I saw them while driving around the Mt Rainier area. I think they're common around any of the active volcanoes - likely St Helens and Baker as well, although I can't say with 100% certainty that I've seen them in each area.
They have evacuation drills once in a while in the towns surrounding Rainier.
Think back to St Helens - the ash cloud went east, but the mudslides/lehars (I think that's the right word) are what caused the destruction, and they flowed down the other side of the mountain. People need to know to evacuate if the volcano is in danger of erupting, although I believe there's only so much that can be done to predict the area of destruction.
Edit: There's tons of info online, but this page has a picture of one of the road signs:
Mount Rainier Volcano Lahar Warning System - Wikipedia, the free encyclopedia
And finally, from the Seattle Times: "Take note, Northwest: Offshore quake would create a tsunami like this." ugh.
Local News | Take note, Northwest: Offshore quake here would create a tsunami like this | Seattle Times Newspaper
I mean, for as long as I've lived here, I've heard that The Big One is a "when" and not an "if." But still - ugh. It makes my stomach hurt to think about either of these disasters happening.