Influence of rainfall on roundabout capacity
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Date
2015
Authors
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Publisher
Universiti Teknologi Malaysia
Abstract
Roundabouts are often installed on urban and Federal roadways in Malaysia. Majority of them are multilane often 2 or 3 carriageway lanes. Roundabouts have some limitations as well as clear advantages. Unlike other intersections roundabouts have no priority traffic flow regime. The basic rule is that entry flows give way to circulating flows. In Malaysia, the capacity of roundabouts is traditionally based on weaving theory and a number of previous studies have measured practical and design capacity. Rainfall has consequences on traffic operations at roundabout which is resulted in greater stopping sight distance, reducing speed and also affecting capacity. However, the extent of which the impact of rainfall has on roundabout has yet to be determined. Rainfall can be light, moderate or heavy. In this study, the influence of rainfall on roundabout entry flow/capacity ratio (RFC) and their implication for gap acceptance was investigated. Two-lane roundabouts at four sites in the Malaysia were investigated under dry and rainy weather conditions. Two automatic traffic counters per roundabout arm as well as two rain gauge stations were used to collect data at each surveyed site for nine weeks between November 2013 and January 2014. Passenger car equivalent values were modified in order to take account of prevailing condition. Regression technique was used to analyze roundabout entry flow/capacity ratio under dry and rainy weather conditions. Gap acceptance technique was used to investigate the implication of rainfall on entry flow/capacity differentials. Peak traffic data were used to model the entry capacities and the ensuing models used to analyze observed off-peak traffic data and determine off peak entry capacities. Observed entry flows were compared with entry capacities and the ensuing ratio used to describe the state of traffic stream under dry, light, moderate and heavy rainfall conditions. Empirical data from observed entry capacity at 2-lane roundabouts have been used to test both stochastic and empirical entry capacity models. In general, the stochastic capacity models seem to have certain limitations when it comes to describing the observed data. Empirical results show that the entry flow/capacity ratio is reduced by up to 10% under heavy rainfall conditions. Reduction in entry flow/capacity ratio induced increase in average gaps. Average gap under dry weather light, moderate and heavy rain conditions are between 3.85s, 4.15 s, 5.78 s and 6.37s respectively. The study concluded that rainfall has significant impact on entry flow/capacity ratio with implication for critical gap at the circulating traffic flow.
Description
Thesis (PhD. (Civil Engineering))
Keywords
Traffic circles, Traffic flow, Traffic engineering—Research