While wind generators are being used experimentally to charge batteries in roadway equipment that can't be reached with power lines, there are at least three reasons why such generators aren't in large scale use. First, wind generators that connect to the AC power grid work most efficiently when they turn at a steady rate—the generator itself must remain in synch with the cyclic alternating current in the electric power lines. The intermittent and sporadic winds produced by passing cars and trucks aren't really suitable for such wind generators.

Second, to make efficient use of the wind created by traffic, hundreds of wind generators would have to be installed on each mile of expressway. Since wind generators are expensive, it's much more cost effective to put them on windy ridges out in the country or by the seashore.

Third, the wind generators you propose would actually extract energy from the cars and trucks and reduce their gas mileages! That fact might surprise you, since it would seem that extracting energy from the wind wouldn't have any effect on the cars and trucks that created that wind. But the wind and the vehicles continue to interact as they move along the expressway—each vehicle drags a pocket of air with it and interfering with this air pocket has the effect of interfering with the vehicle! The vehicle uses energy to maintain this moving air pocket and it burns additional fuel. An aerodynamically well-designed vehicle has a relatively small air pocket, but there is a limit to what can be done. To reduce the energy cost of maintaining the air pocket, the vehicle's driver can steer it into the air pocket behind another vehicle so that the two vehicles share a single air pocket. The lead vehicle then provides most of the energy needed to keep the air pocket moving. This technique of sharing an air pocket is called "drafting" and is frequently used by bicycle racers. But while drafting makes it easier for many vehicles to keep their air pockets moving, the wind generators that you propose would make it harder—they would steal energy from the air pockets of every passing vehicle and make those vehicles fight harder to keep their air pockets moving.

A better way to save energy would be to encourage large-scale drafting in some safe way. Having chains of independent cars tailgate one another would be energy efficient, but would cause horrific accidents. However, assembling those cars into a tightly coupled "train" may someday become possible with advances in technology and computer controls.