1. A computer system for monitoring traffic congestion, comprising: a non-transitory memory device for storing computer-readable program code; and a processor in communication with the memory device, the processor being operative with the computer-readable program code to perform operations including receiving vehicle data from vehicle data sources located in a region of interest, determining a sample size and an average speed for an edge of the region of interest based on the vehicle data, wherein inverse of a variance of the average speed is linearly related to the sample size within a range of sample sizes, determining congestion probability based on the sample size and the average speed, and presenting a report based on the congestion probability.