From the Wikipedia article on “The Wave“:
In 2002, Tamás Vicsek of the Eötvös Loránd University, Hungary along with his colleagues analyzed videos of 14 waves at large Mexican football stadiums, developing a standard model of wave behavior (published in the September 12 issue of Nature). He found that it takes only the actions of a few dozen fans to trigger a wave. Once started, it usually rolls in a clockwise direction at a rate of about 12 m/s (40 ft/s), or about 22 seats per second. At any given time the wave is about 15 seats wide. These observations appear to be applicable across different cultures and sports, though details vary in individual cases. [11]