Bellevue’s adaptive signal technology
Credit: Lizz Giordano

Large five-lane intersections dominate Bellevue. To eke out every little bit of roadway capacity, the city in 2015 finished installing adaptive signal technology at all 203 of its signalized intersections. The system adjusts the timing of the traffic signal cycle based on real-time traffic conditions. In theory, the less unused green left at the end of each light cycle, the better, resulting in more traffic moving through the intersection.

This system, known as the Sydney Coordinated Adaptive Traffic System (SCATS), has reduced afternoon delays at some intersections as much as 43%, but is also benefiting pedestrian and bus riders, the city says.

Traditional traffic signals work on a fixed cycle, which might allow for a couple of settings to be used over the course of the day. But the timing for a traffic signal cycle that works best for the morning commute isn’t always the most efficient for the evening rush or during non-commute hours. Instead, SCATS uses detectors embedded in the roadway to constantly monitor traffic volumes at intersections, adjusting cycle lengths based on current demand. The system tries to decrease delay by reducing the amount of unused green time during each cycle. Generally, the higher the traffic volume through an intersection, the longer the cycle length is to serve the demand.

Bellevue traffic engineers developed ‘personalities’ for each intersection, establishing a minimum and maximum length for each cycle. During non-peak times, intersections operate independently, but to handle commute traffic intersections are ‘married,’ and any cycle length timing adjustments are applied to the entire married group. 

If traffic demand indicates more time is needed for a particular movement in the cycle, the adaptive system adjusts the timing for the next cycle. But if too much time is provided, the system can also trigger a red light early, reducing the amount of unused green.

“Generally, we’re getting cycle lengths that are a lot lower than what we had programmed before for the whole downtown,” said Chris Long, Bellevue’s Traffic Engineering Manager. “Because we would have had to program it for the worst conditions, now we can let it flex to that worst condition then come back down to a more normal state.”

The lower the cycle lengths gives pedestrians more opportunities to cross Bellevue’s massive streets, Long said.

“We have very big intersections. For a downtown environment, it’s unusual to have these five-lane roadways with protected, permissive left turns at all approaches,” Long said. “So it creates very long cycle lengths, so we want to get those as short as possible.”

The system also allows for safety features that give pedestrians an advance green light a few seconds before cars, protecting walkers from potential collisions with right-turning vehicles.

Adaptive technology backfired on pedestrians in Seattle last summer after the city installed it along Mercer Street from 3rd Ave W to Fairview Ave N. Shortened walk times had pedestrians rushing to cross intersections, while the real winners were the drivers whose commute along the corridor was cut in half during peak times, from 34 minutes to 17.

SDOT has since readjusted walk times, giving pedestrians significantly more time to cross the street. At the time, SDOT wrote in a blog post that though the amount of time given to pedestrians to cross Mercer St. may be shorter, the technology will actually provide pedestrians more opportunities to cross the street.

Bellevue has also programmed intersections to provide an early green to buses. Long said there is extra time, ‘slop time’, built into the cycle and if a bus is detected in the queue, the ‘slop time’ goes to the bus.

King County Metro Transit says combining SCATS with Transit Signal Priority (TSP) has improved travel times for buses along the B Line. A retiming of the signals along the B’s route last year resulted in travel time savings of up to 6 percent, according to the transit agency. But just using adaptive technology alone, without TSP, Metro said, may not improve transit travel times.

At some point during rush hour, especially where two large arterials come together, traffic engineers have to decide which street to prioritize. In those cases, the engineers weigh which approach has the capacity to store more cars without impacting other intersections.

The system can’t adapt to huge swings of traffic volumes which occur during special events such as the city’s Fourth of July celebration. Instead, a pre-planned special events strategy is deployed on the system.

“Prior to having the adaptive signals, it could take as much as two hours to clear out all the parking garages,” Long said, “and now it’s down to 45 minutes.”

33 Replies to “Smart Technology Alerting Commutes in Bellevue”

  1. If a pedestrian has to push a button and wait for a walk sign, it’s still a time punishment for pedestrians — even after a car-focused adaptive signal system is operating. Automatic walk signs are still the best priority for riders walking to and from transit stops and stations.

    Hopefully, the nearest intersections to new Link stations will have automatic walk signs, especially during commute hours.

    1. I think even more important than that is the ability to push the beg button 1-3 seconds after parallel traffic gets a green (almost always meaning you missed your cycle and it’s pretty tempting to make a quick hop across the street) and get a guaranteed walk signal in the cycle. That solves a particularly annoying problem, when many people are waiting at an intersection, but no one pushed the button because they all assumed someone else did or they think it’s an automatic pedestrian intersection.

      After that, automatic walk should definitely be everywhere that the timing makes sense. Sometimes it doesn’t make sense because there are some really small roads that get a 1.5 green or something because there is usually one car waiting. Here a beg button that extends the cycle if pressed on green, and reduces the time to the next cycle is sufficient.

      And after that, having a beg button that (on red) reduces the time of the current phase to allow the pedestrian to get a walk sooner is valuable even on 100% automatic walk signs, best case scenario being an instant pedestrian countdown of the other walk signal, or an instant yellow on intersections without automatic walk.

    2. My experience is that the pedestrian crossing lights in downtown Bellevue are terribly unfriendly toward pedestrians and reinforce the fact that pedestrians are second class citizens.

      Specific examples

      At most intersections the Walk light only stays on a very short time before reverting to Don’t Walk, even when the adaptive car cycle continues to give cars a green for a very long time – enough time that it could have remained Walk a lot longer.

      At most intersections, if you request a Pedestrian crossing after the traffic cycle started, you won’t get a Walk signal until it goes through a complete cycle even if it remains green for cars for a long time, long enough to have done a complete pedestrian cycle.

      In many cases it takes an exceedingly long time to get a Pedestrian Walk.

      Software could easily solve the first two problems. If you push for a Walk when it is already green for traffic, the system can decide whether it can give you the [20] seconds, after all it often stays green for cars even when there are no cars. Similarly, once a Walk is given to pedestrians, don’t revert to Don’t Walk until you are read to stop the traffic.

      And it could prioritize pedestrians by saying that no pedestrian should ever had to wait more than (1.5? 2?) minutes for a Walk and speed up the cycle when pedestrians are waiting.

      It is a question of prioritizing walking and pedestrians and making them feel welcome.

      Two more things that I would like to see Bellevue do to improve the pedestrian experience and walking:
      (1) given the pain to cross streets, construction projects should be required to provide a pedestrian walkway when they impinge a sidewalk longer than a week – Bellevue has often allowed projects to close sidewalks for months without providing any alternative besides crossing the streets; and (2) if a merchant provides free parking, explicitly permit people who parked to walk to other businesses not served by the same lot, provided you have done business with one merchant in the lot. Park once, do 3 errands by foot, instead of driving and reparking.

  2. The city of Bellevue can say what it wants, but the fact of the matter is, wait times to cross the street are still excessive, and most of the intersections still have beg buttons, which, if you’re one second late in pressing, you’re expected to wait an entire cycle, which might take several minutes. It is still a common occurrence to walk out of Bellevue Square while the bus you’re trying to catch isn’t even in sight, only to watch the bus come and go, while you’re still standing there, waiting for the light to cross 8th St.

    And there are still many, many streets in Bellevue like this one. Notice that instead of a crosswalk to connect Best Buy with Home Depot, they instead install hostile fencing to “force” people to walk all the way around, basically because the people driving are too important to have to be bothered with stopping for pedestrians. Yet, at the same time, they have an ultra-wide 5-lane street for what’s essentially a minor road serving a couple of shopping centers. And, this is a new street that didn’t even open until a couple years ago. The city of Bellevue may be giving lip service to bikes and pedestrians, but at the end of the day, they are still designing their streets in 2015, like it’s 1950. (I suppose it could be worse, though – at least, the sidewalk has a decent width, and has a landscaped buffer separating it from the street).

    1. I recently spent an evening walking around Bellevue and I was frustrated by how long I had to wait at every intersection. I have a lot of skepticism in Bellevue’s claims. I can’t imagine what it would have been before they “improved it”. I don’t believe it was designed with pedestrians in mind…

    2. I was stunned to learn that 129th and 124th were to be widened to five-lane roads. I first heard about it at a Bellevue open house held jointly with East Link. The roads are clearly for the masses of cars Bellevue predicts will come to the Spring District when it’s finished.

    3. What? The “hostile fencing” is around the Home Depot parking lot. It’s not in the middle of the street. Dude, you can’t blame “them” for the actions of a corporation or its landlord.

      Not to put too fine a point on it, but most people going to Best Buy aren’t then going to Home Depot and vice versa. Especially if they’re going on the bus or walking all the way.

  3. One thing I see more of lately in Seattle is all-walk crossings (Denny & Broadway at CHS is a recent example), which to me seems to be an effective way to pretend to be doing a favor for pedestrians, but these all-walk crossings generally come at the cost of the regular walk period (i.e., the walk signal will be red when parallel traffic is green, even if the beg button was pushed).

    These would be great if all-walk signals augmented the normal pedestrian signal cycle rather than replacing it. It doesn’t seem very hard.

  4. “We have very big intersections. For a downtown environment, it’s unusual to have these five-lane roadways with protected, permissive left turns at all approaches,”

    Of course, downtown Bellevue also has a very nice street grid. Bellevue should do what Bellingham did way back in the early 80s and convert these five-lane streets into alternating two-lane one-way streets, which would slow traffic, reduce pedestrian crossing times, and give everyone more green light (no left turns to accommodate), and even out traffic flows by forcing cars from the busier streets to less-busy ones.

  5. I guess if you start out by punishing pedestrians with ultra-long signal cycles you can give yourself credit later when you “adaptively” shorten them during times of day when the all-important cars don’t need them. SDOT has been operating in this way, too, at times.

    So I can believe that turning on an adaptive system might make signals like Roy/Westlake and Roy/9th more responsive off-peak. I used to live near there and walk across those intersections often; traffic on Westlake and 9th is very peaky, and the signals were originally programmed for the peaks, as described in the article. I still avoid these intersections reflexively, but I imagine they must be better now. Still, during peak hours, pedestrians go to the back of the line; we shouldn’t accept that.

    It sounds impressive that engineers have accommodated more cars, reduced delays, and balanced that with pedestrian needs. But the engineers are solving the wrong problem. This is a political problem, not an engineering problem. Its solutions are political, changing our priorities and values, not engineering solutions. Accommodating more cars is actively harmful. We’ll only build really pedestrian-friendly downtowns if we design for walking first. We’ll only kick car dependence if we stop accommodating as many cars as possible at every opportunity.

    1. Agreed. Encourage density… eliminate unnecessary buffers and parking minimums. Encourage infill… provide incentives for urban development (reduced review times, flexibility, reduced fees). Discourage sprawl… lowest priority review times, added fees. Encourage human-centered design… allow flexibility on city-mandated improvements (road upgrades) in exchange for pedestrian-centered upgrades.

    2. Considering the spectrum of political decisions:

      – Many years ago we made the political decision to remove part of Westlake Avenue, making way for Westlake Mall, a few parcels of “open” public space (including Westlake Park), and later the southern SLU Streetcar terminal.
      – We have continually made the political decision to keep Pike Place, effectively, a “woonerf” by allowing car access but also allowing people to walk down the middle of the street.
      – On a few congested streets we’ve made the political decision to reserve a lane for transit — we decided that transit reliability was more important than undifferentiated vehicle throughput. On most congested streets we’ve made the political decision to allow buses to sit in traffic.
      – On some streets we’ve made the political decision to allow bus lanes to be used for parking some times of the day.
      – On the Roosevelt BRT project we’re probably going to make the political decision to improve cycling conditions on Eastlake at the cost of bus priority.

      In all these cases, engineering analysis could inform the political decisions, and engineering design is needed to implement the political decisions, but ultimately it’s up to us to decide what matters to us. When impressive engineering projects deliver some benefits, but the underlying politics are still car-centric, and the results on the ground still punish pedestrians all too often, these are the politics and the results we question.

  6. There’s also the motor vehicle equivalent of beg buttons, in the form of intersections where you have to come to a complete stop right over the sensor before the cross street light turns yellow, or have to wait an entire additional cycle. This happens a lot to buses when coming out of transit centers. The good news is that the cycle length is reasonable during the midday. But during rush hour,when the bus is carrying the most people, the bus often has to wait for an excessive amount of time.

    1. The left turn signal from eastbound Madison to Lake Washington Blvd is like that, and it’s a turn onto a popular bicycle route. The process of getting to turn if you’re the first to reach the intersection on a bicycle is that you have to know this, pull up into the crosswalk, and gesture to the next car who comes up behind you to make sure they get far enough forward to trigger the signal.

  7. Hate the term “Beg” in any connection, because it implies its user is helpless. We’ve got two incompatible uses for travel, only correctable politically. So counter-movement at least needs a platform-plank or two. For starters:

    First plank, anybody who wants a six-lane boulevard … Fine, just so budget includes a foot and bike bridge over or a tunnel under every intersection or so. Seattle City Council? DSTT 2 means no cars Downtown. Or whenever you want, sooner. Every other measure? Word has it you’ve got more money than God, though less than AARP’s war chest.

    But mainly, given something special to see, buy, or have dinner at,or move to, the trains can start bringing in standing loads of walking and biking people from and to all over the region. Who, being on a train that isn’t under the English Channel, will have already left their cars either home or still at the dealers’.

    Not belittling how hard, and long, the fight is going to be. But if this isn’t already happening, good move is to start meeting with business-people Seattle, Bellevue and similar, just to get their present thinking for strategy next move. Because in addition to everything the NRA says that’s a lie, it wasn’t a gun, registered or otherwise, that “won” the “West”.

    “Big (blue-uniformed) Government” with a lot of registered carbines also brought in genocide, small-pox ,and hordes of undocumented immigrants. And Marshall Wyatt Earp’s registered gun came with his badge. On a train-ticket courtesy of the Chamber of Commerce who’d had one too many drunk cowboy shoot a railroad investor.

    Mark Dublin

    1. Not no cars downtown. There are people in private cars who need to drive downtown to serve customers. There are thousands of businesses in downtown Seattle; they need professional services.

      No cars in bus lanes? Absolutely. No cars on Third Avenue? Certainly. But no cars anywhere downtown? It’s impractical.

      Should they pay for the privilege? If it can be done in a non-intrusive way.

  8. If this is the kind of reporting that we can expect to come out of STB, I might have to rethink my subscription. Bellevue doesn’t need press releases for their terrible technology choices.

    1. Will anyone miss you if you cancel your subscription?

      The Seattle Transit Blog talks about transportation in the Seattle region. Maybe this article should have been more critical. But it was still an interesting article.

      1. That’s the point. This article didn’t bother to counter any of the claims coming from Bellevue’s DOT. No pedestrians who actually walk in Bellevue were asked their opinion. Who actually believes that adaptive signals “gives pedestrians more opportunities to cross”? Shouldn’t a reporter be questioning stuff like that?

      2. When the cycles lengths are shorter (and the City’s engineering dept surely has the data on that), then how can it not be true that pedestrians have more opportunities to cross?

      3. So, one of the tenants of “adaptive signal technology” is that when the light is green and a car is approaching, the signal controller will “hold” the up (up to some maximum amount of time), like a bus driver doing a courtesy wait for someone running to catch the bus. Of course, while cars get this “courtesy hold”, pedestrians don’t. You show up in a car after the light was green for 20 seconds, the light waves you on through. But, the pedestrian who shows up a half second after the light turns green, too bad.

        Obviously, no human police officer directing traffic would be so anti-pedestrian as this. Yet, this is what the software does. Much of the problem has to do that the software is designed to optimize what the engineers told it to optimize for. The traffic engineers say “maximize car throughput”, but they don’t say anything about pedestrians beyond the constraint that someone who wants to cross the street *eventually* can, without getting run over. So, the software, like any software did, as it was told. It optimized for what the engineers said to optimize for, and didn’t optimize for what the engineers didn’t tell it optimize for.

      4. Yes, but the cycles are shorter, not longer. So there is less delay for pedestrians, not more. The non-adaptive signals have longer ‘baseline’ wait times because they’re waiting for cars that might not even be there. The adaptive ones only have longer waits when it’s really necessary to clear traffic backlogs.

      5. As Someone who has walked across 148th 4 to 6 time a week for the past two years i haven’t noticed a difference.

      6. I am, as a rule, extremely skeptical of “man on the street” inteviews to validate whether something has created a quantitative improvement or not. Lizz did go and ask Metro if they’ve seen any improvement, and they have.

        When Metro does a route restructure we like, we don’t go find the rider who doesn’t want to transfer. That’s the opposite of being informative.

        If a program is making things objectively better for pedestrians and transit, we applaud it. It’s not constructive to complain that it doesn’t deliver utopia.

      7. Y’know what? I’ll just quote NACTO here.

        “Adaptive signal control should have limited variation in their cycle length. Operations for adaptive signal control should be limited to suburban settings and event venues where traffic is highly variable. Adaptive signal control can result in a longer cycle length that degrades multi-modal conditions.”

        Why do you suppose NACTO says that, if adaptive signals are so wonderful for pedestrians?

  9. C’mon, Andres, give STB a break. Thanks to Lizz this morning, since we now know the capabilities of this technology, we can also intelligently attack the misuse of it. And also ways we can improve, supersede, or destroy it.

    Notice that we don’t have one single comment today even pretending to support it just to get everybody’s goat (does anybody say that now even if they’re playing an extra in a World War II movie?)

    It’s not like we’re that repulsively liberal National Public Radio fawning over a “start-up” organizing tours to acquaint business representatives with the new residents who have so much more money than the ones they forced out.

    Look at it like a vaccine. Giving the body a deadly infection in tiny amounts to arouse the latent savagery of our immune system. Or like “sparring” to train a boxer. Doubt Mohammed Ali ever tolerated a partner who didn’t want to hurt him.

    So don’t make Lizz rethink herself into becoming chief transit reporter for The Seattle Times. Or special staff assistant to Senator Steve O’ban. Or most horrible of all….

    Issuing TRIGGER warnings! “The following includes material that can induce death by choking on your tongue laughing at it!” Don’t go anywhere, man. Somebody’s got to fill in the space that inevitably follows every comment of mine for three days.


    1. That last line sounds like you are commenting on few people replying to your comments. I think part of the problem is most people can’t figure out what you’re trying to say. I only read your comments on the rare occasion I am in the mood for allegoric prose. Why don’t you put a clear explanation at the start of your comments? You could write your comment, then make a concise explanation at the top. For example, in this one, you could have written the following:

      1) It is good that today’s article positively discusses something anti pedestrian because it sharpens our ability to analyze and critique.
      2) Please criticize ideas not our reporters. STB staff works hard and we would like to keep them.

      Then again, my last comment had one reply that accused me of an opinion I never had, so I guess I probably am not the best person to be giving out communication advice. None the less, I would enjoy it if you made Cliff notes for your comments.

  10. “The system also allows for safety features that give pedestrians an advance green light a few seconds before cars, protecting walkers from potential collisions with right-turning vehicles.”

    This assumes a fact not in evidence: that there are cars that respect either pedestrian right-of-way or a sign saying “no turn on red.” Not in my experience.

    1. The other thing that bothers me about the “advance green light” is that it doesn’t do anything for people who arrive when the light is already green. To say that you need this is to imply that the only time one can safely cross a street is at the beginning of the cycle, which again, leaves to excessive wait times.

      The problem is that people making right turns need to look right and yield to pedestrians in the crosswalk. Hopefully, someday soon, we’ll have a critical mass of robot-driven cars to make this happen. (All it takes is one polite robot to stop and yield, and those impatient humans behind it in line will have to put up with it and wait).

      1. Yes!

        Unless the robots are programmed to analyze the pedestrian’s clothes and general demeanor and run over the “losers” who won’t be able to sue for much. Or may not have any family TO sue.

        Don’t laugh; it could happen though grant if it got out it would be a huge scandal.

  11. Funny – I distinctly remember a communication from the City of Seattle, I think it was from a City Council Member or Seattle Department of Transportation saying the timing lights would not help traffic flow in Seattle. Can we hire some of the Bellevue Transportation people to fix our traffic woes? and kick out the Seattle people who don’t know what they are talking about? Also, charge some impact fees to help pay for a timed system?

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