TRH17 (1988) Geometric Design of Rural Roads

0'798833122
retoria, South Africa, 198%

Geometric des~gn
TRHI 7, Pretoria, South Africa, 1988

n of surfaced rural roa
on the geometric desi n manuals of
ic parameters of driver, vehicle
to horizontal and vertical ali
design of intersections
ion is paid to drainage ele-
ntwerp van buitestedelike dek-
gebaseer op die han
it in die basiese parameters van
ord riglyne wat betrekking
elei. Die ligging en ont-
eskenk aan dreine-
nment, cross-sec
Geometric design
TRH17, Pretoria, South Africa, 1988

The geometric desi
ere derived lar
e years, however
by the various a
The Committee o
practice will be of
creasingly mobile
for practice to be
tions. It has therefore
evance should be
H1 7 represents the a
TRH17 is base
statements on
The wide variety
TRH1 7 therefor
The Committee res
resentatives of the
Geometric design
TRH 1 7, Pretoria, South Africa, 1988

cope of the document
7 3 Traffic volumes
1.4 Traffic speed
7 Capacity and level of service
I. Classification of rural roads
2. "i
The driver
e road surface
kt distance
uccessive curves
Geometric design
YRHI 7, Pretoria, South Africa, 1988

Geometric design
TRH17, Pretoria, South Africa, 1988

Geometric design
TRH17, Pretoria, S ~uth Africa, 1988

Gsnmelric de?lsiign
TRH 1 7. Pretoria, South Africa, 1

nents of the four
rmity of geometry
current practices and standards. The
endment as research results, tailorin
F the conse-
Geometric design
TWHf 7, Pretoria, South Africa, 1988 1

The design of new routes or improvements to existing route
traffic vo!umes. A design life of 20 years is often
eriod may be altered subject to the planning of
cerned, and the evaluation of the economic consequences of
suggested timespan. For example, a relatively low-cost road
volumes may justify a shorter design life because of the sa
er number of axle-load repetitions in the shorter pe
hilly or mountainous terrain may require a longer design li
sonable return on the initial cost of construction.
Projected traffic volumes should prefera
growth factor to present-day traffic coun re an alternative
able, an origin-destination survey may be nec
possible alternatives, a full-scale transporfation
ered. A useful input to the determination of proj
Traffic Demand Model administered by the Department of Tran
Traffic volumes are often e essed in terms of average dail
sured in vehicles per day. ADT does no
hourly fluctuations in traffic volume.
frequently assumed to be the 30th
n, ie the hourly volume exceeded
hourly volume is expressed as a
tween 12 and 18 per cent. A figur
mally assumed unless actual counting suggests an
nual basis, the directional split on most rural roads i
ever, during any specific hour the volume in one direction may
than in the other. The directional split is often in the ratio o
heavier flow is the design criterion.
Traffic speeds are me kilometres per hour.

Geometric design
W 1 7, Pretoria, South Africa, 1988
is affects visi

Geometric design
TRM4 7, Pretoria, South Africa, 1988

Geometric design
TRH17, Pretoria, South Africa, 1988

Traffic volumes ve

iscusses the parameters from se guidelines are derive
the design vehicle, its dimen performance characteri
ssary before climbing lanes, maximum permissible grades, intersec-
nd turning roadway radii and widths can be decided on. The driver's
ht above the road surface and his reaction ime are used to derive stop-
other sight distances.
ht distances are known, rates veqical curvature can, in
icient of friction of the roa ace, in conjunction with the
relating to the driver, determines the various sight distances, and
superelevation rates, from which minimum horizontal radii for the
gn speeds are calculated.
ation of the recommended values is given so that the designer dealin
other design vehicle or circumstance ill be in a position to calculate
n vehicle for which dimensions have been estab-
); the single unit truck ( is the subject of
vely established for the , although they a l
ensions are not avails the dimensions of the
een adopted.
icle most frequently ernploye in the design of rural roads is the single
Geometric design
TRW 17, Pretoria, South Africa, 1988
for the various desi n vehicles are given in Table 2.2.1.

units -i- trailers.
In constricted situations \M
Geometric design
TRH 1 B, Pretoria, South Africa, 1988

Geometric design
TRHI 7, Pretoria, South Africa, 1988

her

Geometric design
H1 7, Pretoria, South

The road surface has
lion of the situation a
qualities taken into ac
rake force coe
safety factor, as
smooth wet surf
Geornetrrc design
7, Pretoria, South Africa, I

Geometric desi
rehorm, Solath Africa, 1 9

ESlG DISTANCE (m)
Geornelrrc des~gn
PRH-17, Prelorta, South Africa, 1988
-

At a st
to see
stopped vehicle starts to cross.
of the intersecting roads.
t distances, rec
be studied.
Geometric design
TRH I 7, Pretoria, South A

9-3
South Africa, 1 988

ic desi
T M 1 7, Pretoria, ica, 19

TRW 1 7, Pretor~a, South Aka, 1988

of a cuwe shoul
TRH17, Pretoria,

Transition curves are r
this basis and rounded g, in m

003';
OOSP
OOOP'
035:
OOOE
OOS-2
OSOZ
OOOF
00s
0

nce from this sec
table feature of the road to the driver.
I = lane factor
sured by the relative sic
at the cornme
Geometric design
TRMl7, Pretoria, South AMca, 19

SIG
Geornetrlc desrgn
TRH17, PreQorra, South Afr~ca, l988
V'. TIOIV RUN-OFF FOR NE ROADS
e lengths based on such direct ratios,
relevation run-off lengths should be
for wider pavements should
, the median is disr election of the lane
. If the median is wi ,2 m, the two car-
rate entities. For i widths of rnedian
actor may be selected.

a l
fall

ility to accelerate and decelerate.
ife economy of the roa
nment should
urves and tangent

tance of the same
gence angle of 1"

met n
4 7, South Africa, 198

GRADIENT (%) LENGTH OF GRAD
ths can be rea from Figure 2.2.4, and are
conventence.
tent.
es by removing slo
Geometric design
TRH17, Pretoria, South Africa, 19

Geometric design
TWMI S, Pretoria; South Africa, 1988

South Africa, 1988

ic de
rim, 1

S-section of a road
vehicles, drain es and, to a lesser extent in the r
destrians. For convenience of drivers, wide lanes a
and gently sloping border areas are
The selection of lane
mended values.
ese conditions, and c
cross-sectional ele
er one lane in each direction
n each direction (multi-lane ro
irection and ar
divided or six-l
ddition of an auxiliary I
Geometric
TRH17, Pretoria, South Afric:

Geornelrrc. design
TRM 17, Pretoria, Soitth Africa, I

Geometr~c des~gn
"TRI 7, Pretorra. South Airrca, 19

Geometric design
TRH17, Pretoria. South Africa, 1988

drains should preferably be raised rather than depressed in
drain 1oca"td almo heighten the possibility
wheel might s
are constructed of either concrete or pre ixed bitumen. The premix
normally has a height of 75 to mm, and IS trapezoidal in prof~le with a
h of 250 mm and a top wid f l00 mm. The concrete edge drain is a
rrler kerb and channel as specifled in SABS 927-19693. This requires a
rnpacted backing for stability and is therefore less convenient to con-
the premix berm.
e top of a cut, is to the cut face w
o deflect away from the cut overland
e area outsid through material
ce the volume of
ved by the side drain.
drain is seldom, if ever, lined. It is constructed h the undis-
soil of the area as it n readily be grass as a protec-
inst scour. Transverse weirs can also be cted to reduce flow ve-
the restrictions mentioned rn Se 2 do not apply to the
er drain. The cut face and the profile of reduce the probability
tering the drain, but, should this h e speed of the vehicle
designer is not directly

in invert are prefe
Underground reticulation is costly both to provide and to m
therefore, without vio
e use of undergroun
of surface draina
Chutes are intended to convey a concentration of water down
without such protection, would be su
large structure to a half-round prec
channels. Flow velocities are high, so that
stream erosion is to be avoided. An exampl
discharge of water down a fill slope from an e
require attention to ensure that water is de
chute, particularly v~here the road is on a ste
It is important that chutes be adequately space
the shoulders of the road. Fu hermore, the dimensions of t
basin should be such that these drainage elements do not
tion,
Because of the suggested shallow de
the design and construction of chutes to ensur
IS not deflected out of the chute. This is a serious
obviated by replacin the chute with a pipe.
bank.

orr~etrrtc:
des~qii
1-4l1'7. Pretoria, South Africa, 1988

FOR GIVEN FILL SLOPE
CHECK NATURAL SLOPE
BELOW FILL SLOPE

R
South Africa, 1988
AROWOOO SPACER BLOCK
25 X 150 X 100
ered 70" or domed
EDGE Of
1USABL.E
SHOULDER
R SA8S 1350-I9

LAST POINT
OF HAZARD
19050

TRH17, Pretoria.
rlc de

DIRECTION OF TRAVEL -

manoeuvres.
ant part of a roa
of vehicles on t
tiveness of its intersections.

last cutve
TRH 17, Pretoria,

TRH 1 7, Pretoria,

ie where there are low volumes
vehicle on traffic flow may be
eed traffic strea
erefore normally provided at major rural int
and right turns from the major to the nor road. These
ned in accordance with the sugges ns contained in
nts of right-turn lanes can be cons~dere both of which in-
s are high and
ht and the lane
is made can be designated for both through and turn-
he flow of throu
y mandatory arrows, possibly reinfor
length of the right-turn lane.
ge for the number of vehicl
-minute period in the peak hour. As a minimum re-
t two passenger cars should be provided; when over
ic consists of trucks, provision should be made for at least
two-minute waiting time is arbitrary, and some other
largely on the opportunities for completin
n the volume of opposing traffic.
natural and expecte
to channelization of t
tion

74 TRHI 7, Pretoria, Sou

connection cornmen
as the diamond ramp
rminal. This makes it possible to accommodate
can be consi ered for a right turn.
p is often employed. Turning vehicles are re-
of direction at relatively low
not require an ad
main structure. Loops

,
tlpap- "3""
, ,a sin
-xA
ir*@^*-
a*(;-" t:
P bbi **;:IP lii"l

MPLE DIAMOND

fic fl
fic S
There are three generally us
tructure, and the Parclo-
has the loops on the sa
tion has the loops on the same
loops are forced to weave either on the freeway
tributor road. This configuration, like the tran
with one Loop.

ssion of access and syste s interchanges ha
es. A freeway terminating at its i
would however glve rise to a three-legged systems inte
access to a local area on one side of a freeway would req
access interchange, although, in t h case, ~ it is often a
development of the area on the other side of the freew
ed interchange in the first instance.
Left turns take place on outer connections and right
. It is not custom
o weaving on the
mi- directional ram
separation struct
If both right turns are on semi-directional ramps, the third layou
9.3.3 results.
tion of continul 0

of lanes at interc
and drop lanes over short
ility that a given exit fr
nance, or because of
that would normally have exi
next exit under circumstances of extr
number of lanes over a reasonable
f operation on the freewa
tes to his chosen destination.
ation of lane balance and continuity of th
positioned to ca

07~6, but no continui ask number of lanes
osic number of lanes and ions balance

two main factors to consider in determinin
o exit at the next inter-
e use of an interchange is determined in part by the clarity of its
since the driver, particularly a driver unfamiliar with the area, must
te advance warning of the location of exits to specific destina-
of interchanges is dealt with in the South African Road Traffic
nua12. In that document it is suggested that a Class I Sign Sequence
a Pre-Advance Sign mounted two kilometres in advance of
n Sign (depending on the nce to the next
ounted about m beyond the entrance. ning will be
ation Sign IS replaced by a P nce Sign to
two consecutive interchanges, the designer the
ed to eliminate weaving between them, and t i red
ning, taking the greater of these two as the minimum acceptable
n the interchanges. As a very rough guide, a distance of about 5
Ily be the minimum acceptable spacing between successive in-
on rural freeways.
rea is more int sively developed, as in the Pretoria-
area, for exa le the spacing of successive interchanges may
S than the 5 km suggested above. Under these circumstances a
km could be considered the minimum for successive access in-
minimum spacing be een an access interch ge and a sys-
should not be less t n 3,6 km. As these di nces are mea-
oad to intersecting road, it is clear t
the effective signin of the freeway will
ir locat~on have an impact on signing requirements. This is
inimurn distance between edge-line breakpoints, ei
tapers of on- and off-ramps of successive inte S
o access conditions, being:
e the rest area h access directly to the free
ss to the crossin oad of the interchange.
ases are also considered. T ese refer to the other element
onsidered in n sequence between it an the rest area in
crossing road;
freeway.
r element is an interchan ajor road as its
is an interc minor road as its

The relevant istances are given in Ta
these distances are measure
IN1 S AND
D (km/h) AMP (k

pound curve WO
long enough to allow t
The rates of super
S
tl

VALUES OF K FOR VERTI ON
The minimum length of vertical curvature suggested for a ram
on aesthetics, but it ill very often not be possible to achieve
gested in Table 4.2.2. Lower values are proposed in
length in metres ecual to 0,6 o
rounded off to the nearest 10 m.
DESIGN SPEED (krnh)
If a stalled vehicle blocks an off-ramp,
blocking of the stop
for future conversion of the
TRH 1 7, Prelor~

ths are used in t e design of the terminal where the ram
ic enters the intersecting road at an
for vehicles entering or exiting from the through
minal of the intersecting road should be designed in accordance with
idelines in Chapter 8. Through road ramp terminals are discussed below.
cing of successive terminals should be such that the manoeuvres carby
a driver entering at one terminal are not hampered by vehicles entere
next terminal downstream, The distance between an entrance and the
exit should allow for weaving between the two terminals. An ex~t foldoes
not caus
successive exits could be cl
river to be able to differenti
nt design speeds that are likely to apply to the various cir-
s of free-flowing ramp terminal are discussed in Subsection 9.2.1,
o 1000 m, Th

at moderate to heavy pedestrian and pedal cycle traf-
accidents are experienced along some tarred rural
areas: in the proximity of towns, in densely
Is, bus stops, shops and housing next to the
ining compounds, industrial plants, a ricultural de-
. Usually, no specific provision is mad for pedestri-
e places - pedestrians are ent~rely dependent on
g to a bus stop or from one place to another. Th~s
er which pedestrian a1 cyclist facill-
ays depend on the vehic
volumes of pedes
eed of vehicular t
itions specified in
he purpose intended, footways must have all-
strians will choose to walk on the carriageway.
ated at least 3
the reserve boundary
mstances be preferabl

walk on. Paved shoulders. 1 3 m
Road shoulders musr also be me
em, thereby forcing
of the road shot~lder are pa
pedestrians with
road surface.
or reconstructeo, a part of the old road
for pedestr~ans or as a cycle lane. Where a
, consideration should be given to erecting
utder and the footwa
existing or new bri ninium structure
separate walkway
The layout of a bus Stop consists of t
oach footways provid
I or a 2011 transition provid
route in the foreseea
mmirnum width of th
occur at bus S ops because buses stop t
of drivers of pedestrians crossin
to the road.
Typical layouts sho the minimum require
pletely. The yello edge line should be replaced with a broken y
line where the bus exits or re-e