Things don’t always work as planned.  That certainly must be the feeling at the Seattle Department of Transportation (SDOT) after the rollout of high-tech adaptive traffic signals along Mercer Street.

Often called “the Mercer Mess,” Mercer Street is a major east-west route that bisects two dynamic downtown Seattle neighborhoods: Uptown, home to the city’s most prized cultural and philanthropic organizations, and South Lake Union, arguably the engine of Seattle’s recent economic growth.

Ever since Mercer was completely rebuilt in a massive project from 2010-2015, the City of Seattle has embarked on a program intended to expedite vehicles through the corridor.  Alternately known as SCOOT (Split Cycle and Offset Optimization Technique, a trademarked system sold by Siemens) or ITS (Intelligent Transportation System, a general term which applies to a variety of technologies), adaptive signals use sensors to track the movement of cars and adjusts signals to move more of them.  In theory, it’s like widening the street without pouring a single bucket of concrete.

The problem is, SDOT, in their rush to move more cars, failed to consider that people need to cross the street.  The cultural and economic value created in these neighborhoods has very little to do with cars and, in fact, the presence of what amounts to a divided highway inhibits the innovation, collaboration, and social interaction that is the true benefit of co-locating so much creative energy in one place.  Furthermore, there is mounting evidence that adaptive signals aren’t the high-tech panacea they are touted to be.

So, let’s take a little walk down Mercer and look at some of the ways adaptive signals frustrate, discourage, and endanger people who walk.

1.  They periodically skip the pedestrian walk phase.  You wait your turn but, for some reason, your turn never comes.  Some people take a chance and cross against the light anyway.  Older, younger or slower people don’t have this option.

A real head-scratcher. Periodically, adaptive signals appear to go haywire, creating moments of traffic anarchy that put pedestrians at risk.  In this example, pedestrians pushed the beg button but the signals skipped the walk phase.  (Photos: Mark Ostrow)

2.  They require you to push the beg button.  Some people push it, some people don’t. Some intersections require it, some intersections don’t.  There is no way to know for sure until your walk signal fails to materialize.  Accessible pedestrian signals, like the one shown below, help people with visual impairments.  However, when pedestrians are required to push the button to get a walk signal, it can be an unnecessary restriction of pedestrian mobility. 

Frustration and indignity has boiled over into citizen activism along Mercer, evidenced by these stickers protesting SDOT’s signal priorities.  (Photo: Mark Ostrow)

3.  They behave erratically.  Typically, a person expects the signal to show walk, then flashing don’t walk, then solid don’t walk.  Signals follow a familiar pattern and people embed that pattern in their muscle memory.  Adaptive signals, however, can flip from walk to don’t walk to walk again without warning and their length can vary considerably.  This makes the signals frustrating and unpredictable.  National Association of City Traffic Officials (NACTO) guidance urges cities to use signals with fixed intervals that are predictable and intuitive to users.  According to the NACTO Urban Street Design Guide:

“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 longer cycle length that degrades multi-modal conditions.”

4.  They don’t give people with mobility challenges enough time to cross safely.  SDOT has worked to address this issue by increasing the minimum crossing time at certain intersections along Mercer but the problem still is not solved.  If you have a spring in your step and assume everyone else does too, sit at a busy intersection sometime and watch people cross.  You might be surprised how many people have mobility challenges.  Note that your sample won’t include people whose challenges are severe enough that they don’t even try.  A humane city works to accommodate everyone.

(Click to enlarge, infographic by Mark Ostrow)

5.  They give pedestrians a “don’t walk” signal even when cars moving in the parallel direction get a green light.  This is the most common, frustrating, and dangerous behavior of adaptive signals.  It is a reasonable expectation that if cars have a green light that pedestrians moving in a parallel direction should have a walk signal for the same amount of time but this is no longer the case along Mercer.  The people who live and work in Uptown and South Lake Union are regular jaywalkers now, since nobody can trust the signals.

(Click to enlarge, infographic by Mark Ostrow)

6.  They prioritize travel in the direction with the most vehicular demand.  This is not always the direction with the most pedestrian, cyclist, or transit rider demand.  On Mercer, cars primarily want to get to and from the freeway (east-west); however, pedestrians want to circulate in their neighborhood, bikes want to travel in and out of downtown (north-south), and the busiest transit routes cross Mercer perpendicular to the direction prioritized by SDOT.  As a result, the most sustainable and space-efficient modes have to get in line behind private automobiles.

Evening commute.  Cyclists and pedestrians are traveling out of downtown but first they need to get across Mercer.  (Photo: Mark Ostrow)

7.  They give cars the green light to block the box.  When you think about blocking the box (a form of gridlock where cars get stranded in the intersection), what Seattle street comes to mind?  If you live here, your answer is probably Mercer.  If this was truly the sophisticated system it is touted to be, wouldn’t you expect it to detect and prevent this kind of gridlock?  Perhaps it’s not so intelligent after all.

Every day.  Cars blocking pedestrians, bikes and other cars is a regular sight at several intersections along Mercer.  Motorists hate it but vulnerable pedestrians and cyclists, who must weave into traffic with cars, are most at risk.  (Photo: Mark Ostrow)

8.  They give cars the green light even when there are no cars (and people are waiting to travel in other directions).  Again, isn’t this supposed to be a sophisticated system?  Rather than optimizing our streets, adaptive signals appear to improve vehicular throughput by stealing time from pedestrians and handing it to cars.

Crossing against the light is the only way to walk along or across Mercer in a timely manner.  (Photo: Mark Ostrow)

9.  They give cars the green light even when traffic is stopped and there is nowhere for them to go.  Meanwhile, pedestrians are forced to wait.  Currently, pedestrian delay isn’t measured by SDOT; it isn’t even a success factor they consider.  So, if cars save one minute driving the entire stretch from Uptown to Interstate 5 and that result is achieved by delaying pedestrians for two minutes every time they cross the street, then adaptive signals are functioning as designed.  It’s easy to achieve positive results if you don’t measure the negatives.

When traffic signal effectiveness is evaluated solely based on vehicular speed, nobody wins.  (Photo: Mark Ostrow)

10.  They prioritize cars most during periods of highest vehicular demand, such as before and after events at Seattle Center.  However, these same periods see the highest pedestrian demand, so people on foot get even less opportunity to cross.  Not all Seattle Center events are the same.  Sporting events and concerts draw lots of cars but protest marches, parades, and conventions bring a substantial number of visitors on foot and by bus.

Most of these gun violence protesters arrived on foot.  (Photo: Mark Ostrow)

11.  If they work as advertised (and the jury is still out on that) they are likely to induce additional vehicular trips.  Mercer is clearly a street where demand for space exceeds supply.  Making it easier to drive on Mercer will induce more people behind the wheel and then we’re back where we started.  Except now pedestrians are stuck too.  Considering the global climate crisis, is that a desired outcome?  Seattle should be actively encouraging people to leave their car at home.

Mercer Mess, 2018 edition.  Remember: it’s a mess because it is full of cars.  Seattle could address this mess by making walking, biking, and transit use more appealing along the corridor.  (Photo by Mark Ostrow.)



  1. Thank you for writing this article! I was a pedestrian commuter from LQA to SLU for the past 4 years, and I agree with all of these problems with adaptive signals. It appears to help cars, but is incredibly unhelpful, dangerous, and frustrating to pedestrians.

  2. Great article Mark!, but you did miss an important 12th way that adaptive signals endanger ped safety: at 5th and Mercer (over by QFC) I have observed that the “lagging green” is set up to operate at both ends of the cycle – at least on occasion (again, the adaptive signals change so much it is hard to see recognizable patterns). That is – when the light turns green for cars, the walk signal for peds in the same direction stays at “don’t walk” (this is true after the left turn signal arrow goes off). After a certain amount of time during that cycle – the walk signal comes on. Then it goes off again while the light still continues as green for cars going in that direction. This is all intended of course to get those pesky peds out of the way so as to enable easier rights on red for cars at both ends of signal cycle.

    What I have observed happening is that drivers of cars making rights on red have their automatic ped radar set for the beginning of the signal cycle “peds will cross in front of me, then I can safely turn right on red” is a deeply ingrained driving habit in the big city. But because peds don’t get an x-ing signal at the start of the cycle… the driver sees no peds. So he looks the other way and prepares to turn right on red. As he starts his turn… the ped walk signal comes on in the middle of the cycle. Peds step out in front of the driver just as he starts turning (while looking the other way – because – gee – those peds on the curb aren’t going to cross in front of me for some reason…). I have seen this happen! Adaptive signalization along the Mercer corridor seems to showcase SDOT’s ‘ready, shoot, aim’ process at its finest. Or is it simply a complete disregard of pedestrian safety in favor of getting a few more motor vehicles through the intersection on each cycle?

      • Yeah. That is what I have observed too. I walk with my dog and my toddler and I always wait because cars will turn right in front of us if we step off the curb immediately after the walk sign comes on. We’ve almost been hit many, many times. I wish people couldn’t turn right on red there at that intersection. It’s a disaster waiting to happen.


    While I understand your point, this is NOT the case; and we shouldn’t be encouraging pedestrians to make dangerous decisions. The traffic light tells the cars what to do and the pedestrian signal tells pedestrians what to do.

    Sometimes we want to give the pedestrians advance opportunity to begin walking into the intersection. You wouldn’t want cars thinking they can drive by watching the pedestrian signal switch and/or seeing someone walking, do you?

    And on Second Avenue, we installed bicycle signals for a reason. Whether or not we like that bike lane implementation, we don’t want cars turning there just because they see a green signal or bicyclists moving through, do we?

    How about we train people to expect to watch for the signal directed at them? (Or be really radical and just do away with signals all together?)

    • I didn’t mean to suggest that pedestrians should walk against the ped signal, simply that pedestrians should get a walk signal when parallel cars get a green light.

  4. For starters, you can only optimize for what you can measure, and car traffic is much easier than pedestrian traffic to measure in an automated way (e.g. loop detectors in the pavement). Measuring pedestrian traffic requires either human staff, or a mixture of cameras + AI (for which the technology wasn’t really there yet at the time the Mercer signalling changes were deployed). Counting beg button presses doesn’t work because you might have 20+ people cross the street in one cycle, while only one of them actually presses the button.

    On top this, the whole adaptive signalling system is off-the-shelf technology, whose software is likely written by people far away from Seattle, where transportation = driving, and nobody ever walks. They pay just enough attention to pedestrians so they can say their system is compliant with the law, placing emphasis on audio and tactile button feedback for the blind, over than the need to press the button in the first place, and wait excessively, once the button is pressed.

  5. Seattle is really not the first city to combine traffic lights in a network and let the signal times adapt based on traffic volumes. So what I don’t get is what went wrong? Is it just that SDOT gave the wrong principals to the project (like car traffic comes always first) or is the system just done cheaply without enough data points in it (like loops in the road, camera’s etc.)? I mean network traffic light don’t favor cars on principle. It does so only if you tell the system to do so.

    • Those are super good questions. I can attempt a partial answer. Adaptive signals have been deployed all over the world. I haven’t studied every deployment but the ones I have encountered in my research are in car-dominated environments (suburban areas or car-dependent parts of megacities like Dubai or London). Mercer is the only place, to my knowledge, where the local residents recognized adaptive signals as a threat to pedestrian mobility and fought back in an organized way.

  6. Mark, thanks for your years of trying to make sense of Mercer adaptive signals.

    Can you tell us, after multiple meetings that you and other advocates have had with Seattle DOT officials, have City staff instituted ANY changes that have improved the crossing experience for people who walk, bike, and use transit — or has the SCOOT system continued to default to people in cars?

  7. Beg buttons have got to go. Biggest pet peeve in the world: you approach an intersection with a crowd already standing at it. Everyone assumes everyone else has pushed the button. No one gets to walk. Brave souls cross against the light creating a potentially deadly situation where unaware or aggressive drivers endanger the life safety of the pedestrians. No “adaptive timing” is worth this.

    • Beg buttons are mislabeled, probably by some writer trying to be clever. When you Beg for something, it may or may not be given. At a properly functioning ped crossing, the button should always yield a Walk signal (OK, eventually…). If it’s always provided, it’s not begging.

      • I don’t understand your comment. At many intersections, if you don’t push the button, you don’t get a walk sign. You have to “beg” to cross the street. Those are terrible, and have no place in the built environment. If you are disagreeing with me, please be more clear.

        • I’m applying a traditional definition of the word Beg. When you Beg for something, it may or not be given. Think panhandlers on the street corner.

          When I push the button on my car door handle, I’m not Begging for the door to open. I know it will open. Same with a properly functioning ped signal. Push the button and the light changes to Walk.

          Why would you want a Walk light at times when nobody wants to walk? Just to anger waiting motorists?

          • Why is life about “motorists?” I won’t accept “beg buttons” (or WHATEVER you want to call them), until drivers who arrive at intersections also have to signal to the traffic light in some way, in order to simply cross the street.

          • In many locations, left turn pockets, motorists have to cross a detector loop in the road, in order to call up a green light. Life isn’t just about motorists; it’s about getting along with one another and sharing limited resources. Let’s be trying to make that sharing fairer than it has been, especially for pedestrians.

          • I agree! Let’s make it fairer for pedestrians, in an unfairly and unjust auto-dominated environment. However, your example in this comment just proves my point: a motorist only needs to mindlessly approach an intersection and the signal phase will change for them. A pedestrian has to be more mindful, or risk losing their signal phase. It is very unfair.

          • “I’m applying a traditional definition of the word Beg.” -RDPence

            The rest of us have moved on. They are called beg buttons.

        • In a a properly functioning ped crossing system, the Walk light will always come up. Just like the left-turn light for motorists in the turn pocket.

    • Push the button, lights will change for pedestrians to walk. Don’t push the button, walk into traffic, pedestrian is causing a potentially deadly situation. Time for both pedestrians and drivers to be responsible for safety. No one has the right to just walk into traffic and expect all drivers to notice and avoid, just not smart for the pedestrian and not safe for anyone in the area.

      • I will agree with you someday, when a motorist has to push a button to cross as street. Until then, it is unfair to put blame on pedestrians for what is clearly a faulty, unfair and unjust traffic engineering system.

  8. Thank you for this. I cross (jaywalk mostly) Mercer on foot daily, and this is completely on point.

    What’s the worst anti-city waste of our taxpayer money? Streetcar delay? Mercer adaptive signaling? Mariners stadium improvements?

  9. I think the situation is actually worse than described — That “total crossing time” is misleading. Pedestrians get only a few seconds of the white Walk light before the orange Don’t Walk light starts flashing. It’s only legal to step off the curb with the Walk light, often as short as 5 seconds. Additional pedestrians are supposed to wait until the next Walk light appears, an unknown amount of time.

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