Abstract
Starting from assumptions regarding the arrival process of circulating streams and according to models based on the gap-acceptance theory, the paper is aimed at comparing operational performances between basic turbo-roundabouts and double-lane roundabouts. The paper proposes applications of the Hagring model for entry capacity estimations at double-lane roundabouts and turbo-roundabouts, these latter, in particular, featured by movements with only one or two conflicting traffic streams. This model allows to use, in fact, a bunched exponential distribution to quantify the distribution of major vehicle headways; it also considers specific values different by each lane for behavioural parameters, minimum headway and conflicting traffic flow on circulating lanes. The results obtained for the two cases examined, although influenced by the underlying assumptions, especially with regard to user behaviour at turbo-roundabouts, can give information about the convenience in choosing, at a design level, a basic turbo-roundabout rather than a double-lane roundabout. The comparison developed in this paper, indeed, can be helpful in selecting the type of roundabout and in particular in evaluating performance benefits that are obtainable from the conversion of an existing double-lane roundabout to a turbo-roundabout with similar footprint of space.
Highlights
The results obtained for the two cases examined, influenced by the underlying assumptions, especially with regard to user behaviour at turbo-roundabouts, can give information about the convenience in choosing, at a design level, a basic turbo-roundabout rather than a double-lane roundabout
It should be noted that, this paper reports a comparison between geometric patterns of intersection different from those considered by the above cited Authors, and different assumptions were made on conflict patterns between entering and circulating vehicles; this concerns, in particular, traffic flows faced by left turning movements from minor roads and models adopted to perfom capacity estimations
Low delays are experienced by users at turbo-roundabouts compared to double-lane roundabouts when high traffic volumes come from major roads and low-to-medium traffic flows come from minor roads
Summary
The results obtained for the two cases examined, influenced by the underlying assumptions, especially with regard to user behaviour at turbo-roundabouts, can give information about the convenience in choosing, at a design level, a basic turbo-roundabout rather than a double-lane roundabout. A starting point for evaluating operational performances at roundabouts and turbo-roundabouts can be represented by capacity methods for two-way-stop-controlled intersections, where vehicles on major streams have priority and vehicles on minor streams are controlled by stop. Regression models do not facilitate the comprehension of the underlying traffic flow theory of determining and accepting gaps upon entering the intersection (Rodegerdts et al, 2007). Estimation procedures for critical headway do not require sites with oversaturated conditions Another behavioural parameter is the follow-up time, defined as the time headway between two consecutively entering vehicles, utilizing the same gap in major or circulating traffic flows at roundabouts; it can be directly surveyed on-field (Rodegerdts et al, 2007). Capacity models homogeneous each other should be used by manoeuvre type, especially where intersections perform multiple turning movements
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