Many roads are coated with a mix of compact snow, ice and slush, said Travis Phelps, a state transportation spokesman.
Drivers should expect challenging conditions through this evening and into Friday morning, he said. Crews are seeing numerous spinouts, and they ask drivers to take it easy.
"Keep the speeds down," he said. "That's the No. 1 rule if you're going to drive out in that stuff."
Roads likely will get worse tonight as freezing sets in, he said.
Transportation crews are treating highways with a mix of sand and salt, Phelps said.
"That should help try to keep that from compacting too much," he said.
The Washington State Patrol also has been seeing lots of accidents and other trouble on the roads, trooper Keith Leary said.
"In the last few years, this was probably the most challenging weather we have had," he said.
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