Several years ago, I was at a FEMA workshop where a City Manager from one of the larger cities in my state was trying to pin down FEMA funding to take care of a massive influx of refugees from the coast in the aftermath of a Cascadia Event. The City Manager was concerned about a situation like the Superdome happening here in the Pacific Northwet. An engineer from our State's transportation department who was sitting next to me started laughing out loud, upsetting the City Manager. He then explained, "There will be no massive influx of refugees away from the coast, because there will be no way to leave the coast. The roads and bridges will be all gone." He opined that the first rough road from the coast to the I-5 corridor would probably not be rebuilt for at least a year.
Some telecom engineers who were participating pointed out that the communication and power infrastructure couldn't be started until after passable roads were rebuilt, starting from Klamath Falls, maybe Bend, and Spokane, continuing on to rebuild I-5 (including all the bridges that will be on the bottom of the Columbia and every other major river). After all that was done, they could start in on coast roads. Most of the equipment will have to be staged out of Spokane, Boise, and Salt Lake, which means the roads over the Cascades will have to be repaired before I-5 gets repaired. After the coast roads were finally in place, the heavy equipment needed to build power lines to the coast could be brought in.
Last year I was speaking with a geologist who works for my state, who expressed the opinion that people on the southern end of Cascadia probably wouldn't see regular dependable electricity for years.