Temporary 4-Way Stop at 220th and 58th

Temporary 4-Way Stop at 220th and 58th

The City is currently converting the intersection of 220th St SW and 58th Ave W from a 2-way stop to a 4-way stop, here's why.


Most cities use what is called an intersection Level of Service (LOS) to rate how quickly a vehicle can proceed through an intersection. In Mountlake Terrace, if a vehicle has to wait between 0 and 10 seconds to proceed through an unsignalized intersection, a LOS of A would be assigned. 10 to 15 seconds would be a LOS of B, 15 to 25 seconds would be a LOS of C, etc. There is always some maximum LOS that a city would consider acceptable. The City has in their Transportation Master Plan that a LOS of E (35-50 seconds) is an acceptable Level of Service for an unsignalized intersection. 

Under the Washington State Growth Management Act (GMA), if a development would cause the LOS to fall below a jurisdiction’s adopted standard, it must be denied unless adequate improvements or demand management strategies can be provided. 

This brings us to the intersection of 220th/58th and the Brighton School development. Because the Brighton School would certainly increase vehicular traffic through this intersection a traffic model was performed which looked at the effect the additional vehicles would have. This intersection is currently a 2-way stop for vehicles on 58th. Vehicles on 220th do not currently stop. The video below shows what traffic might look like in the peak PM hour when school is in session. Watch the entire video, it’s about a minute and a half.

As you can see there are some significant backups both northbound and southbound on 58th. The traffic model showed that a car may have to wait 5 minutes or longer before being able to pass through the intersection This would be a LOS of F and would fall below the City’s standard. So, according to state GMA, the proposed development (Brighton School) is required to pay for modifications that would bring the LOS back up to the City’s standard. 

According to the traffic model, a 4-way stop does exactly that; it brings the LOS back up to the City’s standard. See below for animation of this scenario from the traffic model.

Because the 4-way stop beings the LOS back up to the City’s adopted standard, there are a couple of things the City can’t do: 

  • The City can’t allow the development to occur and do nothing with the intersection.
  • The City can’t deny the development on the basis that it drops the LOS below the standard.
  • The City can’t require the development to do something above and beyond that which would bring the LOS back above the standard. For example, the City could not require Brighton School to pay for a signal when the 4-way stop would be adequate.

The City has planned for some time to make changes to that intersection with either a roundabout or traffic signal. The project is on the City’s 6-year Transportation Improvement Program (TIP) and had been scheduled to be designed beginning in 2019. Below is what the traffic model shows if the intersection were to be a roundabout or a signal.

Obviously it’s 2015 right now so the City doesn’t currently have money set aside for a roundabout or signal. So, temporarily we have the 4-way stop. I’m sure the City would have loved to have money in our current budget to do the roundabout or signal. If that were the case, Brighton School’s contribution could go towards the final project. 

I’m guessing that the next time the City updates the 6-year TIP they might re-prioritize projects such that the roundabout/signal can begin earlier than 2019. 

A few other things to note:

  • The signals and the I-5 onramp/offramp are owned and controlled by the Washington State Department of Transportation so the City can’t modify the timing of those signals.
  • The City Council had an in-depth presentation and discussion on this intersection back in January, 2015. There were several public comments in support of a roundabout. It’s worth your time to listen: http://mountlaketerrace.granicus.com/MediaPlayer.php?view_id=2&clip_id=874
  • Here is the City's press release about the change.
  • I’m not a City employee and don't’ represent the City in any way here so take what I say with a grain of salt.
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