Empirical evidence on the impact of improving road networks and transportation systems on migration in developing countries remains scarce and generally provides mixed results. This paper fills part of this empirical gap in the Tanzanian context by answering the following question: do better roads promote migration or do they discourage it? A difference-in-difference estimation is implemented using the Kagera Health and Development Surveys (KHDS), coupled with information on trunk roads upgraded to bitumen standards in the region. On average, migration decreases as upgraded roads improve local living conditions. The impact proves to be greater in absolute value for well-connected communities, as well as for the probability of moving within the Kagera region. However, the effect decreases over time.
That is from a paper (Journal of African Economies, July 2013) by Marie Castaing Gachassin. I did not find the full paper online. By the same author, but from a different paper, I took a table of road density in African countries (other tables and graphs in the paper are also interesting).