Proposed Framework for Evaluating Spillback in the Hcm

1 The procedures detailed in the Highway Capacity Manual (HCM 2010) estimate capacity and 2 several operational measures dictating level of service for freeway facilities and surface streets. 3 However, these methods do not consider cases in which spillback occurs from one facility type 4 to another. The queueing effects in oversaturated conditions as they propagate upstream – onto a 5 freeway mainline or a surface street intersection – are not accounted for. 6 The objective of this paper is to propose a series of modifications to enhance the HCM 7 2010 methodologies in order to address spillback conditions. These modifications consider lane 8 utilization and lane blockage under spillbck conditions, and consist of restructuring existing 9 equations and reference tables as well as development of new procedures. A four-regime 10 methodology is proposed for evaluating spillback effects from urban streets to diverge and 11 weaving segments. Also, a methodology is proposed to account for the spillback effects from 12 freeway on-ramps, by reducing the effective green time as a proportion of the percent of time 13 that the queue is expected to block the upstream signalized intersecton. The framework 14 developed here uses assumptions that should be further explored through an extensive, 15 nationwide data collection effort. 16 17