SPECIALTRANSPORTATION STRUCTURES
(Notes for Guidance )
Highway Design Procedures/Route Geometric
Design/Horizontal Alignment/Sight distance, widening
and superelevation in curves
Radu ANDREI, PhD, P.E.,
Professor of Civil Engineering
Technical University “Gh. Asachi” IASI
Lecture
Ten
Horizontal Alignment/Sight distance, widening and superelevation
in curves
•
Sight distance in horizontal alignment
•
Widening on curves
•
Problems
•
Additional Readings
Horizontal Alignment/Sight distance
• Checking of sight distance across the inside of
curves is another
important
element of
horizontal alignment design.
• Different sight obstructions such as trees in
wooden areas, existing buildings, guard rails
have to be removed or adjusted in order to
provide adequate sight distance. If they can not
be removed , we have to adjust the road route, its
alignment and cross section.
Horizontal Alignment/Sight distance
• The checking of the geometry of the road is usually
done by using the design speed V and the proper
minimum sight distance. Design charts have been
drafted in order to help the designer to check the
geometry of the road, but when such chart do not apply,
it will be necessary to undertake field measurements or
to make measurements on plans.
• The criteria for the determination of the minimum sight
distance on a horizontal curve are derived further in
relation with the figure from the next slide.
Horizontal Alignment/Determination of the minimum sight
distance on a horizontal curve
Horizontal Alignment/Determination of the minimum sight
distance ion a horizontal curve
• In relation with the figure from the previous slide , the driver of the
vehicle from point B, manifest a tendency to shorten the way,
travelling the lane designated to the opposite traffic, this being the
most unfavourable case which may happen in practice. This
behaviour disturbs the driver of the vehicle in point A, who travels
regular on his designated line.
• In order to avoid collision, both drivers have to see each other in
proper time, in order to make the necessary movements for
correction. So the driver in point A has to put the break and the
driver in point B has to leave the undesignated lane.
Horizontal Alignment/Determination of the minimum sight
distance ion a horizontal curve
• Assuming that visibility is assured, the sight distance E,
measured along the travel AB will be given by the
relation:
E = EA + EB = VA* t + d +3,5 VB
• In this relation , the time of perception is : t= 1 second ,
d is the distance travelled by the vehicle A in 2,5
seconds, and 3,5 VB is the distance travelled by the
vehicle B, whose driver needs 1 second for the
perception of the coming of the vehicle A and 2,5
seconds to change the lane.
Horizontal Alignment/Determination of the minimum sight
distance ion a horizontal curve
• using the equation from previous slide , the
sight distance of any horizontal curve can be
determined.
• In order to cover all the possible situations
which may be encountered, the field of
visibility may be determined, by tracing the
all possible visual radii, connecting the two
vehicles, as shown in the next slide.
Horizontal Alignment/Determination of the field of visibility in
horizontal alignment by using the minimum sight distance
Horizontal Alignment/ The minimum recommended visibility and
overpass sight distances ( STAS 863-85)
•
Design speed ( Km/h)/Minimum visibility distance for curves
The overpass distance (m)
•
V
vd
od
•
100 280
400
•
80
230
325
•
60
140
250
•
50
110
200
•
40
70
150
•
30
60
125
•
25
50
100
( m)/
Horizontal Alignment/widening on curves
• In practice the rear wheels of a vehicle will truck
around a curve of a shorter radius than the front wheels.
Thus the vehicle occupies more pavement width than it
does on the tangent. When the curve is of small radius
and the length l of the vehicle is relatively large, the
need for an extra-width may become significant.
• In order to make operating conditions for vehicles on
curves, similar with those encountered on tangents , it is
necessary to widen the carriage way.
Horizontal Alignment/widening on curves
• In current design practice there are various
modes of applying widening of pavements on
curves. An approximate formula for determining
the
widening necessary for the two lane
carriageway is given by literature [2], as follows
: W = l2/R where : l = the length of the
vehicle; R = the radius of curve
Horizontal Alignment/widening on curves
• As usually, in practice, curves with the radius
greater than 300m do not need widening. Below,
are given the recommended values of widening,
for various values of radii
•
Radius (m):
•
---------------------------------------------------------------------------------------------------
•
widening (m):
20
25
30 35
40 60
80
100 125 125…300
2,00 1,60 1,35 1,18 1,00 0.65 0,50 0,40 0,35
030…0,25
Horizontal Alignment/widening on curves
• Widening should be attained gradual on the approaches
to the curves in order to ensure a reasonable smooth
alignment on the edge of pavement and to fit the paths of
vehicles entering or leaving the curve. On simple
curves, widening should be applied on the inside edge of
pavement only. Curve widening should be attained
gradually over a length sufficient to make the whole of
the pavement fully used. The maximum widening
should be used for the entire length of the circular curve
, and should vary from zero at the origin of the spiral to
the full amount of the at the point of circular curve, as
shown in next slide.
Horizontal Alignment/widening on curves
Variation of the
widening in case of a connection of tangents with two
symmetrical clothoids
Problems
• WORKSHOP No.1
• 1. After connecting the tangents with simple circular
arcs, determine the value of the minimum sight
distance for each circular curve and based on this
determine graphically, the field of visibility on the
inside of each curve , by tracing the all possible radii
of visibility ,connecting two vehicles.
Problems
• WORKSHOP No.2
• 1. After introducing symmetrical transition curves,
determine the values of the widening, apply and and
represent them graphically along the symmetrical
spirals and along the remaining circle arc.
Additional Readings
• Andrei R. Land Transportation Engineering,
Technical Publishers, Chisinau, 2002
• Garber j.N., Hoel A.,L, Traffic and Highway
Engineering, revised second edition, PWS
Publishing,1999
Additional Readings
• Zarojanu Gh.H. Popovici D., Drumuri- Trasee, Editura
VENUS, Iasi,1999
• Belc F. Cai de comunicatie terestre. Elemente de
proiectare, Editura Orizonturi Universitare, Timisoara,
1999
• STAS 863-85 Road works. Geometrical elements of Lay
out. Design specifications
Additional Readings
• Hikerson F.T. RouteLocation and Design, Mc GRAWHILL, Fifth Edition, 1967
• Civil Engineer's Reference Book, 3-rd Edition,
Butterworths, London, 1975
• Dorobantu si al. Drumuri. Calcul si Proiectare, Editura
tehnica bucuresti, 1980