Turbo Roundabouts_Design Principles and Safety

Turbo Roundabouts_Design Principles and Safety

L.G.H. Fortuijn
1
Turbo Roundabouts: Design Principles and Safety Performance
Lambertus G.H. Fortuijn, MSc.
Transport & Planning Department
Faculty of Civil Engineering and Geosciences
Delft University of Technology
Stevinweg 1
2628 CN Delft
The Netherlands
/
Province of South Holland
P.O. Box 90602,
2509 LP The Hague
The Netherlands.
Cellphone: +31 65 3232 698
Fax: +31 15 27 83179
E-mail:
[email protected]
[email protected]
[email protected]
TRB Paper #09-2476
Keywords: Turbo roundabout; Two-lane roundabout; Lane marking; Design; Safety
L.G.H. Fortuijn
2
Abstract
While concentric two-lane roundabouts have a higher capacity than single-lane roundabouts, they have
the disadvantage of a higher driving speed through the roundabout. They also reintroduce the
possibility of lane changing on the roundabout, hence raising the risk of accidents. The author
developed the ‘turbo roundabout’ in 1996 in an attempt to deal with these drawbacks. This type of
roundabout has the following features:
- no lane changing on the roundabout;
- no need to yield to traffic on more than two lanes;
- low driving speed through the roundabout by the presence of raised lane dividers.
The paper deals with the concept of the turbo roundabout and the details determining success.
Studies have shown that the risk of accidents associated with injury is greatly reduced on turbo
roundabouts: an 80% reduction has been measured. In view of the known safety trends on single-lane
roundabouts, it is recommended that a slightly lower reduction in the accident rate (70%) should be
assumed in the long term.
Another great advantage of the turbo roundabout is the fact that the traffic flow can be divided over
the lanes of the roundabout in a much more balanced way.
Since 2000, 70 turbo roundabouts have been built in the Netherlands. The Dutch Information and
Technology Platform CROW published its guidelines on Turbo roundabouts in April 2008 (CROW,
2008).
L.G.H. Fortuijn
1
3
INTRODUCTION
The advantages of the single-lane roundabouts are well known:
• because of the rule ‘yield to traffic on roundabout’ gridlock has became a thing of the past;
• the low driving speed is one of the contributory factors of the safety of the single-lane
roundabout (Fortuijn, 2003), contrary to the old traffic circle;
• Relatively high capacity due to the low speeds.
The combination of all these factors has made the modern single-lane roundabout a success, even in
the Netherlands where there are a lot of cyclists.
When the traffic volume is very high, however, additional lanes have to be added. There are two
known ways of expanding the modern single-lane roundabout:
a) add bypass lanes to the roundabout (in four-leg roundabouts only for right-turn movements);
b) turn the roundabout into a two-lane version.
Solution a) is adopted mainly in three-leg roundabouts. In the case of a four-leg roundabout, it is
impossible with bypass lanes to provide two lanes for through traffic movements. Solution b) then
comes into consideration. But concentric two-lane roundabouts introduce safety problems, and lead to
poor use of the inner lane. This paper deals how the turbo roundabout answers these problems.
2
CONCENTRIC TWO-LANE ROUNDABOUT - THE PROBLEM
One type of safety problem concentric two-lane roundabouts introduce may be illustrated by the
following passage taken from a report of the Queensland Department of Main Roads (QDMR) in
Australia. "International studies have shown that increasing the vehicle path curvature decreases the
relative speed between entering and circulating vehicles and thus usually results in decreases in the
entering-circulating and exiting-circulating vehicle crash rates. However, at multilane roundabouts,
increasing vehicle path curvature creates greater side friction between adjacent traffic streams and can
result in more vehicles cutting across lanes and higher potential for sideswipe collisions" (QDRM,
1999). This statement is also cited in "Roundabouts: An Informational Guide", subsection 6.2.1.2
(FHWA, 2000).
It follows that the curvature of vehicle paths on concentric two-lane roundabouts leads to a dilemma.
Sometimes this dilemma is smoothed over, as in the ASSHTO standard for the geometric design of
highways and streets (AASHTO, 2001). This guideline states that "appropriate vehicular speeds
through the roundabout" are "the most critical design objective" (p. 580). Nevertheless it stipulates
tangential alignment flaring entries and exits on two-lane roundabouts if capacity demands this
(Exhibit 9-17, p.572). Also the publication "Roundabouts: An Informational Guide" (FHWA, 2000)
shares the same design principles – all derived from the British roundabout. In general, these design
principles do not include a recommendation for lane marking on the circulating roadway. Brilon &
Bäumer (2004) concluded from their comprehensive analysis of multiple-lane roundabouts in
Germany that lane marking on the roundabout should "be dispensed with in principle" (p. 81) and that
an upper limit of 60 m for the outer diameter of the roundabout is to be preferred.
In the Netherlands, the 1998 CROW publication “Eenheid in rotondes” (A unified approach to
roundabouts) opted for a lower speed, which involved maintaining the traditional Dutch radial access
lanes instead of changing to tangential access lanes as suggested above. This design is equipped with
two-lane entries and one-lane exits, and the circulatory road was provided with lane-marking. But still
it is possible to cut in the path curvature.
L.G.H. Fortuijn
4
Apart from the above-mentioned dilemma associated with the variation in vehicle path curvature, the
concentric two-lane roundabout has another drawback. A driver in the left-hand access lane (assuming
a right-hand rule of the road) has to change lanes over a very short distance on the roundabout in order
to exit. The position becomes even worse if there are two exit lanes: a serious conflict situation then
arises if the driver in the outside roundabout lane wishes to continue along the roundabout, since the
layout of the concentric roundabout means that other drivers have no way of knowing whether he
intends to continue round the roundabout or to exit.
It must be concluded that the concentric two-lane roundabout in fact reintroduces one of the main
drawbacks of the old-style rotary – the need to change lanes in a bend – by the back door. Fortunately,
the modified right-of-way arrangement allows the modern concentric two-lane roundabout to be made
smaller than before (external diameter < 75 m). As a result, driving speeds will generally be lower.
The concentric two-lane roundabout thus remains better than the old rotary. Nevertheless, it lacks
simple resolution of the conflict situations that is strong point of the modern single-lane roundabout.
A third problem of the concentric two-lane roundabout is also related to the need for lane changing:
the use of the inner circulatory lane is poor, which will have a negative impact on the capacity.
These drawbacks led the author to look in another direction for a solution to the capacity deficit of the
single-lane roundabout.
3
DEVELOPMENT OF THE TURBO ROUNDABOUT
The challenge was to develop a roundabout with a same or a higher capacity than the two-lane
roundabout, but with the same safety features as the single-lane roundabout. The new roundabout
would have to meet the following conditions:
- no lane changing on the roundabout;
- no need to yield to more than two lanes;
- low driving speed through the roundabout.
The answer to this challenge is the Turbo roundabout. The key features are:
a) opposite at least one entry inserts a second lane;
b) traffic approaching the roundabout on at least one leg must yield to traffic in two and no more
than two lanes on the roundabout;
c) smooth flow on roundabout by well applied spiral alignment;
d) mountable raised lane dividers discourage impatient drivers from cutting in.
In addition,
e) each segment of the roundabout includes one lane on which traffic can choose whether to exit
or to continue round the roundabout;
f) at least two exit legs are two-lane.
A further important feature of the turbo roundabout is:
g) the diameter of the roundabout is kept small.
The following characteristics of the Dutch single-lane roundabout are retained:
h) approach legs are at right angles to the roundabout;
i) roundabout shields cut off view of horizon
j) mountable aprons offer sufficient width for long vehicles to use the roundabout.
L.G.H. Fortuijn
5
These features are illustrated in FIGURE 1, and each one is discussed separately in section 5 below.
A number of different types of roundabouts can be constructed on the basis of the principles indicated
above (see section 5 and Figure 4). The author initially called one of these types the ‘turbo
roundabout’. The idea of the ‘turbo roundabout’ caught on to such an extent in the Dutch media that
the name was applied to all modern roundabouts following the above-mentioned design criteria. The
variant initially called simply ‘Turbo roundabout’ is now designated the ‘Basic Turbo roundabout’ to
distinguish it from all the others. Types that lack features e) and f) are called a partial turbo
roundabout, as mentioned in CROW publication No. 257 (CROW, 2008).
FIGURE 1
Features of a Turbo roundabout
The first turbo roundabout was built in 2000, and at the time of writing there are 70 turbo roundabouts
in the Netherlands.
4
THE MARKS OF THE TURBO ROUNDABOUT
The turbo roundabout answers three problems of the concentric two-lane roundabout.
o First a reduction in the number of conflicts, including complete elimination of weaving and
cut-in conflicts. FIGURE 2 shows the differences in types of conflict involved in the turbo
roundabout in comparison with the concentric two-lane roundabout.
o
The turbo roundabout offers also a solution for the other safety problem: the dilemma between
sideswipe collisions and collision speed. Because weaving is unnecessary, it is possible to
apply raised lane dividers, making it impossible to cut in to reduce path curvature without
accepting a high level of discomfort. The effect on the pass trough speed is illustrated in
Figure 6.
o
As far as the capacity is concerned, the spiral lane marking, combined with raised lane
dividers offers a positive effect: the disadvantages of using the inner circulatory lane are
removed. Various aspects of the capacity of the turbo roundabout are discussed in a separate
paper entitled ‘Estimation of the capacity of turbo roundabouts’. The main conclusion is that a
L.G.H. Fortuijn
6
turbo roundabout allows the traffic flow to be distributed over the different lanes, which
makes for a high capacity.
Concentric two-lane
roundabout with
two double-lane exits
16 Conflict-points
12 on entry
2 weaving
2 cut-in
Turbo roundabout
10 Conflict-points
on entry
FIGURE 2 Differences in conflict types between two-lane and turbo roundabouts
The following sections clarify how these principles are translated into design aspects.
5
DIFFERENT VARIANTS OF THE TURBO ROUNDABOUT
Different variants of the turbo roundabout are obtained by varying the number of lanes on the access
and exit legs. There is no point in having two lanes if the traffic volume is low. In fact, from the
perspective of cyclist safety, it is then preferable to make the minor street legs single-lane. This was
the first form of turbo roundabout to be developed. It has been given the name ‘Egg roundabout’. This
variant has the unsystematic feature that the number of lanes in the side legs differs from that on the
roundabout itself. The Egg roundabout is nevertheless classified as a main turbo roundabout type,
since it is widely used.
Apart from the Egg roundabout, all the different main variants systematically meet the requirement
that the number of lanes in the legs corresponds with that on the roundabout. The following main
forms may be distinguished:
Three or four legs:
- Egg roundabout (in essence a reduced form of the basic turbo roundabout)
- Basic turbo roundabout
- Spiral roundabout
- Knee roundabout
- Rotor roundabout (less suitable for three-leg junctions)
Three legs only:
- Stretched-knee roundabout
- Star roundabout
The various forms are illustrated in FIGURE 3, with by approximation the capacity and a schema of the
appropriate flow patron.
L.G.H. Fortuijn
7
Variant forms of turbo roundabout
Four-leg
Three-leg
Three-leg Egg roundabout
Egg roundabout
Basic turbo roundabout
Three-leg Basic turbo roundabout
Three-leg Spiral roundabout
Spiral roundabout
L.G.H. Fortuijn
8
Four-leg
Three-leg
Three-leg Knee roundabout
Three-leg Knee roundabout
Knee roundabout
Three-leg Stretched Knee roundabout
Rotor-Roundabout
Three-leg Rotor roundabout
(not functional)
L.G.H. Fortuijn
9
Four-leg
Three-leg
Four-leg Star roundabout does not
exist, because it is impossible to create
that type which fulfills the requirement
not to give priority to more than two
lanes.
Star Roundabout
(three-leg )
(It is possible to enlarge the scale factor for viewing the figure without losing quality.)
FIGURE 3
Variant forms of turbo roundabout
It goes without saying that combinations of forms sharing the same basic geometry or ‘turbo block’
(the significance of this design element is explained in section 6.3) are also possible. The Egg
roundabout, Basic turbo roundabout and Spiral roundabout fall into this category.
Intermediate variants can further be made by reducing the number of lanes in an access leg from two
or three to one, as occurs in the Egg roundabout. Apart from partial solutions, combined forms or
intermediate variants, the above list contains all possible variants meeting the above-mentioned
requirements
6
CHARACTERISTIC FEATURES OF TURBO ROUNDABOUTS
In this section the significance of the several features or basic characteristics of the turbo roundabout
will be discussed.
6.1
Pre-emption of traffic flows
The pre-emption of traffic flows is an important aspect of the turbo roundabout. It is connected with
the features a, b and c. The first part of feature b (at least one leg must yield to traffic in two lanes on
the roundabout) seems self-evident, since without it the roundabout in question is not a turbo
roundabout at all (as defined in CROW, 2008). If all two-lane entries meet only one roundabout lane,
it will be an alternative form of a bypass roundabout. In the past, this type of roundabout was
sometimes called a ‘sail roundabout’ (wiekrotonde in Dutch) (Fortuijn & Harte, 1997).
6.2
Limited number of circulatory lanes
Having to yield to traffic in more than two lanes makes life difficult for the driver. Erné (1994)
suggests in a study of old rotaries that this requirement has an adverse effect on safety. Moreover,
expanding the roundabout from two lanes to three has much less capacity benefit than expansion from
one to two lanes, since it requires there to be a gap in three lanes at the same time before a driver can
get to the inside lane on the roundabout. This is the background of the second part of key feature b.
L.G.H. Fortuijn
6.3
10
Smooth flow on roundabout by well applied spiral alignment
The use of spiral road markings is a characteristic feature of the geometrical design of the turbo
roundabout (key feature c). The difference between the number of conflict points on a concentric twolane roundabout and a turbo roundabout is illustrated in FIGURE 2, for the case where both types of
roundabouts have the same number of exit legs. It will be seen that the concentric two-lane roundabout
has 16 conflict points, and the turbo roundabout 10.
The spiral alignment offers benefits as regards not only safety but also driving comfort. As shown in
FIGURE 4, the turbo roundabout requires fewer steering movements than a concentric roundabout with
spiral road markings.
The design of the turbo roundabout involves the creation of a logical vehicle track by shifting the
centers of the circles representing successive segments of the track along a translation axis. The
summation of these circular segments generates a spiral path. A driver negotiating the roundabout then
gradually moves away from the center of the roundabout as required for exiting simply by following
the spiral track defined in this way.
The combination of the translation axis and the circles forms a design element known as the ‘turbo
block’ in AutoCAD. The position of the translation axis depends on several factors, but in the first
instance on the position of the double-lane exits. It should be chosen in such a way as to ensure that
the vehicle curvature for all through movements should be roughly the same.
After this global positioning, driving comfort demands more detailed positioning of the translation
axis. It would however go beyond the scope of the present paper to give further details of the design
techniques, related to safety, comfort, accessibility and capacity requirements (CROW, 2008).
L indicates points where the driver has to steer left, and R points where he or she has to steer right.
FIGURE 4: Difference in steering movements between a concentric roundabout with spiral road
markings (left) and a turbo roundabout (right).
The way in which the centers of the circles are displaced is related to the extent to which the lanes
shift outwards on moving around the roundabout. The following general remarks concerning the
design technique may be made in this connection:
- lanes in the knee roundabout shift by one lane width over an angle of 3600;
- in the egg roundabout, the basic turbo roundabout and the spiral roundabout, lanes shift by one
lane width over an angle of 1800;
- in the star roundabout, lanes shift by one lane width over an angle of 1200;
- in the rotor roundabout, lanes shift by one lane width over an angle of 900.
L.G.H. Fortuijn
11
This aspect is not only of practical importance. Following from these considerations, the list of
roundabout variants meeting the requirements set, as given in section 5, is complete, when the number
of legs is limited to four.
6.4
Division of lanes
Speed (km/h)
The safety of a roundabout is largely determined by the speed at which vehicles pass through it. In
concentric two-lane roundabouts, drivers are strongly tempted to cut in the bends at times when traffic
is low. The raised lane dividers (key feature d) in turbo roundabouts play an important role in curbing
this behavior. The relationship between speed through the roundabout and its internal diameter is
plotted in FIGURE 5 for three types of roundabouts. The graphs are based on a centripetal acceleration
remaining within acceptable limits and assuming that drivers neither speed up nor brake on the
roundabout. Dutch lane-width standards, depending on the radius of the roundabout, are also followed:
a dual lane on a conventional two-lane roundabout is narrower than two lanes on a turbo roundabout,
since it is assumed that big trucks do not ride two abreast on a conventional two-lane roundabout.
90
80
70
60
50
40
30
20
10
0
0
10
20
Single-lane roundabout
Double-lane roundabout
Turbo roundabout
FIGURE 5
30
40
50
Rinner (m)
Relationship between Pass through Speed and Type of Roundabout (Single-lane, Doublelane and Turbo roundabout); Width of splitter island = 7 m
The curves of FIGURE 5 apply to roundabouts with splitter islands 7 m wide in the minor roads.
Comparison of the curve for the turbo roundabout with that for the conventional two-lane roundabout
shows the great effect of the lane dividers on the speed of passage through the roundabout. It may also
be clearly seen that the optimum speed of passage is reached at a much smaller internal diameter on a
turbo roundabout (feature g) than on a concentric two-lane roundabout.
The design of the mountable lane dividers is a critical success factor. They should be formed so that
vehicles can drive over them without damage, while at the same time producing enough discomfort to
discourage this behavior in most cases. Various types of lane dividers were tested in 1999 by getting
various makes of passenger cars to drive over them.
It was found that all drivers regarded a divider 30 cm wide and 7 cm high, where the total difference in
height was realized. in two steps over a total distance of 10 cm, as the most effective
The shape of the divider can be slightly modified to permit the use of snow ploughs (also lengthwise).
See FIGURE 6.
L.G.H. Fortuijn
12
Default design
Modified to permit use for snow plough
FIGURE 6 Cross-sectional view of raised mountable lane divider
Thanks to the raised lane divider, the optimum value of the inner radius of the turbo roundabout lies
between 12 m (with a splitter island 3 m wide) and 15 m (with a splitter island 7 m wide). See Table 1,
page 13.
6.5
Robust pre-selection of entry lanes with dedicated exits
Requirement e) played an important role in the development of the concept of the turbo roundabout,
since the idea was to design a roundabout that not only had a higher capacity than a single-lane
roundabout but was also robust enough to handle appreciable variations in the loading pattern. This
made it necessary to include a lane where drivers had the option of exiting or continuing round the
roundabout, in each segment.
The inclusion of two-lane exit legs (feature f) is required to give the roundabout the desired capacity.
Conceptually this requirement is connected with offering choice of direction in each segment. On the
other hand, it is desirable to limit the number of lanes in the exit legs as much as possible, in order to
facilitate the crossing of slow traffic. When weighing up these conflicting considerations, it may be
decided that the number of two-lane exit legs should be kept to a minimum. It follows that
characteristics e) and f) listed in section 3 are not regarded as essential features of the turbo
roundabout. CROW publication No. 257 (CROW, 2008) calls roundabouts that lack these
requirements a partial turbo roundabout.
L.G.H. Fortuijn
13
Feature
Radius and measurements
(m)
Inner radius
R1
12
15
Outer radius inner lane
R2
17.15
20.00
Inner radius outer lane
R3
17.45
20.30
Outer radius outer lane
R4
22.45
25.20
Width start inner lane
5.30
5.15
Width end inner lane
5.00
4.90
Width outer lane
5.00
4.90
Difference between lane width and width between lane marking:
0.65
Width lane divider
0.30
Distance between center points for translation inner lane
5.35
5.15
Distance between center points for translation outer lane
5.05
4.95
Largest inscribed diameter
49.95
55.35
Smallest inscribed diameter
45.18
50.64
Entry radius and exit radius
10.00
Radius entering lane divider
12.00
Radius turning off lane divider
15.00
Width mountable central apron (for vehicles over 22 meter)
Width mountable apron in armpit
5.00
1.50 – 3.00m
Width mountable apron intended for vehicles over 22 meter
Path through speed personal car [km/h]
Table 1
6.6
5.00
37-39
38-39
Radii and other dimensions of basic turbo roundabout
Radial connection of entry lanes
In the Netherlands, it has been decided that approach legs should be at right angles to roundabouts
(feature h). The psychological background of this principle is often misunderstood. The underlying
assumption here is that it is completely clear to drivers that they are approaching an obstacle that can
only be negotiated at low speeds. So this principle should not be adopted without understanding the
reason for it, since this could lead to a real risk of more accidents instead of fewer. This means that a
traffic sign placed on the central island of the roundabout should block the view of the horizon in the
direction of travel (see FIGURE 7, feature i). Meanwhile the obstacle must be collision-friendly and the
central island of the roundabout must not be provided with a raised edge. The combination of these
three points is necessary for a sustainable safe solution. Study of the literature shows that the
importance of this point is not always appreciated by researchers from outside the Netherlands (see
e.g. Spahn, 2007, Fig. 5). If these precautions are not taken, the condition for safe use of roundabouts
with approach legs at right angles to the circulatory roadway outside built-up areas is not satisfied.
L.G.H. Fortuijn
horizon
FIGURE 7
6.7
14
horizon
Roundabout shield
Rideability by long vehicle
A number of dilemmas have to be resolved in the design of a roundabout. One important dilemma concerns
the width of the roundabout lanes. Safety requirements call for narrow lanes since they force drivers to
reduce speed, whereas trucks need plenty of room if they are to be able to negotiate the roundabout. How
can these conflicting requirements be reconciled? The solution is to include the following elements in the
roundabout design:
- a 90-degree angle between approach leg and circulatory roadway, for the reasons discussed in the
previous subsection (safety requirement);
- limited width of circulatory roadway (safety requirement);
- central apron offering additional room for trucks using the inner lane (accessibility requirement);
- aprons in the armpits between entrance and roundabout and exit and roundabout, which also offer
additional room for trucks using the outer lane (accessibility requirement).
The constructional detailing of the apron is as important as its geometric design. The version currently
recommended for use in the Netherlands is that developed by the present author after previous guideline
suggestions had turned out to be unsatisfactory. The apron around the central island should not be too
steep: the crown should be less than that of the carriageway to reduce the lateral acceleration-forces at
heavy trucks. But it should moreover be separated from the carriageway by a rumble strip rising by 7 cm
over a lateral dimension of 10 cm. This will prevent cars from using the apron, while not presenting a major
obstacle to trucks. See (CROW, 2008).
Factors h) and j) are consistent with the findings and design recommendations ing. K. J.B Erné developed
in the 1980’s to solve the dilemma of safety and accessibility and admitted in all Dutch roundabout
guidelines between 1989 and 2008 (CROW, 2008).
7
SAFETY EFFECTS
When investigating the effects of a particular measure, it must be borne in mind that some of the
differences observed may not be due to the measure under investigation but to a number of other
factors. In the present case, the following confounding factors were taken into consideration:
- the effect of priority setting;
- general safety trends;
- statistical averaging effects (Hauer,1997).
These three factors will now be discussed in turn.
L.G.H. Fortuijn
15
a) Priority selection. During the ‘90s, the provincial authorities of the Dutch province of Zuid-Holland
operated a system for prioritizing the replacement of traffic intersections by roundabouts whereby the
intersections with a higher level of accidents came into consideration for replacement first. Analysis of
the accident data for this period confirmed the correctness of this policy. Nine intersections where a
yield control was in force were replaced by roundabouts in the period from 1991 to 1994, while 29
similar intersections were replaced in the period from 1995 to 2005. In the former group, the average
accident rate in the period preceding replacement was 1.81 accidents involving injury per annum,
while the corresponding figure for the latter group was 0.98. After replacement, the first group had an
average accident rate of 0.15 accidents with injury per annum, and the second group 0.26. This
represents a decrease of 91.8% and 73.2% respectively. The effect of the priority selection is
unmistakable (Fortuijn, 2005).
The seven turbo roundabouts in this study were all built in the period 2000-2002. It was not possible to
analyze the accident data for them in the same way. But these results of the single-lane roundabouts
have to be kept in mind when interpreting results for turbo roundabouts.
b) General safety trends. The accident rates in the period three years before the construction of each
roundabout were compared with the rates in the period three years after the construction. The
correction for the general safety trend derived from the analysis of these results was found to be slight
in all cases, amounting to +4.4% for the single-lane roundabouts built in the period between 1995 and
2002 and -9.1% for the turbo roundabouts built in the period 2000-2002. That will lead to correction
factors of 0.958 and 1.10 to use as multiply factors for the reduction factor (=100% – reduction
percentage).
c) Statistical averaging effects. Accident black spots are always identified with reference to the
measured accident rates in the previous three years. It takes some time after an intersection has been
identified as a spot with an elevated accident risk before it is actually replaced by a roundabout. It may
be concluded on the basis of analysis of accident data series that a roundabout is often built three years
after a peak is observed in the accident incidence curve. When intersections with yield control were
converted into a roundabout, it was observed that the accident rate was on average subject to a 20%
random rise. The measured accident rate must also be corrected to take this factor into account. The
corresponding correction when an intersection with traffic lights is converted into a roundabout (not
considered further in this paper) is appreciably lower (about 10%), because in this case the
implementation priority is largely determined by the age of the traffic-lights installation.
Application of factors b) and c) leads to the following corrections to the measured reduction in
accident rates:
- For the 39 intersections with yield control converted to single-lane roundabouts in the period
1991-2002, the average measured number of accidents leading to injury fell from 1.24 to 0.23 per
annum, a reduction of 81.7%. The correction factor for points b) and c) together is 0.963, which
means that the corrected reduction in the accident rate is 78.7 % instead of 81.7 %.
- For the 29 intersections with yield control converted to single-lane roundabouts in the period
1995-2002, the average measured number of accidents leading to injury fell from 0.98 to 0.26 per
annum. The correction factor for points b) and c) together is 0.93, which means that the corrected
reduction in the accident rate is 68.1 % instead of 73.2 %.
- For the 7 intersections (including intersections with yield control , intersections with traffic lights
and an old-style rotary) that were converted into turbo roundabouts in the period 2000-2002, the
average measured number of accidents leading to injury fell from 2.429 to 0.438 per annum. The
L.G.H. Fortuijn
16
correction factor for points b) and c) together is 1.32, which means that the corrected reduction in
the accident rate is 76.1 % instead of 82 %.
It will be clear from the above that the turbo roundabout is a safe solution. There is however still some
uncertainty about the precise reduction in the accident rate to be taken into account, since this depends
among other things on the number of accidents with injury in the period before replacement. In order
to eliminate this factor, traffic safety researchers usually relate the number of accidents with injury to
the ‘traffic performance’ of the intersection or roundabout, i.e. the number of accidents with injury
divided by the number of vehicles passing (in millions). Two problems arise in this connection,
however:
- the ratio of traffic volumes along the major and minor legs has a great effect on the values of the
traffic performance found;
- the variation in the amount of slow traffic (bicycles and mopeds) at the intersection cannot be
neglected in the Netherlands.
The author of the present paper drew attention to the first point, using data kindly provided by
ir. S.T.M.C. (Theo) Janssen from the Dutch national road safety research institute (Dutch abbreviation
SWOV) (Fortuijn, 1995). The effect of volume ratios can be clearly illustrated by dividing
intersections with yield control into four categories defined by different values of the constant c in the
equation:
c=
SLO
( I1 + I 2 ).365.106
where:
SLO= number of victims
I1 = traffic intensity on main road
[veh/day]
I2 = traffic intensity on minor road
[veh/day]
This gives the following values of c (= number of victims per million passages):
c = 0.081
Group a:
0 < I2/I1 ≤ 0.2
Group b:
0.2< I2/I1 ≤ 0.4
c = 0.126
Group c:
0.4 < I2/I1 ≤ 0.75
c = 0.272
Group d:
0.75< I2/I1 ≤ 1
c = 0.297
It will be clear that the distribution of traffic between the main and minor road is an important
explanatory factor here. Therefore, cross-sectional studies only yield a useful result when types of
intersections with roughly the same distribution of traffic are compared. Otherwise a rather
complicated approach, reckoning with different traffic patrons, is needed. For that reason before-after
studies are more likely to yield stable, reliable results to investigate the effects of different measures.
Only it is necessary to keep in mind possible differences in point of departure.
It follows from the above analysis that the measured effect of turbo roundabouts on safety is
comparable with that of single-lane roundabouts. It may however be expected that the construction of
turbo roundabouts at intersections where fewer accidents occurred in the past will not have so much
effect. Analysis of data series for single-lane roundabouts compared with turbo roundabouts (where
the mean percentages reduction is considered for groups where the upper limit of the number of
accidents with injury qualifying for replacement is lowered in steps) leads to the provisional
conclusion that turbo roundabouts may be expected to lead to a 70% reduction in accident rates if the
L.G.H. Fortuijn
17
mean number of accidents with injury per annum at the intersection in question before conversion was
greater than 1.
8
CONCLUSIONS
The drawbacks of the two-lane roundabouts have been a challenge to develop a new type of multi-lane
roundabout without:
- need for lane changing;
- high pass-through speeds.
The concept of the turbo roundabout meets these requirements. In the Netherlands many turbo
roundabouts have been constructed.
The safety effects come close to that of single-lane roundabouts. On seven spots a reduction of 82% of
casualties is measured. Corrected for other influence factors, the reduction was 72%.
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