Room for Improvement: A Critique of the HCM Freeway Analysis Procedure

With an update of the Highway Capacity Manual (HCM) scheduled for 2015, now is good time to revisit the freeway analysis procedures. In this paper, three areas for improvement are identified, some of which will require further research. Recently released clarifications and corrections have provided additional guidance on selecting the appropriate analysis procedure for a given freeway segment. While the manual discusses the overlap of a merge segment followed by a diverge segment, the overlap of influence areas for other combinations are not covered. An approach for handling adjacent segments separated by less than 1,500 feet is suggested based on the configuration and analysis results. Capacity checkpoints are used in the merge and diverge segment procedures to determine if the ramp roadway, upstream freeway, or downstream freeway volumes exceed certain capacity thresholds. These checkpoints should be added to the weaving procedure since the same capacity constraints may occur. In fact, the capacity checkpoints use general threshold values, so a better approach would be to substitute the basic segment procedure for the capacity checkpoints. Despite the 2010 update, the weaving segment procedure remains insensitive to the split in weaving volume between the freeway-to-off-ramp and on-ramp-to-freeway volumes. A suggested approach would be to use the merge or diverge segment procedure when the volumes are particularly imbalanced. Additionally, more guidance is needed for complex weaving areas that may not be adequately capture the capacity effects of lane changing if split into separate basic, merge, and diverge segments.