[kijin]

Average speed cameras are used in a number of countries, though I'm not sure about the U.S.

They are useful for catching speeders in a relatively short stretch of highway, but useless for medium-to-long stretches. Even if you constantly drive at 20km/h over the limit, the average can easily dip below the limit if you stop at a service station for a few minutes or if there's heavy traffic at some point. And if the highway in question doesn't have toll booths, the police will have to install a bunch of expensive cameras at every single interchange.