Photo by Mike Bjork

The two main takeaways from my interview with SDOT Senior Engineer Darlene Pahlman:

  1. The City of Seattle’s policy is that Link trains always receive signal priority, regardless of time of day, location, or direction of travel.  The objective is that trains never stop between stations, aside from stops caused by unavoidable human factors.
  2. The signal settings and algorithm is very complicated, and due to fine-tuning has many exceptions. It’s therefore very difficult to generalize into general principles. There are dozens of variables and thousands of permutations and it’s impossible to definitively state how the system will react without precisely defining the scenario and checking the code.

Reasons your train might stop

Ideally, as the train prepares to depart a station the operator presses a request button.  This sets up a chain of requests for a green signal that enables travel at or just under 35mph, which should allow uninterrupted travel to the next station.  Below is a (non-exhaustive) list of some reasons your train might have stopped, or not gotten its permission to leave the station immediately:

  • The operator has delayed the train.  This could be due to something on the tracks, a late runner, or merely some difficulty keeping the train at the optimal speed.  This often results in a “phantom train” of requests propagating up the line for several signals.
  • Headways less than four minutes.  If trains in the same direction are coming less than four minutes apart, the system will deny the later train priority.  Early on, revenue trains were having this problem fairly often.  According to Pahlman, however, “over the last few months the headways have seen big improvements and are much more on target than during the first few months.” Today, close headways usually result from either trailing an out-of-service train or a “phantom train.”
  • Pedestrian Signal in Progress.  Obviously, pedestrians must be given the chance to complete their crossing.
  • Skipped Pedestrians. If a pedestrian signal has been skipped in the cycle, it will not be skipped a second consecutive time, “to encourage compliance with the signal display.”
  • Emergency Vehicles.  Priority requests from Link and from emergency vehicles are resolved on a first-come first-served basis.    If the emergency vehicle gets there before the Link request does, the emergency vehicle will go first.

SDOT Wants to Know

The process of adjusting signal timing continues.  If you operate a train or automobile and notice an abnormally long wait, please document the time of day, location, direction of movement, and length of wait and email it to  All that detail will allow engineers to check the logs and evaluate if changes need to be made.

SDOT doesn’t need to hear about every time a Link train stops at a light, but if you are a regular passenger on a certain trip and notice the same thing happening day after day, SDOT would like to know that as well.

In Part II, I’ll share answers to some of your specific questions.

66 Replies to “Signaling on MLK (I)”

  1. I know that Link has to obey the speed limit when its ROW is part of a street, due to state law. But are there safety reasons why Link should be limited to 35 MPH through the entire MLK corridor? If there are good reasons, so be it. If it is just a matter of obeying existing traffic laws, has there been talk of modernizing the relevant laws?

    1. I’ve heard that the reason is that if people see a train going 40mph, they’ll think it’s OK for them to also go that fast.

    2. Are the trains really limited to the speed limit by state law? Does that apply to heavy rail running in a similar sort of alignment? What about if all of the intersections had lights and 4 quadrant gates? I know with lights and gates the FTA and AASHTO rules/guidelines allow higher speeds for the rail line.

      1. I can’t think of any heavy rail running in a similar alignment, except maybe the Ballard Terminal Railway. And that goes way slower than the speed limit for cars!

  2. just like to point out that currently it sounds like if Eastlink is surface in DT Bellevue it sounds like it’s up to the city of Bellevue to control signaling, and they are making it sound like rail will not have priority… I am liking surface more but not if it has to wait for traffic on 4th all the time.

    1. There will be significant pressure to give it priority, and there won’t be an anti-rail city council for that long.

      1. Calling someone “anti_rail” because they don’t agree with your preferred alignment is ignorant. Let me correct Ben. The Bellevue city council is not anti-rail. They are pro-rail. They simply have legitimate concerns about the impacts various …. brb … international business call.

      2. then why would they try to push the line as far as possible from the CBD?

        the city council should be concentrating their efforts on how to get the line right under the center of the CBD or close to it.

        That is what Ben means by Anti-Rail.

      3. “Legitimate concerns about the impacts” –

        You should add “to their preferred constituents” to that statement. B7 impacts just as many households, if not more than B3. They just happen to be condo owners vs. mostly single family households in Surrey Downs.

      4. Sam, you must be kidding. They are doing nothing constructive to create a Link alignment that will serve riders. Four members are blindly sticking to B7 without articulating any credible benefits of that alignment while ignoring significant drawbacks. I’m afraid they may do the same downtown.

      5. I hope my question regarding the MOA between City of Seattle and ST will be answered in Part II, that spells out how many trains and how often they may use MLK and when priority is granted. That sets the minimum headway for the system, and prevents a rogue mayor or council from derailing surface running trains in the future.
        I can’t imagine Bellevue agreeing to priority in advance, knowing that LOS F++ will occur as a result, according to the last breifing on the subject to the council by staff. That pretty much trashes any consistent headways during the peaks, and really screws up the timing of merging trains at IDS, with Central Link.
        As for pressure, the motorist still have you outnumbered by 9:1. Prepare for some pushback.

      6. LOS F will occur whether or not we build the train. Please remember that.

      7. I’m not Ben. But without an interlocal agreement in place to ensure signal priority, trains will arrive at IDS pretty much on a random basis. That’s why I think it’s so critical to have it in place from the git-go, and why my question to Martin about Seattle’s agreement with ST.
        I’m sure the $50 mil dev fund for MLK improvements and displacements was part of the bargain, and that’s with a willing participant.
        How much will Bellevue want in exchange for running trains at grade with signal priority? I haven’t a clue, but they are NOT motivated sellers of surface rights.

      8. Actually, as a project of regional significance, Link could be given priority later if there’s a problem interlining.

      9. Well, that’s probably true.
        I had a flash vision of a Bellevue Transportation Dept. Tech, tied to the signal timing control box, with Don Billon standing ready, with whip in hand, to administer lashes for messing with the timing sequence :)

      10. FWIW I’m told that Bellevue has one of the most sophisticated traffic light control systems in the region. I don’t know if it is up to full ITS standards as seen in Asia and Europe, but at the very least the building blocks are there.

        Given the 8 years that is likely to elapse between making a decision on the downtown Bellevue alignment for Link and when it actually opens I suspect there probably won’t be an issue with the City of Bellevue refusing to grant Link signal priority. The City Council very likely won’t have the same members. Besides as East Link nears completion the prospect of a ribbon cutting and the large transportation investment Link represents provides a powerful incentive to make it a success.

        Look at Tukwilla who fought the SR-99 alignment of Central Link as well as the 599/I-5 alignment that was ultimately chosen tooth and nail. Years later when it came time for Link’s grand opening Tukwilla city officials were all crowing about what a great thing it was.

  3. “Emergency Vehicles. Priority requests from Link and from emergency vehicles are resolved on a first-come first-served basis. If the emergency vehicle gets there before the Link request does, the emergency vehicle will go first.”

    This is insane. The Link trains get priority over emergency vehicles at times? So an ambulance or fire truck has to sit at an intersection and wait while a Link train goes through an intersection? What possible justification could there be for that?

    The only reason I can think of why this might happen is if the train is too close to the intersection to stop before entering that intersection. Otherwise, emergencey vehicles should always have priority over Link trains, obviously.

    1. I agree with you last paragraph in principle. I can only imagine this is what the case is, but I’m sure Martin probably knows the answer for sure. Otherwise, I’d say that priority needs to change to only give Link priority if it is a stopping distance issue.

    2. I bet 99.9% of the time it is a non-issue.

      More than likely the driver of the emergency vehicle will be able to verify no train is in their path make a judgment call weather to turn left or go across the tracks just fine without screwing up the signals.

    3. It hard to know with our more details, but it might not be that crazy. Here’s an interpretation with some speculation.

      When the request for signal priority at each section is granted the train is green lighted go. Requests are NOT granted when something is knowingly obstructing the tracks (like a pedestrian crossing, etc.). So, this sounds to me like once a request is granted, it’s basically assumed that section is clear, short of true emergencies. This is would be a pretty safe system because the operator can then assume that *anything* out-of-the-ordinary requires an emergency stop. So by making emergency vehicles work within this same request based system, this keeps the entire system operating in a consistent (and therefore safe) manner.

      The only issue would be in a scenario where a train gets a request granted several minutes out for a particular section and the emergency vehicle, having submitted a request to late, is required to wait for that duration. This isn’t a problem if the sections are small enough and requests are granted only a short time out.

      Emergency vehicles do tend to get into a lot of accidents (I can’t back that statistically, only anecdotally) presumably because they’re navigating traffic in an unusual manner. And, because emergency vehicles are so common, the safest solution would seem to me to have them follow the signal priority system already in place for the train. If they don’t do that, then BOTH the train operator AND the emergency vehicle operator have an additional scenario to consider. I suspect that would minimize the number of accidents.

      Pure speculation however :-)

    4. This isn’t SDOT wanting to give Link priority over emergency vehicles– it’s also safety protocol. I’d assume the system can’t retract a signal request, so at any time a train has already requested priority, it can be assumed that it may already be traveling through X intersection at 35 mph. It might be more dangerous for the operator to brake and risk plowing into the emergency vehicle.

      1. That is accurate. The train signal will stay on until the train clears the intersection or times out at which time the signal would immediately give the emergency vehicle a preempt signal.

        A minor note: Only emergency vehicles with opticom equipment work with this system. Private ambulances and some police vehicles operate in Seattle without this equipment.

    5. There is a difference in the signal world between “pre-emption” and “priority.” Pre-emption is reserved for applications like railroads and emergency vehicles. Priority is for modes such as light rail, street car, and buses. So pre-emption is always on top, priority comes second, and then comes the additional calls for service.

      Hopefully, that clears some items up.

    6. Emergency vehicles … when their lights are flashing ALWAYS have to make sure that their ROW is clear of other vehicles … they cannot rely that other drivers will see or hear them. It is their responsibility.

      The train, on fixed rails cannot move out of the way to avoid a collision nor can they stop on a dime (not that the LINK LRVs stopping distance is something to joke at (believe me I’ve been on board a few times where they drop the track brakes due to an idiot running across the ROW (and one time it was a car crossing over the curb for a U-Turn) … ergo if they are already cleared through an intersection they have the right of way.

      1. they usually have alternate routes to places … and there are always other emergency vehicles.

        if they are blocked … they radio that in for another responder to take the call.

    7. “The only reason I can think of why this might happen is if the train is too close to the intersection to stop before entering that intersection. Otherwise, emergencey vehicles should always have priority over Link trains, obviously.”

      I’m pretty sure this is why Link can get priority. Trains take a LONG time to stop.

  4. Norman,

    Do you know what the algorithm is? (I don’t). I would have to assume it works on the same parameters as what emergency vehicle drivers have to do when they approach a non-Link intersection now, Proceed Cautiously.

    I personally would rather know the facts, and then rant, rather than the other way around.


  5. Common sense dictates that if a train is less-than-safe stopping distance from an intersection, the smaller, more lithe emergency vehicle will slow/stop for the train for the 15 or fewer seconds it takes to get through the crossing. That said, I’ve been on a train that braked sharply and stopped at S Orcas (IIRC) for a fire truck.

  6. Let’s not forget the flagrant, daily violations of the “Move Right for Emergency Vehicles” laws that occur on every major arterial in the city.

  7. This has nothing to do with drivers who fail to move right for emergency vehicles. This is stated policy of SDOT and Metro to give Link trains priority at intersections over emergency vehicles, if the Link trains give the traffic signal a priorty request before the emergency vehicle does (according to the article here).

    That is just crazy, unless the Link train is too close to the intersection to stop. But, from this article, it sounds like Link trains request priority at intersections well before the trains are even near the intersections. If this is the case, then the stated policy regarding emergency vehicles and signal priority relating to Link trains is absurd.

    1. She need to clarify what does she mean by priority request, on an entire segment or a signal-by-signal basis? A few weeks ago, my train left the station normally, went through several signals and then stopped for emergency vehicles crossing at Graham St. So I would assume the latter.

      1. Rule book – R 2.5 EMERGENCY VEHICLES – When obseving emergency vehicles, with lights and sirens, operators shall stop their train clear of intersections, if safe to do so, until emergency vehicles have passed.

        Link operators are to yield to emergency vehicles when it is safe to do so, even if we have a priorty light. This is another reason we might lose the cascade.

    2. Don’t you get tired of looking for things to be indignant about? I’m sure there’s more to this policy than what’s been stated here and I doubt if they’re doing anything that would put people’s lives at risk or impede emergency responses. I know for a fact that both the Seattle Police and Fire departments were involved in the design of the MLK alignment. The tracks along MLK were designed specifically with emergency vehicle access in mind. That’s why the tracks are embedded in concrete and there are no fences or median barriers along the alignment, so emergency vehicles can cross the tracks between intersections.

      Emergency vehicles can still move through an intersection even if they have a red light, just like any other intersection in the city. It’s not like they have to wait there for a green light. 99% of the traffic signals in Seattle don’t have signal preemption for emergency vehicles to begin with.

    3. I didn’t ask this question explicitly, but my understanding is that it’s first come first served on a signal-by-signal basis, so the train miles away isn’t getting priority over an emergency vehicle sitting at the intersection. That would be kind of silly.

      The software is always trying to figure out if it can serve any demands before the train arrives, and obviously ambulances would be at the head of that queue.

    4. But freight trains and open bridges have priority over emergency vehiceles. Light rail in Minneapolis always has priority over emergency vehicles at most intersections (outside of downtown). The key thing is that light rail trains cross fairly quickly, so the time disruption to these emergency vehicles is minimal.

  8. other reason your train might stop: to change operators. This has happened to me several times between SoDo and Beacon Hill and I have wondered what other alternative might exist?

    1. An alternative would be to have operator changes at SODO Station which is a 7 minute walk from the current relief point at the Operations and Maintenance Base. They should get creative and have bicycles for operators to use as a shuttle between the two. That would cut the walk time by more than half and not significantly increase labor costs.

      1. Bikes are nice, but a lot of operators won’t want to deal with the perceived risks in this form of transport to which they are not accustomed.

        I suspect more operators will opt to drive or walk to SODO Station.

        Making staff changes at SODO Station will raise labor costs or possibly cut service hours, though. Nevertheless, making staff changes at the base seems both unnecessarily unsafe and a barrier to decreasing headway.

      2. Think about it this way. Would you have bus drivers change at 3rd and Pike while the bus is full of riders? No way! This is a high capacity line and it needs to be operated in that way. None of this monkey business.

      3. Adam,

        The Link operator changes are much better than forcing everyone out at Beacon Hill station. This was what was done for the first several months of operation.

        For your bus example, the 36 regularly does driver changes at 5th & Jackson inbound to downtown. I do not miss riding the 36.

      4. A couple of weeks ago my #26 local driver changed on 3rd and Pike at peak rush hour.

      5. Route #5 always seems to have a bus driver change at peak hour at 3rd and Pine. Sometimes there’s a lot of people getting on/off so there’s not much of an extra wait, but other times, we do have to wait quite a bit as the new driver gets adjusted to the bus.

      6. Routes 14 and 7 (and likely others) routinely change operators at 5th & Jackson, typically one of the fullest points for both routes. However, they do it in abut as much time as it takes a bus to load and wait for a light there.

      7. Another alternative would be to have the operators clock in somewhere other than the OMF. Maybe they could clock in at ST headquarters at Union Station, just steps from International District Station. What do they do at the OMF before and after taking over a train?

      8. Matt, as a Link Rail Operator, the offical clock is right next to the sign in window so all us operators can set their watches within 20 seconds of that clock before we leave the building and go to the trains. (rule book R1.21.1) We have 20 minutes from our sign in time to read all the new operation notices, get our train orders, and proceed out the building to the relief shack on the elevated track near Forest Street.

        And psf, driver changes were not the reason you were let off at Beacon Hill, the train and operator were going into the yard at OMF and done for the day. That problem has been solved by running those trains empty and out of service from SeaTac or the Pine Street Stub Tunnel

        The reason we make road reliefs at the shack is simple…money. ST doen’t want to pay us for travel time pay to go between Sodo and OMF, or any other station.

      9. I guess my question was more about why ST can’t put the official clock, service bulletins, etc. at Union Station, or somewhere like that.

  9. They should’ve never put it at-grade in the first place. It belonged underground like all the other subway systems worldwide. I hope we’ve learned from that mistake.

    I never knew that at-grade light-rail could be such a problem. Ugh.

    1. This brings up something I’ve meant to ask for a while but always forget.

      A few times here and there I’ve seen people mention either segments that are at grade now, or ones that will be built at grade, with the idea that they can/will be either elevated or tunneled later. In recent US history has this ever happened? Where a line was built at grade, and then separated later?

      1. Heavy rail with WSDOT has done this at several points in Kent, running roads under or over existing grades… but just at some key major intersections

      2. The only example I can think of is Muni moving from at grade on Market to the upper level of the shared tunnel with BART.

        Past that there are some examples way back when and some examples of elevated lines moving to subways.

        I believe Calgary is talking about putting their downtown segment in a tunnel, but so far I think it is only talk.

        But the safe way to bet is to assume if something gets built at-grade we’re going to be stuck with it for a while.

      3. Not 100% applicable, because they changed modes, but the Canada Line up in Vancouver is elevated in Richmond, and replaced an at-grade BRT line that had been built just a few years before to run separate from regular traffic in the median with signal priority.

      4. What’s “recent”?

        Most of the major subway systems dating from the 1900s were explicitly replacements for surface streetcar lines or elevateds, and most of the elevateds were replacements for streetcar lines. Boston’s Green Line is mostly streetcar routes with tunnels built to replace surface running.

        A number of commuter railroads have been grade-separated through various cities in recent decades (too many to mention, including Boston), as have some freight railroads. (e.g. Alameda Corridor.) Cascades is getting some grade separations.

        *Light rail*? It’s too new as a concept, and there hasn’t been enough of it built yet, for it to reach the point where it’s worth conversion from at-grade to grade-separated.

        You wouldn’t expect it to happen until the infrastructure was at least 20 years old, probably a lot more. SF’s Market Street Subway is a good “recent” example, however.

      1. Wow. Don’t get me wrong. I love Link. But dang, some of that stuff in the EIS is sweet.

      2. Would love to have a Granham Street station built someday. I don’t mean underground as in the old diagram, but at-grade with the rest of the existing system.

    2. I hate to break it to you but the vast majority of light rail lines around the world have at least some or all portions at-grade. If you’re going to take the trouble to build a 100% grade separated system you might as well go for high floors, third rail, automated operation, and 75 mph top speeds. That is usually called a “metro” or “heavy-rail” system though.

      The problem is such beasts are incredibly expensive.

    3. In Philadelphia, parts of the elevated line was turned into a subway line. It probably wasn’t cheap to do this, but it has been done before.


    There is no way to communicate to the ped what is happening, and you’ll just encourage jaywalking, despite the engineering opinion. Two minutes is an eternity–four minutes is ridiculous.

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