Abstract-Road traffic congestion is a major issue in most mega cities. The traffic jams are often exacerbated by drivers habitually following the same routes. A real-time, optimum vehicle routing system that takes traffic density into account can be a possible solution to this problem. This paper presents RouteFinder, a system of providing real-time traffic density mapping on the driver's smartphone using an in-vehicle module built using inexpensive components that communicates with the existing mobile telephone network, to enable the driver to choose the least congested route to a desired destination. The hardware module contains the essential elements of a cellular handset such as a SIM card. The location of the vehicle is determined through a standard triangulation algorithm performed using signals from the three nearest cellular base stations, and the location information is constantly updated at relevant Home Location Registers (HLRs) / Visitor Location Register (VLRs) at the telecom service provider using an intermediate MySQL database. An Android application developed for the driver's smartphone shows the present locations of all vehicles in all routes from the origin to the selected destination, with colour codes distinguishing between moving and stationary vehicles. We have implemented our device in 10 vehicles in Dhaka city. Our sample calculations have shown significant savings not only in terms of time, but also in fuel consumption.