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 non-transitory memory device, the processor being operative with the computer-readable program code to perform operations including: receiving, by a congestion monitor of the computer system, vehicle data within a time slot from vehicle data sources located in a region of interest, each vehicle data source being located in a vehicle in the region of interest; determining, by the congestion monitor, a sample size and an average speed for an edge of the region of interest based on the vehicle data, wherein the sample size corresponds to a number of vehicles located in the edge of the region of interest within the time slot and the edge of the region of interest is a predefined segment of a road network of the region of interest and wherein an inverse of a variance of the average speed is linearly related to the sample size when the sample size is less than a predetermined value, determining, by the congestion monitor, a congestion probability based on the sample size and the average speed and by: determining the congestion probability comprises determining the variance of the average speed, determining a slope of a graph of the inverse of the variance against the sample size, and modeling a distribution of true speed using a normal probability model, wherein a mean of the normal probability model is the average speed and a variance of the normal probability model is based on the slope, and presenting a report in a graphical user interface based on the congestion probability of the edge of the region of interest.