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.