Some factors affecting merging traffic on the outer ramps of highway interchanges.

The primary purpose of this research project was to examine vehicle operating characteristics at the outer merging ramp of grade-separated interchanges and to examine factors relating to merging capacity. Data from two interchanges on Highway 401 in the Toronto area have been examined and analyzed and various relationships have been developed. Considerable variability in gap acceptance by the merging driver has been shown to occur. However, it has been possible to establish limiting curves which indicate acceptable and non-acceptable gaps as a function of speed difference between the merging and through vehicle. The principle of critical lags as proposed by Raff has been examined and it appeared that when the merging vehicle travels at a speed equal to or less than the through vehicle the major merging conflict was with the trailing through vehicle. Analysis of the critical lag related to the trailing through vehicle produced a linear relationship between critical lag and the speed difference between the through vehicle and the merging vehicle. A theory of merging which permits computation of merging capacity has been proposed. This theory is based on the probability of a merging vehicle finding a suitable gap in the through stream and is dependent upon the critical lag concept and and the available merging time. In the development of the theory certain rigid assumption were necessary which could be questioned. However, the merging capacity estimates obtained appeared to be reasonable within the range of the data available. Typical merging capacity curves have been plotted to show the effect of entry speed, merging speed, through speed, acceleration lane length and driving lane length and driving lane volume. Although it is recognized that the theory upon which the merging capacity has been estimated needs refinement there is definite evidence that merging capacity is indeed complex and a knowledge of driving lane volume and 'going-away' volume will not provide an answer to merging capacity. The information contained in this report provides but one possible approach to the solution of the merging capacity problem. It is hoped that others will pursue this matter further in order to develop a more through understanding of the merging manoeuvre and hence provide a basis for a more scientific design of merging areas.