gridlock

A condition of total, interlocking traffic congestion on the streets or highways of a crowded city, in which no one can move because everyone is in someone else's way.

On a smaller scale: the situation in which cars enter a signal-controlled intersection too late during the green light cycle, and are unable to clear the intersection (due to congestion in the next block) when the light turns red, thus blocking the cross traffic when it's their turn to go. Repeated at enough intersections, this phenomenon can lead to citywide gridlock.

(figuratively, by extension) any paralysis of a complex system due to severe congestion, conflict, or deadlock.