Traffic Control in Oversaturated Street Networks

The scope and magnitude of the oversaturation problem was defined and the way it may best be combatted with existing methods was determined. The paper reports the beginning of a research process leading to improved operation and controls for oversaturated networks. Six specific objectives of the research are listed. The project was planned in three phases. Phase I was concerned with the definition of the problem and determination of its scope and magnitude. Phase II covered the examination and evaluation of existing traffic control techniques. The third phase dealt with the description of advanced methods of traffic control and with the formulation of a detailed plan and program for test and application of the recommended procedures. The first phase was accomplished by a questionnaire surveys of cities, counties and states in the U.S. and Canada. A figure is presented which illustrates the variation of saturation reported by city size for CBD signalized intersections and for all intersections on the arterial system. The que length characteristics at congested intersections is illustrated by a figure which gives an indication of the range of oversaturated intersection experiences by city size. Information on the means used for timing signals is tabulated. Measures of effectiveness to characterize particular aspects of traffic performance are categorized, and a set of que-related measures was selected as being the most promising for study. In the second phase of the study, a literature review and survey response, identified four causes of congestion. The most effective measures for alleviating the problem are listed. It was found that operational measures leading to increased link storage were more effective in delaying or eliminating oversaturation than were measures aimed at increasing intersection capacity. Some particular findings with respect to signal techniques are presented. In the third phase, 4 relevant advanced control concepts were selected from a review of the literature and each was subjected to an extensive simulation evaluation using three different test networks. The research conducted as part of this study has demonstrated that many existing operations and control techniques can be applied to the reduction or elimination of oversaturation.