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Winter driving - Winter plowing and road treatment


Our crews start working long before snowflakes fall to keep roads open during inclement weather. We use advanced weather forecasting to predict where snow and ice will accumulate and use the information to pre-treat high traffic corridors. The anti-icing chemicals we apply help prevent frost and ice from bonding to the pavement. Once snow has started falling and accumulating, we switch to a salt pre-wet with a corrosion-inhibited liquid deicer that helps snow and ice to melt, making it easier to remove with snowplows – but it takes time to work.


A lot of ground to cover
Our winter snow and ice response is a balancing act between the forces of nature, available crews and keeping equipment running.

While we work 24/7 during winter storms, we can’t be everywhere at once. During heavy storms, for example, a road may need several passes with a plow to completely clear snow and ice. This means roads are prioritized when it comes to plowing.

Priority plow routes
To strategically deploy our resources, equipment and supplies during inclement weather, priority plow levels are assigned to all of the highways we maintain. The determination is based on the following factors:

Knowing the plowing priorities along your route can help you plan your trips, including knowing to expect winter conditions on certain roads or possibly delaying travel during particularly heavy storms.

Priority plow route maps

Our priority plow route maps (pdf 1.9MB) show which roads are prioritized for treatment during severe storms and can help travelers plan routes or make decisions on traveling.

Western WA treatment level goals
Eastern WA treatment level goals
Puget Sound WA treatment level goals

What do the plow map colors mean?


Our highest-priority routes as our primary focus is to keep interstate corridors open. Interstate 5 is the busiest roadway on the west coast and is vital for moving people and goods to support the economy. From the onset of an event, our goal is to keep at least a single lane open in each direction and work towards bare and wet pavement across all lanes.


These are important intercity and local routes, which carry between 20,000 and 80,000 vehicles per day. These stretches of highway include many of the east/west routes like sections of US 2, US 12 and SR 14.


These routes generally carry fewer than 20,000 vehicles a day. The priority levels of some of the green highways change, as the number of people using the highway and geography changes, like US 101 near Discovery Bay.


Winter climates differ greatly on either side of the Cascades. You'll likely notice more orange routes in Eastern Washington, like SR 21. That's because snowfall and freezing temps often occur throughout the winter on the east side of the Cascades. Our priority is to keep traffic moving under normal expected winter conditions for the 5,000 to 10,000 vehicles that use these roadways daily.


Less than 5,000 vehicles use these routes per day. Some of these roads, like the upper reaches of SR 504 on Mount St. Helens, are often closed during the winter months.


We partner with agencies around the state to manage sections of highways which pass through many cities and counties.

Road treatment

We use plowing and a variety of products to pre-treat and clear roadways – balancing the materials and amounts used as conditions change.


Liquid calcium chloride, sodium chloride or magnesium chloride is applied to a bare road before a storm to:


Solid sodium chloride (salt) is applied to remove a thin layer of snow or ice already on the road. It can also be very effective for melting and preventing black ice and freezing rain from adhering to the road


Wetting solid chemical material and sand with calcium or magnesium chloride causes them to stick to the road surface better, helps activate the solid salt and reduces corrosion.


While not used as much as years past, sand is still used in road treatment, particularly in colder weather. Sand can blow off roadways fairly, quickly, however and can cause cleanup and water quality concerns if it enters nearby waterways.

Corrosion inhibitors

We add corrosion inhibitors – products to help prevent corrosion – to our treatment products to reduce corrosion on vehicles traveling over the roadways. Motorists concerned about salt or chemicals on their vehicles may also want to wash the vehicles to remove any material.

Environmental factors

Salt, chemicals and sand all have potential environmental concerns, which is why we train operators to apply only as much as necessary. Our precision equipment is also calibrated regularly to ensure we're applying treatments in carefully measured doses. We also annually monitor more than 50 sites for chloride levels in soil and water near our roadways.

We need your help

Our priority plow route maps (pdf 1.9MB) show which roads are prioritized for treatment during severe storms and can help travelers plan routes or make decisions on traveling.