Gap-accepteance parameters for roundabouts: a systematic review

Abstract

Purpose 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. Methods 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. Results 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. Conclusions 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.

Exploring the uncertainty in capacity estimation at roundabouts

“…The results of the analysis were expressed probabilistically, meaning that the probability distributions of the capacity at each entry lane rather than the simple point estimates of the performance measure were obtained. At last, the comparison was also done between the capacity estimations based on the mean values of behavioral parameters -as derived from a systematic review performed in a previous study by the Authors [31] -and capacity functions based on the probability distributions of the model parameters.…”

“…Bearing in mind that the synthesis of empirical estimates of the behavioral parameters at roundabouts should be based on published researches, in a previous work the Authors undertook an extensive review of the benchmark studies currently available in the worldwide literature to collect the mean values of the critical headway and the follow-up headway at roundabouts and address the variation of each parameter across studies [31]. Thus, we focused on a meta-analysis of effect sizes, that is the analysis where each (primary) study yields an estimate of statistical mean values of the critical headway and the follow-up headway (hereinafter the effect sizes), assessed the dispersion in these effects and then computed a summary effect [32].…”

“…The reader is referred to Giuffrè et al [31] both for further details on the set of rules used for doing the literature search and determining which studies should be included in or excluded from the analysis, the total data set extracted from the available relevant literature on the topic, the methods applied to obtain the values of the critical and follow up headways considered in the calculations, and for a more effective presentation of the results of the meta-analysis of effect sizes that was performed as part of the literature review through the random-effects model. It is noteworthy that some important studies were excluded from our investigation, since experiences in capacity estimation at roundabouts carried out in countries as Great Britain and France were based on a capacity formula which is not a gap acceptance-based model [31].…”

“…It is noteworthy that some important studies were excluded from our investigation, since experiences in capacity estimation at roundabouts carried out in countries as Great Britain and France were based on a capacity formula which is not a gap acceptance-based model [31]. It should be also noted that the research did not attempt to answer the question of how drivers change their acceptance characteristics and adapt themselves to accept shorter or large headways based on traffic levels of the circulating volume.…”

“…Since each effect size varied from a study to another study, the meta-analysis of effect sizes was carried out to combine the data used in all selected studies through the random-effects model. In order to produce a more precise estimation of the mean value of the distribution of effect sizes (namely the summary effect) both the original variance within-study and the variance between-studies were considered [31,32]. Thus, we computed a weighted mean value, assuming that the weight for each study was the inverse of the study's variance; the last one is the sum of the two components of the variance as above introduced.…”