Air quality analysis for highway projects are conducted to determine if projects have the potential to cause exceedances of the National Ambient Air Quality Standards (NAAQS). Prior to construction, detailed air quality modeling analysis is often used to estimate a project's impact on the ambient concentration of atmospheric pollutants. One pollutant of concern is carbon monoxide (CO). A detailed CO analysis is performed using computer models that require extensive input data. Performing this type of modeling is time consuming and increases project expenditures. Because the NAAQS for CO are expressed as maximum concentrations not to be exceeded more than once a year, a screening analysis may be an appropriate tool which can be used to determine if a detailed analysis is necessary. The screening analysis is used to determine if a project may cause a NAAQS violation. A screening model uses readily available data to make a conservative worst-case estimate of a project's impact. Projects that pass worst-case screening analysis would not require a detailed analysis. (The screening procedure must be approved by the appropriate regulatory agencies.) This report summarizes the methods used for establishing a computer CO screening model (Illinois CO Screen for Intersection Modeling, COSIM) that can be used to determine when a detailed CO analysis is needed for highway projects in Illinois, for the Illinois Department of Transportation. This report also evaluates the United States Environmental Protection Agency's model, CAL3QHC, to determine CO levels at three intersections in Illinois.