Green Lights

Suppose you have a train moving along (parallel to) an East-West (EW) signalized arterial.

Westgate Station

Westgate Station

Case 1: If the signals are pre-timed, and the timings are known in advance, the train should never have to stop for the signals (aside from emergency signal pre-emptions and other edge cases). Instead, the train should be able to adjust its speed so that it doesn’t have to stop. It might go at an average speed of say 10, 20, 30, or 40 MPH in order to ensure it hits a green light or better a green wave from whenever it departs a station. The train driver can be apprised of the optimal time to leave the previous (upstream) station, and the speed to travel to hit “green” lights.

Typical Signal Schedule and Traffic Flow Diagram, North-South across Market (1929)

Green waves have been around since the 1920s (See Henry Barnes’s autobiography: The Man with the Red and Green Eyes. Dutton. 1965. OCLC 522406). Static signs to tell travelers the speed of the green wave has been in standard use in some places (e.g. Connecticut Avenue in Washington, DC) for almost as long. Dynamic real-time signs which tell travelers what speed to adjust to to make the green wave has been recently patented and tested in simulation for automobiles: Always Green Traffic Control. The time is ripe for some carefully controlled field experimentation.

Still, pre-timing with information certainly doesn’t guarantee the fastest speed possible for the train, but it does guarantee no stops except at stations, which is good for a variety of reasons, including both travel time (avoid acceleration/deceleration loss), traveler comfort, energy use, and train wear and tear.

Case 2: If the signals are actuated, that is, their phase and perhaps cycle timings depend on traffic levels, and traffic “actuates” the signal, usually through an in-ground loop detectortransit signal priority from a fixed upstream distance should be sufficient to ensure the train doesn’t stop at a “red” light. The traffic light controller would know that a train was coming, and either keep the lights in the direction of the train green (if they are green), or change them to green and hold them, if it is currently red and the green is coming up. The train, knowing when the green will be on, should be able to adjust its speed (faster or slower) to make the green without stopping.

The distance that trains can currently notify a downstream signal controller is when they depart the upstream station, which is up to 1/2 mile or so (the spacing between stations). 1/2 mile at 30 mph takes 1 minute. With a cycle time of 2 minutes, and at least half the green time (1 minute) for the signalized arterial, a green can be guaranteed. If the light is currently red, it will be green within a minute. If it is currently green, it can be kept green for up to a minute. The worst case is it was just about to turn red and instead the green is extended for an additional minute. Alternatively, if it is currently green, a shorter than usual red phase can be inserted to clear the crossing traffic, before the light is turned back to green.

For traffic signals less than 1/2 mile downstream (say 1/4 mile) the warning time is only 30 seconds at 30 MPH. The same logic applies, but it is potentially more problematic as there is less lead time to adjust the timings, so the phase shortenings might be more severe. On the other hand, if more than 50% of the green time goes to the EW movement (say 75%) you aren’t really any worse off.

At 1/10 of a mile the warning time is less, but train departure from the station should be able to be coordinated with the light directly.

Case 3: But let’s say your traffic engineers are incapable of making this work. Should the train and its passengers suffer? This is where traffic signal pre-emption comes in. Most widely used for emergency vehicles, this potentially changes the sequence of phases, so maybe a phase is dropped (it doesn’t occur within the cycle, or within the usual place in the cycle).

This system does ensure that the vehicle requesting the pre-emption gets a green light as quickly as possible (safely turning the conflicting movements to a red phase) and thus can drive at as high a speed as possible. While trains should not need to stop at traffic lights with priority and speed adjustments, with pre-emption, they neither need to stop nor adjust their speed.

What could go wrong?

Pedestrians. Thus far we have been talking about a system with cars and trains. Pedestrians too can actuate signals, though “beg buttons“. These may function similar to vehicle actuators, in telling the traffic signal there is someone who wants to cross. The difficulty for priority or pre-emption is that a pedestrian phase may need to be longer since pedestrians take longer to cross the street than a vehicle does, especially if the street is very wide. So a pedestrian actuator may also extend the green time, in addition to calling for green time. This makes it more difficult to quickly change lights from red to green, since for safety reasons you can’t strand a pedestrian. This makes the ability to adjust train speeds in concert with the traffic signals more important.

Firetruck on University Avenue blocked by LRT train

Firetruck on University Avenue blocked by LRT train

Emergency vehicles. Emergency vehicle on emergency vehicle crashes are a known problem, and pre-emption may make it worse as firetrucks approaching a scene from two directions may both demand a green light, but only one gets it. The driver of one vehicle, not realizing he didn’t get the green (especially if he had the green as he was approaching), fails to yield. There are solutions to these problems.

Any of this will likely lead to additional delays for conflicting vehicle movements (cars making left turns or North-South traffic crossing our East-West arterial). With priority, this may even lead to extra delay for some vehicles on the parallel arterial who have been given a short green so the conflicting traffic can also get a short green before the EW arterial returns to green.

However the train usually has more people on it than are queued up at the other directions, so total *person* delay will generally be reduced.

For a variety of reasons, delay is bad (unless your goal is punishing drivers and air-breathers), we want to minimize total person time (or weighted total person time – recognizing long weights are more onerous than short weights) in the system (because time is money), and minimize pollution outcomes as well.

In short, the Green Line not getting green lights on University Avenue is a solvable problem. It should have been solved already. It eventually will be solved.

Further reading, with math: See Fundamentals of Transportation/Traffic Signals is a non-profit and is volunteer run. We rely on your support to keep the servers running. If you value what you read, please consider becoming a member.

11 Responses to Green Line, Green Lights

  1. Jim August 15, 2014 at 10:21 am #

    “It eventually will be solved.”

    I have my doubts. The train is close to beating 2030 ridership levels right out the box (38,000 weekday riders currently vs 41,000 projected by 2030). I remember the outrage when the Blue Line first started. All those drivers complaining about the long waits at intersections. We aren’t hearing those complaints today with the Green Line. So why should the city change it if ridership is way over projections and drivers aren’t complaining?

    Mind you I ‘m not opposed to preemption. I’m just trying to point out the attitude those in charge are probably feeling.

    • Matt Steele
      Matt Steele August 15, 2014 at 1:25 pm #

      Anyone who has such an attitude should not be in the business of working on transit or public works in our cities. Because ridership could be even higher if we made it faster. Because we should get the most we can out of our $1 billion investment. Because good enough isn’t good enough.

      • Matt Brillhart August 15, 2014 at 1:41 pm #

        Actually the biggest reason is that faster trains cost Metro Transit (e.g. taxpayers) less money. Whatever capital expense is necessary to speed these trains up is justified (within reason, i.e. not tunnels), because it will bring labor costs down and save the agency/taxpayers more money. Of course Metro Transit is going to celebrate high ridership numbers, but they aren’t for one second celebrating how the Green Line’s sluggishness is blowing a massive hole in their operating budget.

  2. Nick Musachio August 15, 2014 at 5:19 pm #

    I’m the inventor of the Always Green Traffic Control (AGTC) system mentioned above. Traffic engineer Derek Lehrke, under the guidance of Dr. John Hourdos at the UMN Minnesota Traffic Observatory has successfully modeled an intersection of AGTC for light rail/Green Line application. I am meeting with St Paul Traffic Engineering on August 20th to present the system to them with the hope of further modeling, leading to implementation. AGTC could eliminate traffic light stops without increasing cross traffic congestion. 1/4 mile long queues are forming on Snelling, Dale, and Lexington during rush hour as it is.

    I will keep appraised of any new developments.
    Nick Musachio

    • Acs August 15, 2014 at 8:39 pm #

      Wow, thank you for your efforts. From everyone at we wish you good luck!

    • P August 17, 2014 at 10:58 pm #

      Great idea, after reading your patent though, I think you might have oversimplified it a bit. It looks like it only works for pretimed signals, correct? What about the vast majority of signals that are actually actuated? If you could find an arterial that had pretimed only, it could be a hit, good luck though. Keep us posted on any word!

      • Nick Musachio August 18, 2014 at 5:36 pm #

        Wow. I’m flattered that you read the patent. That said, Always Green Traffic Control requires a predictable guaranteed green portion of the coming cycle in order to display the proper advisory speed to make the next green light. It does not preclude actuated signaling, but the directive speed will only be for the guaranteed green portion of the light. Gotta guarantee it you know.
        After having talked to many traffic engineers, one would have guessed that the Traffic Moses came down from mount congestion and proclaimed all intersections be fully actuated. Not so, all traffic programs have the capability to have a varied guaranteed green portion of the cycle. It’s simply a programming thing. This system will work with existing signals and controller and improve intersection’s capacities.
        UMN’s Minnesota Traffic Observatory’s traffic engineer Derek Lehrke under Dr John Hourdos has recently modeled an intersection with light rail vehicles going in both directions and never having to stop using this system, thus solving the Green Line’s double stop problem.
        Additionally, we have modeled a new intersection using AGTC in all four directions. The system can carry tremendously heavy traffic loads with little to no queueing. The side by side comparison to the non AGTC intersection is dramatic.
        I’ve shown the modeling to David Levinson for review and will present the modeling to St Paul traffic engineering Wednesday. These models will be available for viewing soon.

  3. David Markle
    David Markle August 17, 2014 at 11:43 am #

    Without having carefully studied this thesis, several things occur to me. 1) It appears to assume that no other trains are running nearby on the same tracks. 2) That being said, it’s probably otherwise essentially correct, but if applied to the Green Line might often require extremely fast accelerations and deaccelartions or at other times annoyingly slow speeds. In other words, the theory may be impractical for our example of the inherently problematical Green Line configuration that was, after all, done as a questionable tool to promote development, not to provide any kind of optimal transit service.

    • Nick Musachio August 18, 2014 at 5:39 pm #

      As I indicated above, UMN’s Minnesota Traffic Observatory has just completed modeling of an intersection with trains running both ways and the system worked flawlessly, totally eliminating stopping at traffic lights. The modeling used Green Line acceleration and deceleration parameters. That’s not a problem. The model will be available for viewing soon.

      Nick Musachio


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