Double-lane Roundabouts Research Articles

Comparing Performances of Turbo-roundabouts and Double-lane Roundabouts

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.

Crash Patterns and Potential Engineering Countermeasures at Maryland Roundabouts

Objectives: Each year in the United States more than 1.2 million injuries and 9,000 deaths occur in motor vehicle crashes at intersections. Previous research has found that construction of roundabouts in place of traditional intersections can decrease crash frequency and severity. Despite these safety benefits, some crashes still occur at roundabouts. The present study systematically reviewed police crash reports for a set of roundabouts in Maryland to develop a typology of crashes and identify potential countermeasures. Methods: A total of 283 crash reports were reviewed including 149 crashes at 29 single-lane roundabouts and 134 crashes at 9 double-lane roundabouts. Based on the police reports, crash types were developed and examined by type of roundabout (single-lane, double-lane), crash location within the roundabout (entrance, circular roadway, exit), and other variables. Field observations were conducted at 8 roundabouts with above-average crash histories to aid in identifying potential countermeasures. Results: About three quarters of the crashes involved only property damage. Of the injury crashes, 14% involved at least one disabling injury; the remaining crashes resulted in probable injuries (36%) or nonincapacitating injuries (49%). One common crash pattern at both single- and double-lane roundabouts involved vehicles colliding with the central island, which accounted for almost half of all single-vehicle run-off-road crashes. Other major crash types included rear-end and sideswipe collisions. About three quarters of all collisions occurred at entrances to roundabouts. Based on review of crash reports and visits to several roundabouts, high approach speeds were an important driver crash factor, and some drivers may not have seen the roundabout in time. Conclusions: Increasing the conspicuity of upcoming roundabouts through larger “roundabout ahead” and “yield” signs could reduce speeds by alerting drivers ahead of time, especially at night. Enhanced landscaping of central islands as well as reflective pavement markers and yield signs at the entrance to roundabouts also could help drivers recognize roundabouts and the need to yield to circulating traffic. Certain design features (e.g., entry deflection on approach roads) also may aid in reducing speeds.

Using Simulation to Plan Capacity Models by Lane for Two- and Three-Lane Roundabouts

Recently, capacity models have been developed for single- and double-lane roundabouts by using U.S. data from NCHRP Report 572. The model for double-lane capacity was provided primarily for critical lane (usually right-lane) capacity as a function of total circulating volume. None of the existing international capacity models separate circulating volumes by lane. This study proposes planning capacity models for two-lane and three-lane roundabouts by separate entry-lane and separate circulatory-lane traffic volumes. VISSIM microsimulation software was used first to compare with the new NCHRP models as well as with Australian (SIDRA) and German (Tanner-Wu) models. Given that predictions from VISSIM for overall capacity of one- and two-lane roundabouts are consistent with U.S. data from NCHRP Report 572, new planning capacity models were developed by using VISSIM for the left lane and right lane of double-lane roundabouts that are functions of separate circulatory-lane traffic volume. For three-lane roundabouts, planning capacity models were developed by separate entry lane (left, middle, and right lane) as functions of inner, middle, and outer circulatory-lane volumes. All variable coefficients are statistically significant, and model fits are strong.

Traffic Safety of Older Drivers in Various Types of Road Intersections

In industrialized countries people over 50 years of age represent a rapidly growing part of population. Their lifestyle is also becoming more active, which means that the percentage of older drivers in the population of all drivers is also increasing. Many different studies have shown that elderly drivers are more frequently involved in specific types of accidents, especially at intersections. In the past 15 years there was a trend of increasing popularity of roundabouts in Slovenia. Their introduction was generally supported by the arguments of increased traffic-flow capacity and traffic safety as well. The studies on which these arguments are based were performed with an “ideal” type of driver in mind; the one that fully understands new rules and reacts correctly in all situations that may occur at such intersections, where there are no light signals to guide them. An elderly person does not necessarily conform to that ideal and if the percentage of elderly drivers became significant, the premises of the above mentioned studies may not be correct anymore which in turn implies that their results could also be questioned. The present study concentrated on the evaluation of traffic safety of elderly drivers, at various types of intersection, from their own perspective. Various statistical analyses of obtained data were performed. The most important finding was that we may claim, with high degree of probability, that the average person of the age of over 60 feels more unsafe at double-lane roundabouts than they would feel had the same intersection been equipped with traffic lights. Elderly traffic participants will always cause more accidents or participate in them due to hazardous factors. Challenge, arising from many different studies and researches, is in studying what measures and solutions can reduce the risk for elderly participants. KEYWORDS: traffic safety, road intersections, roundabouts, elderly people, older drivers