27

Abstract
Road curve Horizontal alignment design standards in the United

superelevation States and Australia have two basic common features:

firstly, the absence of a single nationwide maximum
superelevation rate and, secondly, designers'

design: current freedom in applying above-minimum values for

curve radii. Taking into account the proven
dependence of operating speed on curve radius,
practices and Australian standards introduce the concept of speed
environment (characterising highway sections as a
whole) to be used alongside the traditional design
proposed speed concept (corresponding to individual curves)
and incorporate consistency checks as a feedback

approach loop in the design process. Neither of these

safeguards is explicitly included in American
guidelines. Still, in both countries a variety of
maximum-superelevation values is used, meaning
that identically-designed curves (having equal radius
and superelevation values) may have resulted from
application of different design speeds. In this paper
G. Kanellaidis a proposal for simplifying the relationship between
radius and superelevation is applied to Australian
guidelines for speed environments ranging between
60 km/h and 120 km/h. Consistent application of
this proposal, for which specification of nationwide
maximum superelevation rates is a precondition,
would result in curve radius serving the driver both
as a guide for selecting speed and as a signal for the
centrifugal acceleration to be expected, thus
enhancing horizontal alignment consistency.

Refereed Paper
This paper has been critically reviewed by at least two
recognised experts in the field.

Originally submitted: December 1998

Vol 8 No 2 June 1999 Road & Transport Research

Road curve superelevation design: current practices and proposed approach

28

INTRODUCTION design speeds. From Equations (1) and (2) it follows

Horizontal alignment design policy for rural roads that the sum of (e + f) is reduced proportionally to the
has traditionally been based on the concept of design increase of R. To achieve that reduction, it is common
speed. However interesting differences are noted in to both apply lower values for e and assume lower
the way that concept is selected and applied in values for f. However there is a limiting (minimum)
various countries. In several national practices, the value for the superelevation rate (emin), equal to the
need has been recognised for design speeds to be rate applied (for the purpose of drainage) in tangent
more directly based upon actual speed behaviour, as sections.
well as for checking alignment design on the basis of
estimated operating speeds (ERSF 1996; Krammes It is noted that, above a threshold radius value,
and Garnham 1995). cross-slope is designed in the same way as for tangent
cross-sections. Typically this is the so-called crown
The basic principle of horizontal curve design is section which, for these very flat curves, provides
derived from application of the kinematics equation, negative ('adverse') superelevation equalling emin
according to which the total lateral acceleration, for vehicles moving on the outside of a curve.
applied through pavement superelevation and tyre—
pavement friction on a vehicle negotiating a circular Diverse procedures are applied internationally
curve, should be equal to the centrifugal acceleration regarding not only the design speed concept, but
(CA) due to vehicle movement: also:
• maximum superelevation rates
CA = _1( = (e +f )g (1) • values of f in relation to the design speed, and
• application of e and f values in curves of above-
where V is vehicle speed (in m/s), R is curve radius minimum radius.
(in m), e is pavement superelevation rate, f is the The following sections illustrate similarities and
tyre—pavement side-friction factor, and g is differences in the design approaches of the United
acceleration due to gravity (in m/s2). States (AASHTO 1994) and Australia (Austroads
1993) as regards the above elements of horizontal
Application of the design speed concept involves curve design. Based on empirical findings, research
the assumption of a design speed (Vd). For that suggestions have been made for possible
speed, a corresponding side-friction factor value improvements to existing practices. This paper
(fvd) is also specified; fvd is commonly a decreasing presents a proposal for curve superelevation design
function of Vd. The designer may select from different on the basis of a simple and consistent relationship
possible pairs of values for R and e that satisfy between curvature and superelevation. The paper
Equation (1), subject to a number of constraints. The concludes with a summary of main points and
most important constraint is that superelevation identification of issues for further research.
should fall within a minimum/maximum range, the
maximum of which (emax) could reflect the risk of
stationary vehicles sliding on icy or frozen pavement DISCUSSION OF EXISTING
surfaces. Thus, the minimum radius (R min) is HORIZONTAL ALIGNMENT DESIGN
calculated as follows: PRACTICES IN THE UNITED STATES
AND AUSTRALIA
Vd = 127 Rmin (e max fVd (2)

Design speed
where Vd is in kilometres per hour (km/h) and R.in
is in metres (m). [The numerical coefficient results The United States (AASHTO 1994) follows what can
from conversion between different speed units and be called a 'classical implementation' of the design
multiplication by the value for acceleration due to speed concept (Krammes and Garnham 1995). Design
gravity: (3.6)2 x 9.81-127.] speed (Vd) is chosen based on road classification,
land use and terrain, and it is presumed that Vd will
Designers are generally allowed the freedom of not be exceeded.
applying flatter, above-minimum radii for the same

Vol 8 No 2 June 1999 Road & Transport Research

 Road curve superelevation design: current practices and proposed approach

29
Australian guidelines (Austroads 1993), in contrast, design speeds, depending on whether the assumed
define the following four 'speed parameters': maximum superelevation is 0.06, 0.08 or 0.10
(1) desired speed, defined as the free speed likely to (Hayward 1980; Kanellaidis 1991; Krammes 1994).
be adopted by drivers on tangents and other
less-constrained elements; Overall higher emax values (0.10 and above) may be
inadvisable, even for areas with a low probability of
(2) speed environment, which is numerically equal
wet or icy pavement conditions, since excessive
to the desired speed of the 85th percentile driver
superelevation may mean that drivers of slow
and is used to characterise a full road section;
moving vehicles (e.g. trucks) can be subjected to
(3) design speed, applying to individual geometric negative side friction, causing them to steer in the
elements; and opposite direction to that of the curve. Such situations
(4) limiting curve speed standard, defined as the are undesirable and potentially hazardous for the
speed beyond which f will exceed its design drivers involved.
value (for a given design speed).
Side-friction factor in relation to the
Especially for speed environments of less than 100
design speed
km/h, the Australian guidelines recognise that
individual curve geometry is determined by, but In the US, the design values for side-friction factor (f)
also helps to determine, the 85th percentile speed. have been determined on the basis of experiments
Thus consistency must be ensured at an early stage using a 'ball-bank indicator', conducted in the 1930s
by using trial alignments/iterations. and 1940s, in which the main criterion is 'the point at
which the centrifugal force is sufficient to cause
Maximum superelevation feelings of discomfort to drivers' (AASHTO 1994).
According to the AASHTO guidelines the f is a
Both in US (AASHTO 1994) and Australian decreasing function of the design speed, changing
guidelines (Austroads 1993), a range of possible linearly from 0.16 for 50 km/h to 0.14 for 80 km/h
values for the maximum superelevation rate (emax) and then, linear again but with a steeper slope, to
is foreseen. This practice can be attributed to the 0.10 for 110 km/h and 0.087 for 120 km/h.
federal structure of both countries (whereby state-
level authorities have the liberty of setting their own In Australian guidelines (Austroads 1993), the design
limits), as well as their expansion over a large values for f have been derived from observations of
geographical area with varied climatic conditions. driver speed behaviour on rural road curves. For
The latter is an important factor in defining emax speeds above 90 km/h, design values are in excess of
values: in the case of wet or icy pavement conditions, those likely to be required by the 85th percentile
superelevation rates of 0.10 or above may be unsafe driver. The function of design f against design speed
for slow-moving or still-standing vehicles (especially in Australia follows an inverse S-shape; it changes
when tyre quality is poor). linearly from 0.35 (50 km/h) to 0.31 (70 km/h),
falling steeply to 0.12 for 100 km/h and 110 km/h
• In the US, the commonest emax values for (and to 0.11 for 120 km/h and 130 km/h).
interurban links are 0.06, 0.08 and 0.10;
sometimes the more extreme values of 0.12 The main hazards from vehicle movement on a
(provided that snow and ice do not exist) or 0.04 curve at an excessive speed are skidding and rollover.
(preferred in urban design) are used. For the majority of vehicles, the critical f values for
• In Australian guidelines, recommended emax skidding and rollover far exceed the design values
values range from 0.10 (or sometimes 0.12) in assumed by highway design guidelines (Harwood
mountainous terrain to 0.06 to 0.07 in flat terrain. and Mason 1994), with the possible exception of
trucks, where instability can occur at values near the
The observed variability in emax values in both
Australian guidelines' f value for 50 km/h (0.35).
countries, despite having a justification based on the
climatic variability (different probability of ice or
The assumed f values in US and Australian guidelines
snow), does not help achieve nationwide consistency
are decreasing functions of design speed. Empirical
in the countries concerned. It can be proven, for
example, that identical curves may have resulted data reveal thatf values 'acceptable' by drivers are also
from the application of US guidelines for different higher for lower speeds (McLean 1981; Krammes 1994).

Vol 8 No 2 June 1999 Road & Transport Research

Road curve superelevation design: current practices and proposed approach

30
Furthermore, empirical evidence suggests that the f design value off is reached, thus indirectly providing
values accepted by drivers are well in excess of the a lower limit. Minimum superelevation values range
assumed values, especially for lower speeds (below from 0.02 to 0.03, the latter value being typical for
90 kph) (McLean 1981; Lamm et al. 1989; Krammes bituminous pavements.
1994). It may be thus concluded that assumed f
values are, at least at lower speeds, conservative in Discussion on the effect of curvature
comparison to the f values acceptable by the 85th and superelevation on driving
percentile driver (f85). behaviour
Table 1 presents (for the speed range of 60 km/h to There is ample and consistent empirical evidence
120 km/h) a comparison of assumed f values in US linking operating speed to radius of curvature.
and Australian guidelines. Relationships have been developed over the years
showing that the inverse of curve radius, 1/R, can be
Superelevation and side-friction a good predictor of speed (Taragin 1954; Lamm et al.
1989; Kanellaidis et al. 1990; Ottesen and Krammes
values in curves of above-minimum
1994).
radius
The procedure described in United States guidelines In current US design practice (AASHTO 1994), it is
(AASHTO 1994) recognises the possibility that curves usually assumed that the design speed is unchanged,
with above-minimum radius will be overdriven that is unaffected by changes in curvature. Thus
(driven at speeds above the design speed) but makes curves of an above-minimum radius only serve to
no corrections to the design speed. Rather, it provides reduce the total centrifugal acceleration requirement
an increasing ratio of e over f for increasing curve (e + f), which is directly proportional to degree of
radius, resulting in a parabolic relationship between curve (or inversely proportional to curve radius).
e and the inverse of R (1/R). The limiting value emirs However, this consideration does not take into
is equal to 0.02. account the fact that operating speed is affected by
the horizontal alignment. In certain cases, this
In Australian guidelines (Austroads 1993), it is only omission may lead to a serious underestimation of
mentioned that when above-minimum radii are operating speed.
selected, the corresponding superelevation and side
friction values are 'below their maximum values'. This question is addressed in a different way in
No exact calculations are made and no special graphs Australian guidelines (Austroads 1993), where
are developed for above-minimum radii. However, design speed does not correspond to a whole section
the limiting curve speed standard is, for given e and but to individual elements. Thus, given a speed
R values, the speed at which the corresponding environment, design speed for a horizontal curve is

Table 1
Assumed side-friction factors (f) in design guidelines of the United States and Australia
for speeds between 60 km/h and 120 km/h

Design speed Assumed side friction factors (f )

Vd (km/h)
AASHTO (United States) Austroads (Australia)

60 0.153 0.33
70 0.146 0.31
80 0.140 0.26
90 0.127 0.18
100 0.113 0.12
110 0.100 0.12
120 0.087 0.11

Vol 8 No 2 June 1999 Road & Transport Research

 Road curve superelevation design: current practices and proposed approach

31

a function of that speed environment and the curve through what has elsewhere been criticised as an
radius, as shown in Figure 2.2 of the Australian 'arbitrary' process. On the contrary, the influence of
guidelines. With the help of an iterative process alignment design (and especially curve radius) on
(using trial alignments subject to consistency operating speeds is recognised, and the Australian
checking), the Australian guidelines provide the definition of design speed corresponds to individual
potential for abetter matching between design speed curves (as contrasted to 'speed environment', which
and actual operating velocities. characterises a whole highway section). Through an
iterative procedure, alignment is fine-tuned so as to
However, in both countries, given regional help attain both consistency of successive curves and
differences in maximum superelevation rates, it is harmonisation between assumed and operating speeds.
possible that identical curves (that is having equal
radii and superelevation rates) correspond to The concept of using an assumed speed for the
different design speeds. A large diversity of whole section together with assumed speeds for
superelevation rates and the lack of a clear individual elements, the latter being more closely
association between superelevation and curvature related to expected actual speeds, is applied in a
have been observed (Krammes 1994). Also, there is similar form in Italy (Krammes and Garnham 1995),
empirical evidence that superelevation does not where the 'range' of design speeds is the equivalent
have a significant influence on driving behaviour of the speed environment. Kanellaidis and
parameters, such as operating speed (Gambard and Dimitropoulos (1995) have mentioned a similar two-
Louah 1986) or acceptable f values (Kanellaidis and stage concept for curve design, consisting of design
Dimitropoulos 1995). Thus, although the dynamics speeds (defining the minimum radius) and 'speed
of vehicle movement show that the selection of standards' (corresponding to a range of radius
superelevation is important for traffic safety, values); speed standards may be higher (but not
research findings suggest that it does not make lower) than the design speed, reflecting the fact that
much of a difference for drivers, who are primarily at these flatter curves operating speed may exceed
affected by the radius of curvature in choosing their the design speed.
speed.
In addition, the importance of horizontal alignment
Therefore, there appears to be a case for highway consistency (given the effect of horizontal curvature
design practice to provide a clearer link between on operating speeds) should not be underestimated
speed, curvature and superelevation; this would (Leisch and Leisch 1977; Messer et al. 1981; Lamm et
further refine the link between curvature and speed al. 1992; Krammes 1994). Large operating-speed
environment that is already inherent in Australian differentials are known to be correlated to increased
practice. If a given curve radius corresponded to a accident occurrence (Anderson and Krammes 1994).
certain (and appropriately selected) superelevation Australian guidelines are among those incorporating
rate, then the speed chosen because of that radius research proposals for design standards to include
would result in a specific (and acceptable) level of consistency checks as a 'feedback loop' (Lamm et al.
overall lateral acceleration (e + f). Thus, the radius 1992; Kanellaidis 1996).
would serve the driver not only as a guide as to the
speed to be chosen, but also as a signal for the If the designers' liberty of applying above-minimum
centrifugal acceleration to be expected. In this radius values is to be preserved (and indeed it may
manner, the consistency of the alignment could be be unnecessarily restrictive to do away with above-
further enhanced. minimum design altogether), it is important to ensure
that above-minimum design does not lead to
potential safety problems. Flatter curves are
PROPOSAL FOR IMPROVING CURVE associated with higher operating speeds and, due to
SUPERELEVATION DESIGN PRACTICE conservative assumptions for the design f, design
Compared to other countries' practices, the speed is (even at minimum-radius curves, especially
Australian horizontal alignment design procedure in US standards) an underestimation of operating
(Austroads 1993) follows a considerably advanced speed. Therefore, design guidelines should be
approach in dealing with the issue of providing a enhanced with additional consistency safeguards
proper matching between assumed and actual regarding above-minimum design.
speeds. The selection of design speed is not made

Vol 8 No 2 June 1999 Road & Transport Research

Road curve superelevation design: current practices and proposed approach

32
One important inconsistency factor is the variety of that this ratio should be at least 0.25 (Craus and
emax values applicable in the practices of Australia, Livneh 1978; Kanellaidis and Dimitropoulos
the United States and other countries. It has been 1995).
argued that agreement on a single e max value,
• Criterion 4: The 'hands-off' speed, defined as
possibly at the rate of 0.08 (assuming high-type
the threshold speed between positive and
pavements), would be beneficial and feasible,
negative side friction, is an important parameter
contributing to greater consistency (Craus and
regarding the safety and comfort of slower
Livneh 1978; Kanellaidis 1991; Krammes 1994).
drivers. British guidelines (Highway Link
Design 1984) require that the hands-off speed
In addition, simplification of the radius—
should be, at most, equal to the predicted 15th
superelevation relationship, so that there is a one-to-
percentile free speed; the ratio of that speed to
one correspondence between R and e, could also
the design speed is estimated at around 0.60.
lead to greater consistency, as already discussed.
The limiting condition for this criterion is
The idea of simplifying the R—e relationship is not
specified by applying a superelevation rate such
new. There have been arguments and actual
that the ratio of [ e / (e + f) ] equals 0.36, since
proposals for a change in that direction (Koeppel
(e + f) is proportional to the square power of
1986; Kanellaidis 1991; Nicholson 1998). This paper
design speed, (Vd)2. Therefore, the criterion is
presents the possibility of applying such a proposal,
satisfied for [ e / (e + f) < 0.36.
defined originally by Kanellaidis and Dimitropoulos
(1995), to the design of two-lane rural roads in
Australia. If the above criteria are applied for a certain speed
environment (which determines the maximum f
In applying this proposal, it is assumed that the value), then in the corresponding radius—
designer follows the recommendations of Australian superelevation graph (R—e graph) there is an area
guidelines regarding the selection of speed within which all four criteria are satisfied. Within
environment. Within the selected speed this area it is possible to define a simple R—e
environment, design speed for individual curves is relationship by which each allowable radius value
defined in association with the chosen radius corresponds to one single superelevation rate.
(Austroads 1993). The proposal's innovation lies in
the introduction, for each speed environment, of a The paper's proposal specifies possible R—e
one-to-one correspondence between radius and relationships for speed environments between 60
superelevation. km/h and 120 km/h (in increments of 10 km/h).
Figure 1, Figure 2, Figure 3 and Figure 4 illustrate the
The proposal is based on four criteria. The first two application of the four criteria, as well as the proposed
are given in Australian design guidelines, whereas relationships, for the speed environments of 60 km/
the additional criteria correspond to additional safety h, 80 km/h, 100 km/h and 120 km/h. The proposed
considerations suggested by research. relationships consist of linear segments linking
rounded pairs of values (usually with an accuracy of
• Criterion 1: Superelevation should be between 50 or 100 metres for radius and 0.01 for
emin and emax• For Australian nationwide superelevation), intended to provide a practical
consistency, it is proposed that these values are proposal.
set at 0.02 and 0.08 respectively.
Figure 5 presents an overview of the proposal for the
• Criterion 2: The side friction factor should not range of speed environments examined. It is noted
exceed the specified maximum value fmax' that, for speed environments of 100 km/h and above,
corresponding to the speed environment as there are no pairs of R—e values with e > 0.06 satisfying
defined in Australian standards (see Austroads all four criteria. Therefore, for those speed
values in Table 1). environments, the upper parts of the graphs (shown
• Criterion 3: The portion of the total sideways in dotted lines) are replaced by the limiting values of
force that is provided by superelevation, that is Criterion 2 (maximum f value); for these values,
the ratio of e over (e + f) , should be higher than Criterion 4 (maximum 'hands-off speed') is not
a desirable minimum value to provide a satisfied. It may thus be recommended to avoid
satisfactory safety margin. It has been suggested superelevation rates exceeding 0.06 for speed

Vol 8 No 2 June 1999 Road & Transport Research

 Road curve superelevation design: current practices and proposed approach

33

0.08 ,
„ , \
,, \
,, \
0.07
. ,, \
, \
Superelevation rate (m/m)

0.06

, \
0.05 , •, \
Criterion 4
Criterion 2 ;
Criterion 3 s, \
0.04
\
s. ..,
...
0.03
. -..,

. „
0.02
0 100 200 300 400 500 600
Curve radius (m)

Figure 1
Proposed R-e relationship for the speed environment of 60 km/h

0.08 I
II
1 °\,
k
\
0.07 i
\
Superelevation rate (m/m)

\
\
0.06
\ s

`\ \
0.05 + II
\ s \Criterion 4
Criterion 4 \ N,
, Criterion 3
0.04
. It\ .
' . ..
, . .
.
0.03 4
, ----,
----,
, , ----,

0.02
0 100 200 300 400 500 600 700 800 900 1000
Curve radius (m)

Figure 2
Proposed R-e relationship for the speed environment of 80 km/h

Vol 8 No 2 June 1999 Road & Transport Research

Road curve superelevation design: current practices and proposed approach

34

0.08

0.07
Superelevation rate (m/m)

0.06

0.05

0.04
Cnterion 4
•

0.03

Criterion 2 Cnterion 3

0.02
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
Curve radius (m)

Figure 3
Proposed R-e relationship for the speed environment of 100 km/h

0.08

0.07
Superelevation rate (m/m)

0.06

0.05

0.04
.Criterion 4

0.03

Criterion 2 \ Criterion 3
0.02
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 13(X) 1403 1500 1630 11(i) 1833 1933 2330 2100
Curve radius (m)

Figure 4
Proposed R-e relationship for the speed environment of 120 km/h

Vol 8 No 2 June 1999 Road & Transport Research

 89 121158 246 394 476 597
0,08 MI =
i t
I
t
i I t
II I
0,07 , •
I I I
I I I
I I I
I I I
t I I

Road curvesuperelevation design:

1.* 0,06
E

re
o
• 0,05
ca

as
c.
• 0,04
120
90 100 11
Vol 8No2 J une 1999 Road & Trans port Research

currentpracticesandproposedapproach
60 70 80
0,03

510 694 907 1148 141 1714 2040

_aa — .
0,02
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200
Curve Radius (m)

Figure 5
Proposed R-e relationships for speed environments between 60 km/h and 120 km/h
Road curve superelevation design: current practices and proposed approach

36
environments of 100 km/h and above, in order to (4) 'hands-off speed' not exceeding 60 per cent of
provide increased comfort and safety for the slower- the speed environment value (provision for the
moving traffic. safety and comfort of slower drivers).
Criteria (3) and (4) serve to introduce additional
SUMMARY AND FURTHER ISSUES safety elements in curve superelevation design, going
one step further than the Australian guidelines'
The Australian highway design guidelines already substantial existing safeguards of minimum
(Austroads 1993) include certain provisions for design values and consistency checks.
improving the safety of horizontal curve design. In
contrast to US guidelines (AASHTO 1994), where Application of the criteria can be the first step towards
the traditional design-speed approach is utilised, applying one-to-one relationships between radius
Australian standards include four distinct speed and superelevation, for various speed environments.
parameters: desired speed, speed environment, This paper's proposal uses pairs of rounded values,
design speed, and limiting curve speed standard. leading to simple and straightforward relationships.
Thus the two-way interaction between curve It also ensures—with the exception of a few cases
geometry and operating speed is acknowledged, corresponding to maximum or minimum
and trial alignments are used in order to ensure the superelevation rates—that a specific pair of radius—
highest possible degree of horizontal alignment superelevation values corresponds to one single
consistency. speed environment. The proposed approach is a step
towards further enhancing horizontal alignment
However the absence of a nationwide maximum consistency and, as such, can be generally beneficial
superelevation value, in combination with designers' to driver comfort and safety. Driving behaviour
freedom (within the consistency constraints variability implies, of course, that perception of the
specified) to use 'above-minimum' curve radii— 'feedback' from the design would be stronger for
with a corresponding reduction in superelevation some drivers and subtler for others.
rate and side friction factor—may lead to cases where
identical curves (with equal radius and The above criteria, or nationally-defined variants
superelevation values) correspond to different taking into account the different national approaches,
assumed speeds. Since the curve radius has been could be adapted to the horizontal curve design
proven to be the key determinant of actual operating procedures of other countries too. At a next stage, the
speeds, it is advisable to design curves in such a way approach could be extended to also cover very flat
that curve radius can be a signal for the total curves (determination of the threshold for removing
centrifugal acceleration to be expected by the 85th adverse superelevation).
percentile driver; this canbe achieved by more closely
linking curvature and superelevation in design One possible area for further improvement that is
practice. not addressed by this paper's proposal concerns the
fact that f values acceptable by the 85th percentile
This paper has presented a proposal for narrowing driver tend to exceed the design values (especially
down the theoretically infinite choices of above- for speed environments below 90 km/h). Since the
minimum curve radii and corresponding application of higher superelevation rates (0.10 or
superelevation rates. To that end, the following greater) is generally not recommended, the question
criteria are used: to be posed is whether it would be feasible to raise
(1) a nationwide range of possible superelevation the design values for f, in order to achieve a better
rates between 0.02 and 0.08; matching between design and operating speeds.
(2) a maximum side friction factor depending on Considering that the vector of tyre—pavement friction
the speed environment, as specified in on horizontal curves has a tangential and a lateral
Australian guidelines; component, any increase in the allowable side friction
value (lateral component) will mean a decrease in
(3) superelevation rate such as to counter at least the available tangential friction coefficient. Therefore,
one-quarter of the total centrifugal acceleration if side friction factors were to be increased, the
(safety margin); consequences with respect to stopping (that is a

Vol 8 No 2 June 1999 Road & Transport Research

 Road curve superelevation design: current practices and proposed approach

37
reduction of available tangential friction factor KANELLAIDIS, G. and DIMITROPOULOS, I. (1995).
leading to an increase in required stopping sight Investigation of Current and Proposed Superelevation
Design Practices on Roadway Curves, TRB International
distance) should be taken into account and weighed Symposium on Highway Geometric Design Practices, Boston,
against whatever benefits would arise from matching Mass.
design speed and operating speed. Since tyre— KANELLAIDIS, G., GOLIAS, J. and EFSTATHIADIS, S.
pavement friction inventories often reveal large (1990). Drivers' Speed Behaviour on Rural Road Curves,
disparities, the choice of f values is by necessity Traffic Engineering and Control, Vol. 31, No. 7, pp. 414-15.
conservative; in the longer term, improved quality KOEPPEL, G. (1986). Strassenentwurf—Rueckblick und
and consistency in both pavement and tyre quality Ausblick, Strasse und Autobahn, No. 9, pp. 395-402, Federal
may lead to increased design f values, which may Republic of Germany (in German).
correspond more closely to acceptablef values than KRAMMES, R.A. (1994). Design Speed and Operating
is the case today. Speed in Rural Highway Alignment Design, 73rd Annual
Meeting of the Transportation Research Board, Washington,
REFERENCES D.C.
AASHTO (1994). A Policy on Geometric Design of Highways KRAMMES, R.A. and GARNHAM, M.A. (1995). Review of
and Streets, American Association of State Highway and Alignment Design Policies Worldwide, TRB International
Transportation Officials (AASHTO), Washington, D.C. Symposium on Highway Geometric Design Practices, Boston,
ANDERSON, I.B. and KRAMMES, R.A. (1994). Speed Mass.
Eduction as a Surrogate for Accident Experience at LAMM, R., CHOUEIRI, E. and MAILAENDER, T. (1989).
Horizontal Curves on Rural Two-Lane Highways, 73rd Side Friction Demand Versus Side Friction Assumed for
Annual Meeting of the Transportation Research Board, Curve Design on Two-Lane Rural Highways, Transportation
Washington, D.C. Research Record 1303, pp. 11-21, TRB, National Research
AUSTROADS (1993). Rural Road Design—Guide to the Council, Washington, D.C.
Geometric Design of Rural Roads, Austroads, Sydney. LAMM, R., GUENTHER, A. K. and CHOUEIRI, E.M.
CRAUS, J. and LIVNEH, M. (1978). Superelevation and (1992). Safety Module for Highway Design: Applied
Curvature of Horizontal Curves, Transportation Research Manually or Using CAD, 71st Annual Meeting of the
Record 685, pp. 7-13, TRB, National Research Council, Transportation Research Board, Washington, D.C.
Washington, D.C. LEISCH, J.E. and LEISCH, J.P. (1977). New Concepts in
ERSF (EUROPEAN ROAD SAFETY FEDERATION) (1996). Design Speed Application, Transportation Research Record
Intersafe—Technical Guide on Road Safety forInterurban Roads, 631, pp. 4-14, TRB, National Research Council, Washington,
ERSF, Brussels. D.C.

GAMBARD, J.M. and LOUAH, G. (1986). Free Speed as a McLEAN, J.R. (1981). Driver Speed Behaviour and Rural
Function of Road Geometrical Characteristics, 14th PTRC Road Alignment Design, Traffic Engineering and Control,
Annual Meeting, Brighton, United Kingdom. No. 4, pp. 208-11.

HARWOOD, D.W. and MASON, J.M., Jr. (1994). Horizontal MESSER, C.J., MOUNCE, J.M. and BRACKETT, R.Q. (1981).
Curve Design for Passenger Cars and Trucks, 73rd Annual Highway Geometric Design Consistency Related to Driver
Meeting of the Transportation Research Board, Washington, Expectancy, Vols. II and III, Federal Highway
D.C. Administration, Washington, D.C.

HAYWARD, J. (1980). Highway Alignment and NICHOLSON, A. (1998). Superelevation, Side Friction,
Superelevation: Some Design Speed Misconceptions, and Roadway Consistency, Journal of Transportation
Transportation Research Record, 757, pp. 22-5, TRB, National Engineering, Vol. 124, No. 5, pp. 411-18, American Society
Research Council, Washington, D.C. of Civil Engineers.

HIGHWAY LINK DESIGN (1984), Departmental Advice OTTESEN, J.L. and KRAMMES, R.A. (1994). Speed Profile
Note TA 43/84, Department of Transport, London, United Model for a US Operating-Speed-Based Design Consistency
Kingdom. Evaluation Procedure, 73rd Annual Meeting of the
Transportation Research Board, Washington, D.C.
KANELLAIDIS, G. (1991). Aspects of Highway
Superelevation Design, Journal ofTransportation Engineering, TARAGIN, A. (1954). Driver Performance on Horizontal
Vol. 117, No. 6, pp. 624-32, American Society of Civil Curves, Proceedings of the Thirty-Third Annual Meeting of the
Engineers. Highway Research Board, pp. 446-66, TRB, National Research
Council, Washington, D.C.
KANELLAIDIS, G. (1996). Human Factors in Highway
Geometric Design, Journal of Transportation Engineering,
Vol. 122, No. 1, pp. 59-66, American Society of Civil
Engineers.

Vol 8 No 2 June 1999 Road & Transport Research

Road curve superelevation design: current practices and proposed approach

38
George Kanellaidis

is a Civil and Transportation Engineer with more than 25 years of expertise in a large
number of Greek and international projects, and research in the field of highway
engineering, human factors and road safety. His current position is Associate Professor
in the Department of Transportation Planning and Engineering at the National Technical
University of Athens.

Contact

Dr George Kanellaidis
National Technical University of Athens
Department of Transportation Planning and Engineering
Iroon Polytechniou 5,
15773 Zografou / Athens,
Greece
Tel +30 1 772 1283
Fax +30 1 772 1327
E-mail g-kanel@central.ntua.gr

Vol 8 No 2 June 1999 Road & Transport Research