I have found some information about voided slab beams on different state websites. Most of the designs call for an 2-5″ asphalt overlay.  I’m using a 4′ wide by 18″ deep 50′ voided slab, with no overlay, for the county bridge.

Voided slabs speed the construction of a bridge but as one engineer told me, they will never gain acceptance on “real” roads. The longitudinal joints can break down and the lack of cover on the top surface can be problems.