Originally Posted By: philip
>
There are several scenarios which have different likelihoods. I'm in the San Francisco Bay Area on a peninsula. If I can't go south, I'm stuck. There are two major roads to the south. The one nearest me (a block away in fact) is covered in overpasses, so if they drop in an earthquake, that road can't be used.


I have a question for anyone who has lived through a quake scenario with collapsed overpasses - are the roads they overpass actually out of use?

I'm more familiar with overpass situations that come from temporary impassability - the overpass gets hit by an oversize truck, and has to be repaired. When the overpass goes 'down', traffic is re-routed and still viable over the offramp and onramp to either side. In other words, you still can travel a roadway with downed overpasses by exiting and re-entering at each downed overpass.

I-90 in western Washington headed up to Snoqualmie Pass answers this description - except for one overpass that lacks an onramp continuing East. Which scotches any mass evacuation plans to Eastern Washington after a serious shake (assuming the I-90 summit bridges survive).