City’s new concurrency plan doesn’t measure congestion, overall travel time

By Miki Mullor

Deputy Editor

A new traffic concurrency plan for Sammamish appears unlikely to meet the Sammamish City Council target date to lift the building moratorium in July, despite six months of staff and consultant work and expenditures of about $375,000. (Read more.)

Concurrency is a state law requirement to “​prohibit​ ​development​ ​approval​ ​if​ ​the development​ ​causes​ ​the​ ​level​ ​of​ ​service​ ​on​ ​a​ ​locally​ ​owned​ ​transportation​ ​facility​ ​to​ ​decline​ ​below the​ ​standards​ ​adopted”, unless “​transportation​ ​improvements​ ​or​ ​strategies​ ​to​ ​accommodate​ ​the​ ​impacts​ ​of​ ​development​ ​are made​ ​concurrent​ ​with​ ​the​ ​development “. The law allows development to proceed if “a financial commitment is in place to complete​ ​the​ ​improvements​ ​or​ ​strategies​ ​within​ ​six​ ​years​.”  (see RCW 36.70A.070, and a clean indented version)

Accordingly, cities are required to set a level of service standard for their roads, measure traffic and forecast future impact of development on traffic.  

In response to residents’ frustration over traffic congestion in Sammamish, City Council has enacted a moratorium and directed staff to revise the city’s concurrency system to focus on drivers’ experience. 

New plan presented

On May 5th, staff presented to the Council and the Planning Commission the results of the revised concurrency system. The new system is based on measuring the average delay time drivers experience when approaching key intersections throughout the city.  The results drew skepticism in the new system from several council members, as the results showed as if traffic congestion actually improved in the city from 2014 to 2016 despite significant growth inside and outside of the city.

For example, according to the results, it takes only 12 seconds on average to get through 37th Way intersection on Sahalee Way. Sahalee Way is notoriously stand still during the morning commute.

Screen Shot 2018-05-14 at 6.29.16 PM
A long line of cars are backed up on northbound Sahalee Way during a morning rush hour.

Congestion is not being measured

In a two-hour interview with Sammamish Comment on 5/9/2018, Cheryl Paston, Deputy Director of Public Works, acknowledged the proposed concurrency system does not measure congestion.

“[Congestion,] that’s a modeled output and so whenever you can’t directly measure things now, you’re getting into the world of, well, you do have to make some assumptions,” Paston says. “The congestion itself is not directly measured because it’s a time-based, distance-based, volume-based which we can’t have enough people out there with their stopwatches.”

The HCM (Highway Capacity Manual) requires measuring of the congestion when calculating intersection delay LOS.  

Known limitations

The city is using a computer simulation, (known internally as the “model”), to compute the delays drivers experience on Sammamish intersections, which in turn determines the concurrency level of service.

However, the model has limitations, explains Paston.

“If traffic is at a standstill, it [the model] doesn’t know how to manage that, so that’s a limitation of the model. They don’t know how to handle standstill.  Another challenge for the model is if the congestion at the intersection is actually caused by downstream.“ (downstream means that traffic from the next intersection is backed up and prevents vehicles to pass through on a green light).

In a November 2017 memo to City Council, Kendra Breiland of Fehr & Peers, the city’s traffic consultant, promoted this same approach as “Could be done quickly and would simplify the City’s concurrency program” but failed to disclose its limitations in handling congestion, even though she recognized that “driver experience should be a key measure of system performance.” 

Manual process prone to human errors and subjectivity  

The intersection delay model is based on “turning movements” counts that measures how many cars passed through the intersection.   

“There’re the tubes and then there’s the turning movements in the intersections and those are done manually,” explains Paston.  

In reference to a recent council meeting, when Mayor Christie Malchow highlighted the measurements of the intersection at 36th St. and Sahalee Way, Paston acknowledges problems with the process:

“For example, 36th at Sahalee, when we looked at that, frankly the traffic counts were wrong”.

Paston explained that the counts are measured by interns who use clickers to count cars that go through. When looking 36th St., which appeared to be failing in the model, staff speculated that an intern may have hit the “10” digit button instead of the “1” digit, inadvertently causing the data to show a failure at that intersection.

Paston also explains the subjective impact a traffic modeler can have on the model:

“If you have an inexperienced modeler and they look at that just the intersection in isolation and they don’t understand this dynamic of really, it’s the downstream intersection, he’d try to keep tweaking this intersection to make the model work.”

Concurrency tests assumed unconstructed traffic improvements

The term concurrency stems from the legal requirement that “​transportation​ ​improvements​ ​or​ ​strategies​ ​to​ ​accommodate​ ​the​ ​impacts​ ​of​ ​development​ ​are made​ ​concurrent​ ​with​ ​the​ ​development. ​” (RCW 36.70A.070).  The law allows development to proceed if “a financial commitment is in place to complete​ ​the​ ​improvements​ ​or​ ​strategies​ ​within​ ​six​ ​years​.”

Paston explain that the city maintains a concurrency traffic model that is used to approve development in the city.

“We have what’s called the pipeline model, or it’s sometimes called the six-year TIP, the concurrency model. That looks out six years and says you have this many projects that are permitted, they might not be constructed, but they’re permitted.” [TIP stands for Traffic Improvement Projects]

“It’s the city capital list of projects,” she said. However, the listed projects may not be funded and there is no guarantee that any of these will be built.

“Right, but we have to assume that they are,” she said. “[City Council] intention is to fund the whole six years CIP, and we do put some assumptions in there that we’ll go for grant applications, so, hopefully, we’ll get some grants to help pay for projects,” acknowledges Paston.

Sammamish TIP backlog
Click on image to enlarge.

Historically, 70% of the traffic capital projects within the City of Sammamish have not been constructed and several major projects have been on the plan for well over six years.   

When reached for comments, Council member Chris Ross said “It is imperative that we have clarity of assumptions and that the data being used is understood, accurate and reliable, before we can finalize a new concurrency model.”

(Transcription of the interview with Ms. Paston)

6 thoughts on “City’s new concurrency plan doesn’t measure congestion, overall travel time”

  1. I think labeling your link “requires measuring of the congestion” is misleading because the page linked to does not state the requirement to use measurement methodology (demand flow).

    1. From the link:

      “When measured in the field, this flow rate is based on a traffic count taken upstream of the queue associated with the subject intersection. This distinction is important for counts during congested periods because the count of vehicles departing from a congested approach will produce a demand flow rate that is lower than the true rate.”

      The HCM requires measuring cars arriving at the beginning of the queue (which is the congestion), not when they get through the intersection.

  2. The entire infrastructure in Sammamish is going to be very chaotic down the road. Not only the traffic, what about water supply, sewer treatment , growth with no end, etc. The City Council has no vision as they look for permit money and other fees to meet current needs. Such poor planning meeting long term goals.
    H.W. Maine
    Sammamish, WA 98075

    Sent from Mail for Windows 10

  3. The sentence “The new system is based on measuring the average delay time drivers experience when approaching key intersections throughout the city.” would be better as “The new system is based on estimating the average delay time drivers experience when approaching key intersections throughout the city.” The delays assigned to intersections are computed, not measured.

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