No abstract

ABSTRACT

Alternative types of roundabouts are usually more recent and have been implemented only in certain countries. Typically, they differ from "standard" roundabouts in one or more design elements, whilst the purposes of their implementations are also specific. The main reasons for their implementations are particular the disadvantages of "standard" one-or two-lane roundabouts in particular circumstances or changes of "actual circumstances" which in the past has led to roundabout implementations. Today, several different types of roundabouts are well-known ("mini", "double mini", "dumbbell", "with transition central island", "with joint splitter islands", "traffic signal controlled", "assembled roundabouts" …), and it is possible to stipulate that they will be further developed in the future. This paper illustrates two relative new alternative types of roundabouts-"turbo" and "flower" roundabouts and their comparison from design, traffic safety and capacity point of view.

The paper presents a microsimulation-based approach for roundabout safety performance evaluation. Based on a sample of Slovenian roundabouts, the vehicle trajectories exported from AIMSUN and VISSIM were used to estimate traffic conflicts using the Surrogate Safety Assessment Model (SSAM). AIMSUN and VISSIM were calibrated for single-lane, double-lane and turbo roundabouts using the corresponding empirical capacity function which included critical and follow-up headways estimated through meta-analysis. Based on calibration of the microsimulation models, a crash prediction model from simulated peak hour conflicts for a sample of Slovenian roundabouts was developed. A generalized linear model framework was used to estimate the prediction model based on field collected crash data for 26 existing roundabouts across the country. Peak hour traffic distribution was simulated with AIMSUN, and peak hour conflicts were then estimated with the SSAM applying the filters identified by calibrating AIMSUN and VISSIM. The crash prediction model was based on the assumption that the crashes per year are a function of peak hour conflicts, the ratio of peak hour traffic volume to average daily traffic volume and the roundabout outer diameter. Goodness-of-fit criteria highlighted how well the model fitted the set of observations also better than the SSAM predictive model. The results highlighted that the safety assessment of any road unit may rely on surrogate safety measures, but it strongly depends on microscopic traffic simulation model used.

Over the recent decades, roundabouts have been increasingly used when building new at-grade intersections and up-grade junctions all over the world, and also when rebuilding existing intersections. Modern roundabouts exist in all European countries and mainly worldwide, so we can say that today modern roundabouts are a world phenomenon. No uniform guidelines exist in Europe for the geometric designing of roundabouts as specific circumstances (local customs, habits, traffic cultures...) differ from country to country. Certain solutions that are safe in one country could be dangerous in another. Consequently, most countries have their own guidelines for the geometric designs of roundabouts that are, as far as possible, adapted to their circumstances in these countries and are therefore more acceptable within their surroundings. Today, after many years of experience, there are different ideas about the "ideal roundabout", with some consensus on the crucial effect of rules how to negotiate intersections. There are several different types of roundabouts worldwide today, called the alternative types of roundabouts. Some of them are already in frequent use all over the world (hamburger, dumb-bell …), and some of them are recent and have only been implemented within certain countries (turbo roundabout, dog-bone…) or are still at the development phases (turbo-square, flower, target, with segregated left-turn slip lanes…). Both groups typically differ from the "standard" one-or two-lane roundabouts in one or more design elements, as their purposes for implementation are also specific. Therefore, it was decided that it would be useful to collate, in one article, some of the alternative types that are already in frequent use today in some countries and some of them that are "still coming".