During the last three decades, roundabouts have gained increased popular and political acceptance and are now used worldwide. Roundabouts are known by their high performance levels in safety, traffic capacity and in atmospheric emissions, especially when compared with other at-grade intersections.
This research was focused on using appropriate modeling methodologies to understand the effects of turbo-roundabouts (innovative roundabout where lanes are bounded by traffic signs and raised curbs placed at entering and circulating lanes) on capacity, safety and emissions, in comparison with conventional single-lane and double-lane roundabouts.
AROUND focused on the demand for sound analytic and simulation roundabout models. On site work using an equipped vehicle, video cameras, pneumatic tube detectors and viacounts to measure speed and count vehicles was carreid out.
The work comprised five main parts:
- Collection of geometric, traffic and pedestrian data for a set of sites covering a wide range of geometric and operational characteristics;
- Comparative assessment and improvement of existing analytic capacity models, either regression or gap-acceptance based;
- Calibration of microscopic simulation models, with emphasis on the sub-models directly associated to driver behaviour at roundabouts (car-following and gap-acceptance);
- Association of a pollutant emission model with the microscopic models. The estimation of the pollutant emissions will be supported by the Vehicle Specific Power (VSP) concept, which correlates the vehicle speed profiles with the respective consumptions and emissions;
- Application of the improved capacity and pollutant models to test the efficacy of new solutions, such as the turbo-roundabout, now very popular in Netherlands.
- Compared to the conventional double-lane roundabout, the turbo-roundabout has a reduction of the number of conflict points and the deflexion level control justify the reduction of accidents in 80% as some literature shows.
- The performance in terms of capacity is not consensual. However, in some specific conditions the turbo-roundabout solution can be extremely useful, putting together safety and capacity benefits.
- The applicability of the concept was tested by turning a real roundabout into a turbo-roundabout. The intervention tends to be a positive change resulting in speed reductions and in additional free space.
- Among the benefits introduced by the turbo-roundabout, it can be mentioned that the spiral lane marking, together with raised lane dividers, allows the traffic flow to be distributed over the different lanes. Thus, the turbo-roundabout has a higher capacity, compared to the double-lane roundabout.
- The turbo roundabout answers three problems of the double-lane roundabout, such as:
- there are usually two entering lanes continuing into 2 circulating lanes;
- the use of the inner circulating lane is more attractive, since there is no need for weaving;
- the entering flow is no longer hesitant, which increases the entry capacity.
Turbo-roundabout is an innovative roundabout characterized by the following aspects:
- lanes are bounded by traffic signs and raised curbs placed at entering and circulating lanes;
- spiral lines and raised lane separators on the ring require drivers to choose the lane before entering the intersection;
- spiral lines on the ring guide drivers to the correct exit.