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They are white, with HM and a circle with a line through it.

Here is the explanation from Hard Drive in the Oregonian:

Sunset Highway commuter Michelle McNabb writes: “I’ve seen new signs on U.S. 26 eastbound into Portland. The signs read: ‘HM’ and then have a circle with a line through it. Any idea what the signs mean?”

A: “No Handlebar Mustaches?” “No Holding Milk?” Actually, the initials stand for hazardous materials. Basically, the Oregon Department of Transportation installed the signs as an extra reminder that explosive and toxic materials aren’t allowed between Oregon 217 and Interstate 405.

The idea is to keep them away from a possible catastrophe in the Vista Ridge Tunnel. Next time you enter the tunnel, take another gander at how many people live above it and how many drivers are using it — and you’ll understand why.