This paper presents the finding of a computational sensitivity study on the effects of traffic on the turbulent characteristics of air flow and the subsequent effects of contaminant transport within an urban street canyon. The FAST3D-CT CFD model is used to examine these characteristics in a prototypical street canyon modeled after Pennsylvania Avenue in Washington D.C. Comparisons are made with high frequency experimental data that was recorded during the D.C. Winter 2000 Campaign performed by the Defense Threat Reduction Agency. Findings show that in addition to street orientation, traffic can significantly effect the turbulent characteristics of these flows. Both qualitative and quantitative comparisons are provided.