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).

It really depends. The example you give is the best case scenario, where there is an on/off ramp that you can use to drive around the blocked section of freeway. But you can't always count on on/off ramps being close by or surface streets nearby that can detour around the problem.

There was a tunnel fire on the I-5 north of the LA area a couple months ago, which is the primary north-south freeway linking northern and southern California and basically goes up the middle of the state. People had to drive way, way out of their way to bypass that fire, either going all the way around over the high desert to the east, or up the coast instead. If the tunnel fire had been associated with a major earthquake and fire crews and repair crews were tied up elsewhere, there's no telling how much fire damage would've been done to the tunnel or how long it would take to repair and reopen.

Or look at the Bay Bridge collapse at the '89 quake. No easy way around that.

Just to piggy back on this, for those unfamiliar with CA geography, if I-5 is out, you have A) Pacific Coast Highway or Hwy 101, runs along the N/S axis of the state, or I-395, along the east border. Really, that's it in terms of major routes. And there's very little E/W connectors that aren't local routes. No major East west freeways between Los Angeles (maybe Santa Barbara) and the San Jose/Sacramento areas.

So basically, like it was said, it depends. If a big one hits SF or LA, there's probably easy ways around it. If you take out central California, you're going to waste lots of time taking dirt roads or one-laners for maybe hundreds of miles to get around things.

I normally keep 40 gals gas on hand but I'm increasing that to 50 gals since that equals 1 refill for the van and 1 refill for the truck. That gives me about 12 hours of driving time, if I am frugal with the "accelerator".

Today I got another ten gals and thaqt gives me 30 gals at this moment. When I come back home next month, I'll bring that up another 10 gals to 40 gals total.

I have a layered approach. Bobs for the adults in our bedroom, bobs for the kids are spare diaper bags kept in each vehicle and the normal diaper bags. Next layer is our suitcases and the 'pack and play' portable cribs for the kids. Next layer is the gear staged on the shelf in the garage, food, water, portable grill, camping gear, etc.
If something would force us to leave our home its more likely to be localized where we would drive to a nearby hotel so we may use the first couple layers but wouldn't need camping gear and such.

When we bug out to the camp site we will take everything including the kitchen sink