You'd need the equivalent of a 228-lane Brooklyn Bridge to move all those people into Manhattan during Monday morning rush hour.
At best, it would take 167 inbound lanes, or 84 copies of the Queens Midtown Tunnel, to carry what the NYC Subway carries over 22 inbound tracks through 12 tunnels and 2 (partial) bridges. At worst, 200 new copies of 5th Avenue. Somewhere in the middle would be 67 West Side Highways or 76 Brooklyn Bridges. And this neglects the Long Island Railroad, Metro North, NJ Transit, and PATH systems entirely.
Kinda puts the subway in perspective, doesn't it? And don't miss the map at the bottom that shows the size of the parking lots needed for all those cars.