As a California resident I have a question:
CVC 21706.5 says that traffic in the opposite lanes of the freeway is not in an "emergency incident zone". OK, but why are drivers on divided streets ... required to pull over and stop. If it's essentially impossible to cross the median, and an emergency vehicle is going without delay in one direction, why does all traffic going the opposite direction also have to stop?
First, let's note this is a different subject than the Move Over law that started the thread.
To answer your question: On a "freeway" (as that term is defined in CVC) it is essentially impossible for the authorized emergency vehicle to cross the median into your lanes. Yes, there are special crossovers for that purpose, and on freeways with more than 46' of median there might not be a physical divider, so theoretically emergency vehicles could cross anywhere, but practically speaking it is rare, and you'd have plenty of time to see them doing it (thus putting them now on your side, where you are required to stop).
On a regular "divided road," however, there are normally many openings in the median, for cross-streets, driveways, etc., and the emergency vehicle that is coming at you in the opposite direction may very well be going to an emergency on your side of the road, or planning to turn across your path down a side street. With all those flashing lights, you'd never notice his turn signals, if he even has the time to activate them. Lastly, fire apparatus, ambulances, and police cruisers have been known to drive right over concrete medians if that will shave a half minute off the response time. So you are required to stop immediately wherever you are.
Yes, you can argue that some medians are uncrossable for long distances (Jersey barriers, hedge rows, etc.), but that's not universal, and experience has shown that the law can not leave this sort of matter to the judgment of individual drivers, many of whom won't even notice the gaps. So for safety's sake, the law requires the stop in all cases.
I can tell you from experience that it does not matter how many flashing lights and sirens you have, there are drivers out there who will drive right in front of you.
The law in most other states is similar: Red (or Blue) Light, Move Right (and stop when safe to do so).