1. A computer-implemented method for visualizing GPS data on a map, comprising: receiving GPS samples from a plurality of GPS devices, each GPS sample comprising a location of a GPS device from which the GPS sample is received; determining travel speeds of the plurality of GPS devices from the GPS samples; filtering the GPS samples based on the travel speeds of the GPS devices; and generating for display on the map, a plurality of representations for the filtered samples, each representation visually indicating the location of a GPS device that provided a filtered sample.