We are reading Herbst' States and Power in Africa for a grad school class and the author has a chapter that does a poor job of examining the road networks (or lack thereof) in Africa. This 2006 report does a great job of investigating the highway infrastructure (and potential) in Africa. There's a powerpoint and a report at the link below. The next step in this line of thought would be to connect the highway infrastructure development to the needed port infrastructure development.
Time to Build a Trans-Africa Road Network: New Analysis of the Costs and Benefits, and Preliminary Suggestions on How the Donors Could Make it Happen