I remember hearing a story once that went something like this:
A behavior specialist for the US Army was sent to a city in Iraq where civil unrest frequently broke out. While studying a major plaza in the city, he noticed a few things:
First, a crowd would gather. Next, the food trucks would arrive to serve the crowd. Then, a riot would break out.
So, he decided to try prohibiting the food trucks from entering the plaza. His plan worked - the next day's crowd would gather and simply disperse.
I was unable to verify this story, however.