A system and method for determining the flow of traffic in a monitored route segment using global positioning system, or assisted global positioning system, enabled cellular phones as traffic probes takes advantage of the existing cellular infrastructure to deploy traffic probes to determine the flow of traffic based on the speed and direction of probes. Filtering of probe data can be used to remove probes having outlying results such as excessive speed, or probes that do not fit the general trend already observed on a traffic route segment.