It will probably take 12 months just to form the exploratory committee that will form the actual committee that will even begin to consider legalizing fully driverless cars.
This change involves many levels of government bureaucracy. It will happen, but it's definitely not going to be "immediate".
Normally, you would be correct. However, there will be so much at stake that there will be huge amounts of money greasing the skids on any required legislation.
Real Level 3 Autonomy will displace 75% of truckers almost immediately--that's 1.5 million jobs lost. You will have one or two drivers in the front truck and a half-dozen trucks running autonomously but slaved to the front truck. WalMart will almost certainly lead the charge.
Level 4 autonomy will allow the insurance companies to jack insurance rates on normal drivers through the roof. You will get a massive insurance discount if you never touch the wheel (removes human driver error--total monitoring so always other driver's fault).
With that much money at stake, the obstacles will melt like ice in the summertime. See: Fracking in Pennsylvania for an example of how fast legislation can move with enough money behind it.
Maybe, maybe not. We've seen tech outpace regulation many times - Facebook and privacy, Uber and taxis, e-mail and spam, etc.
I mean, I can understand "move fast and break things" when it comes to releasing web applications. My sympathy ends when you're literally moving fast and literally breaking things.
Once self-driving cars hit, the switch will be immediate and massive. I would predict 10% to 90% penetration in probably less than 12 months--the limitation will simply be building the cars fast enough. That's a lot of people suddenly with no income.
Protecting people against that disruption for 50 years? Probably a bad idea. Protecting people for 5 years? Maybe a good idea.
An efficient market is a ruthless market. And people are its victims.