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Turbo Roundabouts

posted by Florin Nicolae
September 05, 2010

A turbo roundabout is a multi lane roundabout with spiral road markings and separated lanes, at which road users have to choose the correct lane before entering the roundabout, in order to leave it in the desired direction.
Turbo roundabout was developed in The Netherlands by Mr. L.G.H. Fortuijn, a lecturer at the Delft University of Technology and member of the Board of Economy and Transport in the province of South Holland. Since 2000, over 70 turbo roundabouts have been built in the Netherlands were multi lane roundabouts are no longer built, and existing multi lane
roundabouts will be reconstructed into turbo roundabouts. The main reason is the disappointing performances of multi lane roundabouts on both capacity and road safety.

The main characteristics of a turbo roundabout are:

1. a turbo roundabout has more than one lane;
2. the correct lane has to be chosen before entering the turbo roundabout;
3. entering traffic has to give way to circulating traffic, which is limited to a maximum of two lanes;
4. within the roundabout itself no weaving or cutting is possible;
5. the roundabout can only be left via the previous chosen lane.

Advantages:

• a surveyable situation when a driver enters the roundabout: drivers need only to give way to traffic in a maximum of two well-demarcated lanes;
• the correct lane has to be chosen before entering the turbo roundabout;
• no risk of accidents due to lane changing on the roundabout;
• low driving speed through the roundabout because of raised lane dividers.

The main reasons to choose a turbo roundabout rather than other intersection types are:

• Increase capacity of the intersection. The capacity of a turbo roundabout is higher than a single lane roundabout (1½ to 2½ times as high) or a two lane roundabout (1 to 1½ time as high).
• The capacity of a turbo roundabout is equal or higher than a signalized intersection. The delays are less than at a signalized intersection.
• Increase road safety on the intersection. A turbo roundabout is safer than a give way intersection (± − 70% in fatal accidents or accidents with hospital treated injuries) and safer than an intersection with traffic signals (about − 50% reduction in fatalities and hospital-treated injuries), although not as safe as a single lane roundabout (turbo roundabout: 20% to 40% greater accident rate).
• The spatial need (m2) of a turbo roundabout is about the same as a signalized intersection (assuming that the signalized intersection would also allow two trucks driving in parallel, in all directions).
• The construction costs of a turbo roundabout are higher than an intersection with traffic signals, but the life cycle costs and social costs are less.

DIFFERENT VARIANTS OF THE TURBO ROUNDABOUT

Five types of four-leg turbo roundabouts can be distinguished, based on differing number of entry and exit lanes and bypasses. The need for these variations has mainly to do with differences in the distribution of traffic volume over the legs of the intersection:

Basic turbo roundabout

Egg roundabout

Knee roundabout

Spiral roundabout

Rotor roundabout

For three-leg roundabouts, there are only two types of turbo roundabouts:

Stretched-knee roundabout

Star roundabout

Essential design features

A number of design features can be called ‘essential’ in the sense that without these elements the intersection is not a turbo roundabout. These essential features of the turbo roundabout are:

1. opposite at least one entry a second lane in inserted on the central island side;
2. at least two entry legs (one leg at a three-leg roundabout) give way to traffic on two but no more than two lanes;
3. spiral marking fluently guide traffic from inside to outside, avoiding weaving and cutting conflicts on the roundabout;
4. mountable-raised lane dividers causes optimal vehicle curvature by keeping vehicles in their lane and by using a small diameter;
5. at least two exit legs are two lane;
6. on every section there is a decision point at which traffic can choose to exit or continue on the roundabout.

REFERENCES:

Fortuijn, L. G. H., Turbo Roundabouts: Design Principles and Safety Performance. Transportation Research Record: Journal of the Transportation Research Board, No. 2096, Transportation Research Board of the National Academies, Washington , D.C. , 2009.

Roundabouts - Application and design; A practical manual. Ministry of Transport, Public Works and Water management, Partners for Roads, June 2009.