Abstract
The critical and follow-up headways are the two gap-acceptance parameters which explain the traffic interaction of a minor street vehicle when enters the roundabout, merging into or crossing one or more circulating (major) streams. Several studies and researches provide measurements of critical and follow-up headways from real data at roundabouts. The objective of our research is to synthesize the data from the series of selected studies to interpret variation across the studies. In order to match the research goal, a systematic literature review on estimations of critical and follow-up headways at roundabouts was undertaken. Since several studies and researches developed worldwide were examined, we were able to note that the effect size varied from study to study. Thus the meta-analysis of effect sizes was performed as part of the literature review through the random-effects model. After discussing the assumptions of this model, the dispersion in effects across studies was assessed and the summary effect for each of the parameters under examination was computed. Calculations were made both for single-lane roundabouts and double-lane roundabouts, as well as for turbo roundabouts. Compared to the results of individual studies, the single (quantitative) meta-analytic estimate provides an accurate and reliable synthesis on the specific issue here addressed, and gives, with greater power of the individual reviewed studies, a comprehensive measure for the parameters of interest.
Highlights
1.1 The backgroundIn recent years, roundabouts have gradually gained great popularity worldwide as they represent a type of intersection control without traffic signals which, by making use of a circular geometric layout, establishes a self-regulated intersection control system
The quantification of the critical and follow-up headways can vary based on the number of lanes, the diameter of the central island, the entry width, the presence of bunched vehicles in the circulating streams, whereas at multi-lane roundabouts, the different demand scenarios can produce dominant and subdominant arrival flows at entries which in their turn can influence the predictions of the gap acceptance parameters [24, 25]
The minimum and maximum values of the critical headway were estimated both for single-lane roundabouts, and for double-lane roundabouts, as well as for turbo roundabouts; the same tables show the values of the follow-up headway
Summary
In order to match the research goal, a systematic literature review on estimations of critical and follow-up headways at roundabouts was undertaken. Since several studies and researches developed worldwide were examined, we were able to note that the effect size varied from study to study. The meta-analysis of effect sizes was performed as part of the literature review through the random-effects model
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