Hi adiatmad, Thats a big question! It really depends on the router and wether you are routing for a vehicle, cycle or foot. A lot of vehicle routers will route along a road no matter what. So the quality of the software is a main concern.

The lack of a layer tag on a bridge or tunnel. Hedges, walls or buildings across highways. Misuse of the oneway tag. Incorrect turn restrictions. Just a few problems there. 

Regards Bernard.