Conferencia "Planning and designing of the planning and designing of the turbo roundabouts" (D.Sc. Tomaž Tollazzi, Universidad de Maribor, Eslovenia)

1. D.Sc. Tomaž TOLLAZZI, Full Professor
PLANNING AND DESIGNING OF THEPLANNING AND DESIGNING OF THE
TURBO ROUNDABOUTS
INVITED LECTURES
Faculty of Civil Engineering

2. 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)
g
roundabout 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.

3. 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.

4. 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.

5. 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 circulatoryy g , g y
carriageway, which increases the capacity of entries.

6. 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!
Merging
Diverging
Merging
DivergingDiverging
Crossing
Diverging
Crossing

7. 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 g
Namely, 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

8. 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!

9. CONDITIONS AND LOCATIONS OF APPROPRIATE USE
Th 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.

10. GEOMETRIC DESIGN ELEMENTS OF THE TURBO ROUNDABOUTS
The 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!

12. POSITION OF THE TRANSLATORY AXLE AND CONTROL OF THE
TURBO BLOCK POSITION
The turbo block contains (besides all radii) also the so-called translatory
axle. A translatory axle is an axle, whereon the movement of certain radii
is carried out.is carried out.
The best position of the translatory
axle is as if the clock hands pointed
the time:the time:
“five minutes to five o’clock” in the
case of four - arm
or
“ten minutes past eight o’clock” in
the case of three- armthe case of three- arm
turbo roundabout!

13. 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 of
construction
4‐5 months 4‐5 months
indicative price 800.000 ‐ 1.500.000,00 600.000 ‐ 1.100.000,00
Turbo the duration of 5 months 5 – 6 months
roundabout 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 %)!

14. 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

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

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

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