The proposal would involve mounting horizontal wind turbines above the roadway that would be driven by the moving air generated by the passing traffic. The electricity generated by spinning these turbines could be fed back into the grid. Joe's analysis indicates that based on vehicle speeds of 70 mph each turbine could produce 9,600 kWh per year.
The wind turbines shown in the proposal are of a quiet running type. Certainly in many built up areas there is enough constant traffic volume to maintain a steady airflow through much of the day. The big question that needs to be answered is whether the nature of the turbulent airflow could keep the turbines turning. If a turbine could be optimized to work in that environment it seems like it might be a very worthwhile investment.
It's certainly deserving of further investigation.