What Does I-5 Ramp Metering Mean for SDOT’s Mercer Corridor?

GE DIGITAL CAMERA Mercer St at Fairview Ave

It looks like the Washington State Department of Transportation (WSDOT) is about to apply the brakes to Seattle Department of Transportation’s car funnel that is the new Mercer Street. A few weeks ago the state transportation agency installed signals that will allow it the ability to meter the five lanes entering I-5 from Mercer, both northbound and southbound. This Saturday, those meters will get turned on, but only on weekends; weekday metering will not start until after April 10th.

For almost a year now, the upgraded Mercer Street traffic signal technology from Uptown to I-5 has been doing what it was intended to: increase vehicle throughput. Seattle Department of Transportation (SDOT) picked the corridor to be the first in the city to get the “adaptive signal” treatment after the complete rebuild of the street from one-way to two-way, completed in 2014, resulted in travel times that were around the same as they were pre-construction due to the much higher volume of cars on the new two-way Mercer. Called SCOOT, the system’s objective is clear, if not linguistically, on the SDOT project page: “to keep people driving moving”.

Because the signals along Mercer keep traffic moving by prioritizing east-west movement for cars, other types of travel are not prioritized. Last summer, Mark Ostrow outlined how pedestrians were being impacted: walking time frequently lags behind the time allocated for traffic in the same direction, leaving many pedestrians waiting at the curb. Because the adaptive signal system measures vehicle throughput and not pedestrian (or bus rider) throughput, it can’t prioritize the latter, except in heavy-handed ways around the margins of the system. In November, SDOT tweaked the amount of time pedestrians were allocated to cross Mercer in one cycle, which also resulted in fewer walk cycles overall. After all, as SDOT’s blog post on the issue mentioned, “As for walk cycles, it’s important to note that…Mercer[‘s eastern portion] has high vehicle volumes.”

With the average daily traffic levels on Mercer at Fairview going from 37,900 vehicles in 2010 to a staggering 66,100 in 2016 (the most recent year for which there’s data available), it’s not surprising that Mercer’s ramps might be dumping more traffic onto I-5 than can safely merge onto the existing lanes. But it begs the question as to how sustainable SDOT’s approach of prioritizing east-west traffic to and from the freeway is long-term. We don’t yet know exactly what the impact to the capacity of Mercer Street will be with these new ramp meters, but it’s beginning to look like the multi-million dollar investment in smart signal technology simply moved the bottleneck onto I-5.

In addition, SDOT has stated the goal of expanding SCOOT technology to another chronically congested corridor nearby: Denny Way. Denny is also congested with vehicles getting to and from I-5: how will adaptive signal technology work with ramp metering? Will the car funnel only work until WSDOT needs to implement freeway demand management on the on-ramps there? Will this issue come up with nearly every corridor that leads to and from I-5?

Since the launch of SCOOT on Mercer, regular statistics have been posted to the project page showing the system’s performance. The data from January of this year shows 7,673 vehicles moving through Mercer during the three hours of PM peak, when there is the most eastbound demand on the street. That’s actually down from the first few weeks of SCOOT’s implementation, where the total numbers were closer to 9,000 vehicles.

January 2018 data from SCOOT along Mercer Street. (City of Seattle)

The most remarkable part of the results of Mercer’s signal upgrade as presented by SDOT is the assertion that carbon emissions are being reduced as a result. According to their numbers, from January, the average vehicle is emitting 316 fewer pounds of carbon per year as a result of the efficiencies gained though these signals. To assert emission savings from a street that has seen overall vehicle numbers grow by almost 30,000 per day in just six years is, well, dishonest. In a imaginary world where the number of cars on a street remains exactly the same all the time, yes: moving cars more efficiently will reduce in less idling and fuel and carbon savings. But as traffic improves, more people will be persuaded that they’d like to drive on Mercer. Improving the “efficiency” of the street has the same induced demand result as adding another lane, just on a slightly smaller scale. On the flip side, we see a de-prioritization of other less carbon-intensive modes, which has the same effect of inducing driving.

As cities around the country become aware that expanding their major streets is a fool’s errand, they increasingly appear to be turning to multi-million dollar signal investments as an alternative to adding lanes. But “intelligent” signal technologies may be the urban freeway expansions of the 21st century: demand inducers that turn cities away from the real cures of urban congestion: safe places to walk and bike, dedicated transit, and demand pricing.

Tomorrow, the City Council’s Transportation and Sustainability Committee will be considering whether to authorize a federal matching grant to implement intelligent traffic systems around the University of Washington. At this point, it’s not exactly clear what a $4 million grant from the Federal Highway Administration (FHWA) would go toward, just that it’s earmarked for “advanced transportation and congestion management technologies”. The matching grant would be coupled with $5.5 million in city funds. If streets in the University District are being readied for the same “upgrades” that Mercer Street received, we should ask ourselves if those improvements are in the long-term interest of our city and our planet, or if we’re just moving bottlenecks around again with the quixotic goal of improving congestion.

Adaptive Signal System Kicks Pedestrians To The Curb

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Ryan Packer lives in the Summit Slope neighborhood of Capitol Hill and has been writing for the blog since 2015. They report on multimodal transportation issues, #VisionZero, preservation, and local politics. They believe in using Seattle's history to help attain the vibrant, diverse city that we all wish to inhabit. In December 2020, Ryan started a three-month stint as editor of Seattle Bike Blog.

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I don’t understand this notion that bottlenecks are not worth fixing and are intrinsically of value because they deter driving. If you want to deter driving then make driving more expensive and provide better transit options. But no one’s goal should be to force people to sit longer in traffic. Especially given a lot of these people are driving because they need to live further out due to a lack of affordable housing… and further out areas have less public transit options. Especially when there is a clear case of roads getting used inefficiently, and a simple fixcgets people off clogged areas faster as opposed to sitting around polluting and wasting people lives.

Yes, the fix is not perfect… and better public transit options should be available… and more people should utilize public transit. All of these things can be true and yet making a simple fix like adaptive lights is the right thing to do. It still is a net positive. I see the reasoning behind choosing to spend your billions on public transit as opposed to adding lanes to highways. But that does not mean you should spend zero on upgrading (and maintaining) current infrastructure when this can lead to greater efficiency and better utilization of what is currently available. Especially when you look at how much time is wasted commuting (in all forms.)

Seattle has many obstacles in terms of geography, demographics, affordability and lack of existing infrastructure for all modes of transportation. The notion that we should not seek efficiencies with what we currently have as a way to punish people seems like a non-progressive approach.


Bottleneck fixes for SOVs that actively make all other modes worse are not worth doing. How is it a net positive to make Mercer flow better at the expense of people walking, bicycling, and taking bus lines that go north/south across it?

Vincent Adultman

Bellevue has implemented the system city-wide and it’s great. Seattle’s problem is mainly that they’ve only implemented it on one corridor.


Yes, Bellevue is a pedestrian wonderland. Sometimes I like to just go and walk around downtown Bellevue for the joyous experience. ?

Vincent Adultman

Completely agree. Nice clean parks, great library, awesome shops, restaurants, and quite a bit less human feces on the sidewalks. And I’ll take crossing 8th over crossing Mercer anyday.

Benjamin Plotke

I think the author, recognizing all the negative externality of cars, doesn’t want to do anything to encourage them, including making signals better for cars. But I think the argument Ryan makes to support his dislike of the adaptive signaling is also valid. Even if your goal is better car throughput, these adaptive signals are mostly to help roads to the freeway. Since the freeway is already over capacity, the adaptive signals are not useful for cars.

Personally i don’t have a strong opinion on the matter. I would love if we setup our streets like Stockholm, poke around on Street view, it’s amazing. But we are so far from that philosophy I hardly think some adaptive signals will really make much difference. Really I think our traffic problem is too much parking. We allow more parking than the streets have capacity for. Of course the argument right now is where to set the parking minimums so I don’t expect many people would be receptive to neighborhood parking caps. Guess we’ll just keep triaging our traffic flesh wound with adaptive signal bandaids.


I spent several weeks in Stockholm, and agree it is a multi-modal transit dream. Great options for bikes, pedestrians, bus, tram and even ferries. Stockholm has housing affordability crisis similar to Seattle, but the main difference is Stockholm (an most legacy cities) have existing options to transport large populations to the unaffordable downtown via rail. So you can start at baseline with less cars on the road.

Seattle currently lacks viable means of rapidly getting large numbers of people from affordable areas to the downtown. It is trying to get there through admirable expenditures towards light rail and BRT, but until that needed infrastructure is in place, I think the region needs to try and support its car commuters too.

Put yourself in the shoes of someone with a family who can only afford a <400K home commuting to a job in central Seattle. You likely are living in Renton, Burien, White Center even further out in Everett or Sumner. Unless you can somehow make closer in neighborhoods affordable for families, you should support all means to get those people into Seattle to work, or else Seattle becomes the Bay Area, with a core of wealthy to the exclusion of all else… and even the Bay Area has BART and Caltrain.

I think there are public transit solutions to make this work (likely more via BRT and somehow upgrading Sounder as opposed to massively expensive new light rail), but for the time being the car is one of the only options. And just because it is not the preferred environment option does not mean it should be completely neglected on principle. Because a lot of people advocating that are people who are fortunate enough to live in wealthier close in neighborhoods and are solely focused on their bike lanes, and not the ones forced to sit in their cares for 80 minutes because of a lack of other current options. (And I'm one who lives close in and loves the idea of more bike lanes. I just think some people are a bit self righteous when they want to punish people for using highways.)