Why Does Seattle Have So Many Potholes Right Now?

Last week, and unbeknownst to each other, my brother and mother called me on the same day with the exact same question: “Why does Seattle have so many potholes right now?” My knee-jerk reaction was to rant against our city council and say it’s their fault, but it turns out there are several reasons why Seattle is particularly susceptible to potholes.

⁦@SDOTtraffic⁩ massive pothole at 2nd and McGraw on Queen Anne. Let’s get this patched. pic.twitter.com/Bkv6dIRWQC

— Andrew J. Lewis (@LewisforSeattle) January 16, 2022

Potholes are created by the cycle of water seeping into cracks in pavement, freezing/expanding and weakening the pavement structure, and then thawing, leaving additional cracks behind and allowing the process to begin anew. In the meantime, cars and trucks passing over this weakened pavement further break the pavement down, and over time, they cause it to collapse completely, creating a pothole.

Western Washington’s climate is ideal for potholes. The steady drip of rain, cool temperatures, and practically non-existent sunshine during the winter means that it’s pretty easy for water to get deep into the pavement, and even during our dry periods, it’s too humid and our sun is too weak to evaporate any of the subsoil moisture. Additionally, temperatures will often dip slightly below freezing during our cold snaps, so there is a lot of freezing and thawing of the road surface compared to other parts of the country.

Still, Seattle sees an abnormally large amount of potholes compared to some of the newer cities and suburbs in the Puget Sound region. This is due to two reasons. First and foremost, Seattle’s roads are layered. Although most are covered in asphalt, many have concrete below that, and some, particularly from Pioneer Square north to Ballard and east to Capitol Hill, have bricks below the concrete. This means that any water that flows through a crack in the asphalt at the surface sits at the base of a layer and collects instead of draining into the soil, and when the region experiences a hard freeze, this water freezes, expands, and thaws, leaving weakened pavement that eventually turns into a pothole as traffic passes over it.

Additionally, Seattle’s roads are constantly being loaded by very heavy vehicles. King County metro operates 1,540 buses, and there are a ton of construction vehicles as the city continues its tech-led construction boom. The most heavily-trafficked roads are also the soonest to be repaired, but I remember how quickly potholes began to pop up on Montlake Blvd just to the south of the Montlake Cut, where Bus Route 48, one of King County Metro’s most active lines, passes by 1019 times a week.

The weather in late December 2021 was perfect for giving us so many potholes in January 2022. On the heels of a stormy November and a drippy, cool December, an arctic front moved through during the morning of December 26th, bringing 4-6” of snow to much of the city and keeping highs below freezing. After the initial burst of snow, Seattle went into a hard freeze between 12/26 and 12/31, with highs in Seattle proper staying below freezing all days except 12/30 and lows dropping into the teens on the 27th. With such cold weather, all the water in and underneath the top layer of pavement completely froze solid. By early January, temperatures slowly rose above freezing as a series of increasingly strong storms moved inland, and since then, potholes have repeatedly occurred as vehicles pass over spots in the pavement weakened by the deep freeze.

If you see a pothole on the road, you can report it by going to https://seattle-cwiprod.motorolasolutions.com/cwi/tile , selecting “Pothole Report” from the dropdown, and filling out the form. Even though it’s fun to blame our city council, it’s up to us to give WSDOT the info they need to they can patch these potholes!