BACKGROUND

Freeway congestion usually occurs at freeway merge, diverge, and weaving segments that have the potential to develop bottlenecks. To alleviate or mitigate the impacts of congestion at these segments, a number of active management operational strategies have been implemented such as ramp metering, hard shoulder running, managed lanes, etc. The current freeway merge and diverge methodologies in Chapter 14 of the 6th edition of the Highway Capacity Manual (HCM) were developed over 25 years ago using limited field collected data. Although weaving segment analysis was updated more recently, the relationship with the merge and diverge methodologies have not been clearly addressed. In addition to limited data, the methodology does not conform to the fundamental relationship of traffic flow, namely that flow is the product of speed and density. The HCM does not offer any methodology for lane drops or additions, which often occur in the vicinity of freeway merge/diverge segments. In the past decade, the data available to traffic engineers have expanded exponentially with ubiquitous sensor coverage of urban freeways and probe vehicle coverage of entire roadway networks.  These new datasets provide a wealth of information to support the development of updates or changes to the merge, diverge, and weaving segment methodologies, and potentially complement traditional data sources.

OBJECTIVES

The objectives of this research were to (1) develop methodologies to update the HCM related to merge, diverge, and weaving methodologies; and (2) pilot the developed methodologies to demonstrate the full range of applicability of the proposed updates to the HCM.  

STATUS: Published as NCHRP Research Report 1038. 

The research report can be downloaded here