This thesis reviews the concept of critical gap as a parameter within the stochastic modeling of a single lane roundabout and its adjustment to represent real world local conditions. Existing traffic conditions of a single lane four-legged roundabout in O’Fallon, Illinois, was modeled using VISSIM microsimulation software with traffic turn movement volumes. Field observations were made to obtain a critical gap measurement of the roundabout and reflect this within the modeling and compare the model to the observed traffic flow conditions. The VISSIM microsimulation software modeling results were found to be consistent with field observations for both modeling methods of conflict areas and priority rules. The default critical gap parameter used for single lane roundabouts is based on a limited site inventory from roundabouts within the United States of America. The outcome of this study determined a critical gap estimate of 4.2 seconds, specific to Southern Illinois. In addition, this study confirmed that VISSIM microsimulation software, with minimal calibration, can realistically estimate performance for a single lane roundabout in Southern Illinois.