100% gridlock

[snopes]

Comment: Here is one sort-of legend that I always wondered if
it were true: On 9/16/1984, (probably in the evening) Manhattan
island reached 100% gridlock. That is, not a single car got in or
out of the island.

I was actually in Manhattan that day, stuck in traffic. I think it was
days like that that lead NYC to put "gridlock boxes" across many
intersections-these are white grids painted over the intersection, and no
car is supposed to be stopped over it, lest you get a ticket and points.
As a matter of fact, they JUST raised inbound tolls again for the same
reason.

[Aureal]

Originally Posted by snopes

Comment: Here is one sort-of legend that I always wondered if it were true:

I was actually in Manhattan that day, stuck in traffic.

So... if he was there, why is this person asking us if it's true? Wouldn't s/he be in a better position to say, having first-hand experience? >.>

[snopes]

Originally Posted by Aureal

So... if he was there, why is this person asking us if it's true? Wouldn't s/he be in a better position to say, having first-hand experience?

If he was stuck in traffic, it's unlikely he was in a position to observe all of Manhattan and determine whether any vehicles entered or left that day.

- snopes

[charlie23]

The New York Times has no mention of it in their archives, which is a bit strange if it were true. "100%" gridlock in Manhattan would certainly seem newsworthy.
The "Antigridlock Boxes" actually seem to date from 1981.
http://www.barrypopik.com/index.php/...block_the_box/:
"30 March 1981, New York Times, pg. B6:
They are called "antigridlock boxes," and they look like football end-zones plunked down in the middle of busy intersections in midtown Manhattan. (...) The idea of an antigridlock box is to keep traffic out of intersections to prevent "spillback," the backup that occurs when vehicles do not get all the way through an intersection before the traffic light changes."

[Mycroft]

The 'Anti-Gridlock' boxes are very common in the UK at junctions to ease traffic flow generally as you are not permitted to stop in one, or to enter it if the exit is not clear.
wrt the OP, I do not believe it is possible to have complete gridlock on an island the size of Manhatten as all tunnels and bridges would have to be closed, however a limited area could certainly become gridlocked for a considerable period

[Richard W]

Originally Posted by Mycroft

The 'Anti-Gridlock' boxes are very common in the UK at junctions to ease traffic flow generally as you are not permitted to stop in one, or to enter it if the exit is not clear.

Yes, they're called box junctions and they're a standard road feature. (Except they're yellow here, not white).

Like Mycroft said, the gridlock would have to be wider than just Manhattan to prevent people getting across the bridges and tunnels out of Manhattan itself.

And if all the bridges and tunnels were closed, then no cars would get on or off regardless of the state of traffic on the island.

A similar story that I've heard is that there are more cars than there is road space on the island of Jersey (in the Channel Islands, UK-ish), so if everybody took all their cars out of their driveways or garages at once, the island would be completely gridlocked.

(edit) Also, what time period are they talking about here? "Not a single car got in or out of the island" for how long? Since they say "probably in the evening" they can't mean all day.