Exploring the uncertainty in capacity estimation at roundabouts

European Transport Research Review

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Table 2 The meta-analytic estimates for follow-up headway at roundabouts [31]

Roundabout	Entry lane	Random estimate (se)	95% LL and UL	Z-value	Q	I ²
Single-lane		3.10 (0.07)	(2.96; 3.25)	41.82	33.90	20.37
Double lane	Left	2.85 (0.10)	(2.66; 3.04)	29.58	22.32	19.36
Double lane	Right	2.72 (0.08)	(2.57; 2.87)	35.74	17.77	9.98

95% LL and UL stand for the 95% lower and upper limits for the summary effect; Z-value corresponding to the confidence limits of 95% tests the null hypothesis that the common true effect is zero; p – value tells us only that the effect may be or is probably not zero; Q stands for the Cochran’s Q test [34], that represents a measure of heterogeneity and is the sum of the squared deviation of each effect size from the mean, weighted by the inverse-variance for each study; I² stand for the Higgin’s index, or the ratio of true heterogeneity to total observed variation [35].