At a time when mobility in Brussels is becoming increasingly critical from the standpoints of the environment and efficiency, this article gives a detailed and exhaustive analysis of the geography of traffic conditions affecting the trams and buses of Brussels’ main mass transit network. The finely disaggregated data that we were able to obtain from the Brussels Interborough Transport Company (STIB/MIVB) enabled us to calculate and map three indicators (commercial speed, irregularity, and los...