D.Sc. Tomaž TOLLAZZI, Full Professor

PLANNING AND DESIGNING OF THEPLANNING AND DESIGNING OF THE TURBO ROUNDABOUTS

INVITED LECTURES

Faculty of Civil Engineering

What is turbo roundabout?

is a canalized multilane oval intersection with a non traversable oris a canalized multilane oval intersection with a non-traversable or partially traversable centre island and with a spiral circulatory carriageway, where into three or more roads enter, with multiple centres of outer and inner diameters and traffic lanes (spiral course of the

The turbo roundabout is an innovative arrangement of the two lane

of outer and inner diameters and traffic lanes (spiral course of the carriageway)

groundabout that has revolutionised roundabout design in The Netherlands. The turbo roundabout is a specific kind of spiralling roundabout that was developed by Mr. L.G.H. Fortuijn. Turbo roundabout is a special type of the two lane roundabout, where some direction flows are separated or run on physically separated lanes.

In the turbo roundabout the traffic flows run separately even before the entry into the roundabout, they occupy separate lanes all the wayentry into the roundabout, they occupy separate lanes all the way throughout the roundabout, whereas traffic flows run separately also at the exit from the roundabout. Physical separation of traffic lanes is interrupted only in places of entryPhysical separation of traffic lanes is interrupted only in places of entry into the inner roundabout traffic lane.

Physical separation is achieved by specially shaped elements, which hinder (but not prevent) the change of traffic lanes in the roundabout.hinder (but not prevent) the change of traffic lanes in the roundabout.

The turbo roundabout has a bigger capacity, compared to the “normal” two-lane roundabout. There are several reasons for that:two lane roundabout. There are several reasons for that:

• at the roundabout entry of this type, there are usually two traffic lanes, which directly continue into two circulatory traffic lanes,• the use of the inner circulatory traffic lane becomes more attractive, since there is no need for weaving,• the entry traffic flow is no longer hesitant, when entering the circulatory y g , g ycarriageway, which increases the capacity of entries.

The turbo roundabout has a higher level of traffic safety, compared to the “normal” two-lane roundabout.normal two lane roundabout. There are several reasons for that. The most important is lower number of conflict points!

MergingDivergingMergingDivergingDivergingCrossingDivergingCrossing

DRIVING RULERS IN THE TURBO ROUNDABOUT

Phisical separation of traffic flows requires the driver to decide on the direction of exit from the roundabout or to select an appropriate traffic lane already at the entry into the roundabout or even a little before the entry. When the driver, before entering the turbo roundabout, decides on which exit he shall leave the roundabout, he can no longer change his decision. , g gNamely, the driver must choose an appropriate traffic lane even before the entry into the roundabout, because the turbo roundabout does not permit changes of traffic lanes in the circulatory carriageway!p g y g y

Possible directions of driving within the roundabout and possible exits from the individual lane of the circulatory carriageway are indicated infrom the individual lane of the circulatory carriageway are indicated in signs at the portals.

Drivers should be assisted by clear signposting and lane marking - a i l f f ki h b d l d f t bspecial form of arrow marking has been developed for turbo

roundabouts, which makes for clearer lane selection!

CONDITIONS AND LOCATIONS OF APPROPRIATE USETh t b d b t i i t l ti l ti t id thThe turbo roundabout is an appropriate solution on locations outside the urban areas, when there is normally one main and one side traffic route, regarding the intensity of the traffic flows.

The turbo roundabout in the urban area is just a conditionally appropriate solution. If a roundabout with two entry and two exit lanes is considered, which would be located in the urban area, we must first solve the

The turbo roundabout is a conditionally appropriate solution in the case

which would be located in the urban area, we must first solve the problem of traffic-safe leading of non-motorised traffic participants.

The turbo roundabout is a conditionally appropriate solution in the case of:- existing traffic-overloaded one-lane roundabouts, the size of which ( t di ) bl th i l t ti f dditi l i l t(outer radius) enables the implementation of an additional circulatory lane inwards (better solution) or there is space for the implementation of another circulatory lane outwards (somewhat less appealing and more expensive solution)expensive solution),- existing traffic-overloaded two-lane roundabouts,- existing traffic less safe two-lane roundabouts,- reconstruction of the classic intersection with a predominant main traffic direction, with a very strong traffic flow.

GEOMETRIC DESIGN ELEMENTS OF THE TURBO ROUNDABOUTSThe geometrical form of the turbo roundabout is formed by the so-calledThe geometrical form of the turbo roundabout is formed by the so called’’turbo block’’.

This is a block or a formation of all the necessary radii, which must be rotated in a certain way, thereby obtaining traffic lanes or driving lines.

The size of radii of the standard roundabout with a spiral course of the circulatory carriageway and the width of the circulatory traffic lane mustcirculatory carriageway and the width of the circulatory traffic lane must be selected in a way that the speed of driving through the roundabout does not exceed or equals 40 km/h!

POSITION OF THE TRANSLATORY AXLE AND CONTROL OF THE TURBO BLOCK POSITIONThe turbo block contains (besides all radii) also the so-called translatoryaxle. A translatory axle is an axle, whereon the movement of certain radii is carried out.is carried out.

The best position of the translatoryaxle is as if the clock hands pointed the time:the time:

“five minutes to five o’clock” in the case of four - armor “ten minutes past eight o’clock” in the case of three- armthe case of three- arm

turbo roundabout!

From this step, the procedure is nearly the same like in ’’normal’’ roundabouts!

At the end – a short comparation of prices (expences):- this comparation is stupid because every case is unique

depends also of pipelines below at grade- depends also of pipelines below, at grade- depends also of the dimensions of the working areas- ...

But, however: 2008‐2009  2010 ‐ 2012

''Standard'' double  ‐ lane 

the duration ofconstruction

4‐5 months 4‐5 months

indicative price 800.000 ‐ 1.500.000,00 600.000 ‐ 1.100.000,00Turbo  the duration of 5 months 5 – 6 monthsroundabout  construction

indicative price 800.000 ‐ 1.650.000,00  500.000 ‐ 1.200.000,00  

So, we can conclude that, in global, turbo is a little bit more expensive solution than ‘‘standard‘‘ double- lane roundabouts (10 – 15 %)!

On this moment as I know there are just few countries with turboOn this moment, as I know, there are just few countries with turbo roundabouts:

The Netherlands about 80- The Netherlands – about 80 - Slovenia – 11- Germany – 1- Hungary – 1- Belgium – 1

Indiana (USA) 2- Indiana (USA) – 2

Germany: Baden Württemberg (without kerbs, just painted – bad experiences)

Hungary: city Szolnok; from 2009; too much divided kerbs – bad experiences!

Indiana (USA): city Carmel – very good experiences in both turbo roundabouts!!!roundabouts!!!