Unless the road was built in a tunnel in the first place, there are structural issues as well. You can't just put some concrete structure over the top of a road and have it support multiple stories of building on top of it. Plus, streets work better as open-air outdoor spaces, especially for pedestrians.

I-290 in Chicago runs through the Old Chicago Main Post Office: http://cdn.abclocal.go.com/content/wls/images/cms/124812_128...

There are already buildings built over railways (as well as many railways tunnelled under things). But they're much safer (hardly any accidents, trained people who know evacuation routes etc) and the noise and smell isn't as bad as it would be for a road.

The examples I think I've seen are hard to verify on Google Maps, it's a brown-grey mush, and in any case they're all odd buildings rather than a 'tunnel' created out of buildings.