Assessment of Basic Freeway Segments in the German Highway Capacity Manual HBS 2015 and beyond

Abstract

The second edition of the German Highway Capacity Manual HBS was published in 2015. The paper presents the HBS procedure for the assessment of basic freeway segments and discusses challenges for the future development of the methodology. As in the former edition of the HBS, the volume-to-capacity ratio is used as measure of effectiveness for basic freeway segments. The design capacities and speed-flow diagrams for basic freeway segments were completely revised and supplemented by new design values for segments with four-lane carriageways and segments with hard shoulder running. Besides the provision of specific design capacities, the reduced capacity variance on segments with variable speed limits, which was ascertained in recent investigations, is considered by adjusting the lower threshold value of LOS E. The new edition of the HBS also provides a framework for the use of specific parameters and the application of alternative methods for the assessment of traffic flow quality. Overall, the revised procedure for the analysis of basic freeway segments includes major enhancements and covers a larger number of segment types, but the main concept still follows the tradition of using deterministic capacities and providing rather simple analytical procedures to assess the traffic flow in one specific peak hour. For the future development of the HBS, the application of computer-based simulation models as well as the use of stochastic traffic flow parameters that better represent traffic reliability will increasingly emerge.