Depends on the roadway (type of road) and the size and type of the sewer system. Not all roads are created equally with differing standards for subgrade, sub-base, and the physical pavement structure as well as different pavement mixes. Likewise, most cities don't really have the type of sewer system that one can "fall into". New York, Chicago, London, Paris... in areas. Nashville, Atlanta and the like... nope. You'll hit some large pipes if you hit the mains; but those are nothing anyone is going to be walking through. Even the storm water systems.
