Abstract
Entry capacity loss at multilane roundabouts caused by rainfall was investigated. Traffic and rainfall data were collected continuously for eight weeks at selected sites in South Africa and Malaysia. Three classes of rainfall (light, moderate, and heavy) were used. Both linear and exponential techniques were used to model the relationship between entry and circulating traffic flows. Correction factor for entry angle and radius was introduced in the linear model equations to allow for variations in geometric data. Correction factor is not required for the exponential model technique. Notwithstanding, both techniques predicted that entry capacity loss would result from rainfalls irrespective of their intensity. Results show that light rainfall may cause about 5% entry and circulating capacity loss, moderate rainfall may cause about 12% capacity loss, and heavy rainfall may cause about 17% capacity loss. This paper proposed further studies into the effect of two circulating lanes rather than one on entry capacity at roundabouts.
Highlights
According to the HCM 2010 [1], roundabout entry capacity is “the maximum sustainable flow rate that can be achieved during a specified time period under prevailing road, traffic, and ambient conditions.” us, suggesting that entry capacity is not a fixed number and that rainfall can cause entry capacity loss, the extent of loss has yet to be established
Model entry and circulating traffic flows in Step 1 using both the linear and exponential regressions as shown in Figure 3 and test for statistical fitness. e coefficients of determinant (R2) are above 0.5, which indicate that the model equations are reliable. e T-test result was greater than 2.2 at 95% level of confidence which shows that the parameters used are significant, and the F-test result indicates that the model equations did not occur by chance. e statistical results are satisfactory. e model equations are Exponential model: QE 2551e −0.001Qc, (14)
Linear circulating flow is 1708 pcu/h when entry flow is zero, whereas the exponential curve is discontinuous at 500 pcu/h, suggesting that small entry traffic flows are inconsequential. e exponential equation becomes nearly asymptotic to the x-axis making it unreliable to model small entry flows when the circulating traffic volume is high. e absence of the y-intercept means that the geometric influence of a roundabout is unexplained
Summary
According to the HCM 2010 [1], roundabout entry capacity is “the maximum sustainable flow rate that can be achieved during a specified time period under prevailing road, traffic, and ambient conditions.” us, suggesting that entry capacity is not a fixed number and that rainfall can cause entry capacity loss, the extent of loss has yet to be established. In most countries including South Africa and Malaysia, the “yield-at-entry” rule in which approaching vehicles must wait for a gap in the circulating flow before entering the circle is followed. If there is no traffic flow in the roundabout, drivers may enter cautiously without yielding. E “yield-at-entry” rule holds irrespective of prevailing road, traffic, and ambient conditions except in circumstances where the roundabout is signalized or directions are given by traffic officers. Since circulating traffic flow is not burdened with responsibilities of the yield rules, it begs the question, to what extent is the circulating capacity affected and how will that in turn affect the entry capacity? Rainfall intensity is classified in many studies as light, moderate, heavy, or very heavy. Rainfall intensity greater than 50 mm per hour is classified as very heavy and violent, omitted in the study
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