[ZeroGravitas]

In london they have "Yellow Box Cameras" which detect people blocking four-way junctions. I don't know if it's the same in other countries but you're not supposed to enter these yellow marked boxes unless you are sure you can get to the other side, so you won't block cross traffic.

However, my first result of Googling claims that they're actually causing longer jams. Sound like it could be fixed with slightly smaller no-stop zones.

http://www.timesonline.co.uk/tol/news/uk/article673093.ece