Globally consistent spatial road data sets are indeed scarce. Many of the available data sets has data with highly variable consistency.
A new project hosted at CIESIN is due to release a new globally consistent road data set named gRoads.
For road density mapping, this will provide a good alternative. However, the data is not meant to cover street data, but roads between settlements.
The project is due to release in early 2010, which is hopefully about now.
More information here.