All of the major cities have done studies and have their own plans. Some I've heard of include emptying certain areas first to allow other areas to more effectively pass, or of evacuating by order according to the first letter of their last name. None of them can be evacuated very quickly. Especially when you consider the number of morons who normally drive around with 2 gallons of fuel in the tank, running out of fuel on the freeway and clogging it up for everyone else. There's simply no excuse for that. It costs the same to keep a tank full as it does to keep it empty.
There's no quick way out in an evacuation other than getting out before it starts. Or to be the first car on the road when the order is given, and only if you live near the edge of the city. Those in the middle of it are screwed.
I've always said keep your eyes and ears open and be ready to go at a moment's notice. If it looks bad, bail. Don't "wait and see". Better to have a few false starts than a single late one. If you want to "wait and see", drive out of town, pull over and listen to the radio or something. At least you're out of the traffic jams if it starts up, and you have a headstart.