I recall seeing this idea on RealNEO a while back. Check the following links for means by which we could potentially utilize our existing highway infrastructure to generate electricity.
Harvesting energy from any body of mass in motion:
http://www.kinergypower.com [1]
The new 'jersey barrier': exploring the urban highway's potential for wind power: http://www.metropolismag.com/cda/story.php?artid=2466 [2]
Links:
[1] http://www.kinergypower.com
[2] http://www.metropolismag.com/cda/story.php?artid=2466