I hadn't expected to get so many questions regarding my thoughts on large/extended family evacuations. Actually, truth be told, I wasn't expecting questions at all. Let's take a look.
What suggestions do you have for the masses living in crowded northeastern metropolitan where often "alternate routes" aren't such an option?
Given human nature, it seems to me that most folks tend to wait until the last moment to start running. Having watched a couple of natural disasters as they unfolded, so to speak, I have noticed that the roads appear to be relatively empty during the first hours, or sometimes even days, after people are informed that the Fit Is About To Hit The Shan.
Getting into the wind as soon as possible after discovering that Mother Nature is winding up a right hook will probably leave you with less-congested roads.
There are only a certain number of routes into/out of Manhattan. DC isn't much better.
Let me preface this by saying I've never been on the ground in Manhattan, I don't know anything about Manhattan, and I'm working from a bit of a disadvantage here.
Assuming that circumstances conspire to force you to leave later, rather than sooner, how to evac?
So. Let us pull up a topo map of Manhattan and the surrounding area, pour a mug of Earl Grey, and let the mind wander.
You know what comes to mind? Evacuation routes are only limited if you limit yourself to ground transportation only.
It would take a bit more prep, but would it be possible to arrange for the use of a boat to ferry your and your across the water? Manhattan is an island. I would imagine that the places for a boat to dock and take on you and yours should be fairly numerous, even without Improvising, Adapting, and Overcoming.
The simplest iteration is to arrange for a friend and/or family member to be waiting at a pre-determined point on the mainland for pick-up. This allows you to by-pass congested bridges, but gets you back on roadways as quick as possible.
Alternately, depending upon variables I don't have access to, it might be possible to use the boat itself as the evac. Both the East River and the Hudson River appear to be navigable for a good part of the year. Assuming that you begin your evac with a safe margin, there doesn't appear to be any great obstacle to riding one of the rivers right out of the danger zone, to a destination point on one of the rivers itself, or disembarking and proceeding with your evac well-clear of any panic zones.
Again, I don't know the area, but it's an idea to mind-wrestle.
If boats aren't to your liking, as long as you leave at first warning, the roads and highways should be relatively clear.