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Driver gap acceptance behavior is a function of geometric and operational factors. An essential measure of gap acceptance is the critical gap. At signalized intersections with permissive left turn movements, the critical gap is the foundation to estimate left turn capacity. Several studies have identified geometric, temporal, spatial, and operational effects on the critical gap estimates. However, recommended critical gap values have remained unchanged for decades. The Highway Capacity Manual recommended critical gap for left turns at signalized intersections is 4.5 seconds and the follow-up headway is 2.5 seconds. Most conventional signal optimization software (PASSER, SHYCHRO, VISSIM, SIDRA, HCS) use these outdated critical gap and follow-up headway estimates without accounting for specific traffic conditions such as large vehicles affecting gap acceptance. As part of this study, observational data and gap acceptance estimates with the presence of large vehicles will be integrated with microsimulation and signal optimization software to evaluate the effect of large vehicles in operational performance of left turning vehicles at signalized intersections.