This paper describes a path tracking algorithm implemented in an Intelligent Transportation System in order to validate if a vehicle is following properly a certain path previously assigned. The algorithm is based on a geometrical analysis and GPS data specified to trace in real-time the current position of the vehicle according to a previous digital map of the path to be tracked. As a case study, the tracking algorithm has been developed following the regulations of the public transportation of Medellin, important Colombian city with a large number of public buses and a significant complexity in the mobility within the city.