California Partners for Advanced Transportation Technology

This report documents work accomplished during Fiscal Year 2005-2005 as a part of a research project entitled “New Approach to Bottleneck Capacity.” This project is developing an alternative to the traditional Highway Capacity Manual approach to capacity analysis in which capacity flow [either pre-queue flow (PQF) or queue discharge flow (QDF)] is related to a set of intervening variables, including the average time gaps in the critical lane (i. e., that with the highest flow rate) and the distribution of flow across the lanes, represented by the critical lane flow ratio (i. e., the flow in the critical lane divided by the average flow per lane). These intervening variables, in turn, are to be related to the geometric characteristics of bottleneck sites, their vehicle populations, and their driver populations. Work to date has included the collection and analysis of data, analysis of traffic data to document flow characteristics at individual study sites, and an analysis of the relationships among the various traffic flow characteristics, including relationships among the intervening variables and between the intervening variables and capacity flows. Major findings to date are that (a) there are significant differences in the mean values of the flow characteristics during different episodes of PQF and QDF at individual sites;(b) means of flow characteristics are significantly different among the sites (with the exception of critical lane average time gaps in PQF); (c) flow variances also differ significantly among the sites;(d) QDF appears to vary by time of dayat some sites; (e) critical lane average time gaps and critical lane flow ratios are not correlated with one another in either PQF or QDF; (f) there is a significant negative correlation between the time gaps and the flow per lane; and (g) there is a very strong negative correlation between flow in the critical lane and critical lane average time gaps; when plotted, this relationship is virtually linear. On the basis of these findings, models relating flow per lane (for PQF and QDF) to critical lane flow ratios and critical lane average time gaps are proposed for use in the next stage of the research, which will focus on relating the flow ratios and time gaps to the geometric, vehicle-population, and driver population characteristics of the study sites.