A Rationale for Enhancing the German Highway Capacity Manual to Incorporate Oversaturated Freeway Facility Analysis

Unlike the U.S. 2010 Highway Capacity Manual, the most recent edition of German Highway Capacity Manual does not include methodology to evaluate the facility within oversaturated time periods. To incorporate this traffic flow regime within the methodology, we identify several aspects of the methodology that require enhancements and propose modifications. Next, we implement these modifications in a macroscopic traffic model developed as a software implementation of the German guideline. In this manner, the HBS user can conduct traffic flow analysis that includes volume-to-capacity ratios above 1.00 which has not been foreseen by the guideline until now. The main contribution of this paper is the identification of methods within the German Highway Capacity Manual that require modification to enable incorporation of oversaturated traffic flow within the analysis. Further, we introduce traffic density as a variable within the HBS methodology and show benefits of its incorporation during the oversaturated time periods. Finally, the paper includes illustrative examples of empirical and model-based evaluation of congested freeway facility in Germany. The paper concludes with a discussion of proposed methods and their limitations.