Design Speed
Layout constraint
Layout constraint assesses the degree of constraint resulting from the
road cross-section, verge width and frequency of junctions and
accesses.
Both carriageway width and verge width are measured in meters.
Density of access is expressed in terms of the total number of junctions,
laybys and commercial accesses per kilometre, summed for both sides of
the road using the three gradings low, medium and high, defined as:
Low = Between 2 and 5 accesses per kilometre
Medium = Between 6 and 8 accesses per kilometre
High = Between 9 and 12 accesses per kilometre.
Design Speed
Layout constraint (Lc)
The layout constraints for different combinations of the above relevant parameters are
defined in the Table below for seven different road types.
Having sketched a trial alignment on paper, the table is utilized to estimate Lc,
whose value will range from zero for a 3-lane motorway (D3M) to 33 for a 6m single
carriageway road with a high level of access to it and narrow verges (S2, 6m, H, 0.5m
verge). Where the exact conditions as defined on the table do not apply, interpolation
between the given figures can be employed.
Design Speed
Layout constraint (Lc)
Design Speed
● Alignment constraint measures the degree of constraint resulting
from the alignment of the highway. It is assessed for both dual
carriageways and single carriageways:
Dual carriageways:
● Ac = 6.6 + B/10
● Single carriageways:
● Ac = 12 - VISI/60 + 2B/45
● Where, B = Bendiness in degrees per kilometre (°/km) VISI =
Harmonic mean visibility. VISI can be estimated from the empirical
formula: Log10 VISI = 2.46 + VW/25 - B/400 Where: VW = Average
verge width averaged for both sides of the road.
Design Speed
● Alignment constraint
Design Speed
Design Speed
Design Speed
The design speed calculated from the resulting alignment and layout
constraints is then checked against the originally assumed
design speed.
Equally, it may be necessary to upgrade the design if the resulting
design speed dictates this.
If any alterations to the geometry of the highway are undertaken, it
will be necessary to recalculate the design speed in order to make
sure that its value has not changed.
While the design speeds for two sections running into each other
need not be the same, it is advisable that their design speeds
differ by no more than 10km/hr.
Examples
Example 1
An existing 7.3 m wide single carriageway road with 1.5m wide verges (see figure
below) has a layout as indicated in figure 1 above. The length of the section of highway
under examination is 4 km (X to Y). The relevant angles are (refer to previous figure):
● A = 95°
● B = 65°
● C = 60°
There are a total of 24 access points and minor junctions along the length of the
highway. Calculate the design speed.
Examples
Example 2 – Comparison of observed speeds with calculated design speed
Taking the existing stretch of highway referred to in Example 1, Table below shows the
results from a speed survey taken along the route.
Determine the 85th percentile speed and compare it with the derived design speed.
Design Capacity
Basic considerations
The term capacity is used here to define the
ability of a road to accommodate traffic under
given circumstances. Factors normally taken into
account are the physical features of the road and
prevailing traffic conditions.
Design Capacity
Prevailing road conditions
Capacity figures for uninterrupted flows on highway have to be
modified if certain physical design features are not adhered to. Poor
physical features, which reduce capacity include:
Narrow traffic lanes. Lane widths of 3.65 m are now accepted as
being the minimum necessary for heavy volumes of mixed traffic.
In adequate shoulders. Too narrow or lack of shoulders alongside a
road cause vehicles to travel closer to the Centre line thereby increasing
the medial traffic friction. Also, vehicles making emergency stops must
park on the carriageway reducing the road capacity.
Design Capacity
Prevailing road conditions
Side obstructions. Obstructions such as poles, bridge abutments,
retaining walls, parked cars located within 1.75 m of the edge of
carriageway contribute towards reduction in the effective width of the
outside traffic lane.
Imperfect horizontal or vertical curvature. Long and/or steep hills
and sharp bends result in adequate sight distance. Drivers are then
restricted in opportunities to pass and hence the capacity of the facility
will be reduced.
Design Capacity
Prevailing road conditions
Prevailing traffic conditions. Prevailing traffic conditions are
not fixed but vary from hour to hour throughout the day. Hence
the flows at anytime are a function of the speed of vehicles,
the composition of the traffic streams and the manner in
which vehicles interact with each other.
CROSS-SECTIONAL ELEMENTS
Cross – section elements are those features of the highway which
form its effective width, and which affect vehicle movement. Of
primary interest are
The number and width of traffic lanes.
The central reservation
Shoulders
Camber of the carriageway
Lay – bys
Side slopes
CROSS-SECTIONAL ELEMENTS
CROSS-SECTIONAL ELEMENTS
Factors controlling space for cross-sectional elements
Functional requirements
Road category, regional, city, area connectivity
Mobility requirements
Speed categorization
Operational requirements
Traffic volume, nature of traffic, composition
Factors controlling space for cross-sectional elements
Specific requirements
Parking, loading-unloading, exclusive facilities, high
banks, deep cuts, unstable land, landslide etc
Social requirements
Ease of use, comfort, convenience, access to
property
Service requirements
Utilities, drains, illumination
Factors controlling space for cross-sectional elements
Safety requirements
Road signs, signals, barriers, medians
Future expansion
Extra land
Cross-sectional Elements
Cross-sectional Elements
Cross-sectional Elements
Cross-sectional Elements
Cross-sectional Elements
Traffic lanes
The number of traffic lanes to be used in a specific
situation is depended on the volume and type of
traffic that has to be handled.
Normally the minimum number is two. Even
though traffic volumes may be light, safety
considerations and easy of traffic operation
require two lanes.
Traffic lanes
3-lane roads are constructed when the design
volume exceeds the capacity of a 2 – lane
road but is not sufficient to justify a 4 – lane
facility.
3 lane road should only be constructed in rural
areas where it is practical to provide nearly
continuous overtaking sight distances.
Traffic lanes
3-lane two-way facilities should never be constructed
in urban areas because of their high accident
potential ie in heavy traffic . Drivers often want to use the
middle lane as the opposing traffic and attempt to overtake
regardless of the hazardous conditions.
Four or more lanes are needed to enable vehicles to
overtake on lanes not used by opposing traffic.
Normally these highways have dual carriageways
separated by a central reservation.
Traffic lanes
Central reservations/ medians
Most dual carriageways have at least two traffic lanes in
each direction divided by a central reservation at least
1.75 m wide even in urban areas where space is
restricted.
While it has been proved that the wider the central
reservation the greater the reduction in head-on
collisions, it has not been possible to establish any
overall relationship between total number of accidents
and the reservation width.
A reservation width of 12 –15m is needed to bring an
encroaching high-speed vehicle under control to avoid
the possibility of becoming involved in a head-on
collision on the other carriageway.
Central reservations/ medians
In rural areas consideration should always be
given to varying the width to obtain a safe,
pleasing and economic design that fit the
topography. Where possible shrubs should be grown
in as this reduces headlight glare and acts as cross
barriers.
The reservation surfacing should contrast with
the carriageway and distinctly visible during day
and night in wet and dry weather. For widths
wider than 1.75m, grass is most suitable, but
below 1.75m, use raised medians with contrasting
bituminous or concrete surfaces.
Central reservations/ medians
Central reservations/ medians
Central reservations/ medians
Central reservations/ medians
Central reservations/ medians
Central reservations/ medians
Central reservations/ medians
Shoulders
A shoulder is that portion of a road way adjacent to the
travelled way that is primarily used as a refuge area by
parked vehicles. Well designed and properly maintained shoulders
are necessary on all rural and suburban arterial roads
carrying appreciable amounts of high-speed traffic.
The shoulder widths vary. About 3.35m is needed by a lorry to
enable a tyre to be changed without danger to operator.
Passenger cars require less but form greater part of the total
stoppages hence widths of 3m are usually recommended for
major highway design purposes. This allows a 1m gap between
the parked cars and the edge of the carriageway.
Shoulders
The best though expensive is to extend the road base
beyond the edge of the carriageway. This makes the
shoulder stable, high structural strength to carriageway and
shoulder can be used as a slow traffic lane when carrying out
maintenance work on carriageway.
Shoulder surfacing must be distinctly different from the
carriage way otherwise motorists will use it as a
regular traffic lane. Surfacing can be of grass,
bituminous with coloured stone chippings.
Shoulders
Shoulders
Shoulders
Shoulders
Shoulders
Shoulders
Lay-bys
When economic considerations do not allow the use of full
length shoulders then lay-bys should be built at favorable
locations along the highway.
If located on high-speed roads they should be provided with
adequate acceleration and deceleration lanes on either
end.
There spacing a long the road should be related to the
volume of traffic i.e. one for every 1.6 km on each side of a
highway carrying 6000 pcu/day and one at 4.8 km intervals
on roads carrying 3000 – 6000 pcu/day.
Lay-bys
Lay-bys
Lay-bys
Lay-bys
Bus -Stops
Bus -Stops
Camber
Camber
The term camber is used in highway engineering to
describe the convexity of the carriageway cross – section.
The object of cambering is to drain water to avoid ponding
on the road surface. Today high-quality roads have average
cross – slopes of between 1 in 40 and 1 in 48 …1 in 60.
On dual carriageways, it is desirable for each carriageway to
be cambered. This minimizes the sheeting of water during
rainstorms and keeps the difference between the low and high
points in the carriageway cross - section to minimum.
Camber
Where carriageways are sloped in one direction to drain from the
median space to the outside, savings are effected in drainage
structures and treatment of intersecting roadways is easier.
Another possible arrangement for dual carriageways is that each
carriageway should have a one – way cross – slope draining
towards the central reservation.
Advantage – Outer lanes used by commercial vehicles are more free
of surface water.
All surface water collected in a single drainage (more economic}
Disadvantage – Drainage passing over the inner high-speed lanes
causes splashing on windscreens.
Camber
Kerbs
Kerbs
Kerbs
Kerbs
Side slope
Side slope
Side drains
Side drains
Bicycle facilities / NMT
Pedestrian facilities
Pedestrian facilities
Pedestrian facilities
Pedestrian facilities
Pedestrian facilities
Pedestrian facilities
Pedestrian facilities
Pedestrian facilities
Pedestrian facilities
Cycle tracks
