Lecture 1 Highway Geometric Design
Lecture 1 Highway Geometric Design
Lecture 1 Highway Geometric Design
LECTURE 1
MISS EDNA
INTRODUCTION
Geometric design relates dimensions and the layout of the road in the terrain with
the requirements of the driver and the vehicle.
Main features considered in the geometric design of roads are
horizontal and vertical curvature and
the visible features of the road cross-section.
We also factor in
Sight distances
Intersections
Good geometric design ensures
adequate levels of safety and comfort are provided to drivers for vehicle
maneuvers at the design speed,
the road is designed uniformly and economically, and
The road blends harmoniously with the landscape.
Therefore, Geometric Design of Highways deals with
Cross section Elements
Sight Distance
Horizontal Alignment
Vertical Alignment
Intersection Elements
THE ROAD SYSTEM
Roads have two basic traffic service functions which, from a design standpoint, are
incompatible. These functions are:
(i) to provide traffic mobility between centers and areas
For roads whose major function is to provide mobility, i.e. to cater for
through and long-distance traffic. high and uniform speeds and
uninterrupted traffic flows are desirable.
For roads whose major function is to provide land access, high speeds are
unnecessary and, for safety reasons, undesirable.
Thus, the function of a particular road in the national, regional and local
road network has a significant impact on the design criteria chosen, and the
design engineer has to give careful consideration to this aspect in the early
stages of the design process.
The following steps are required--
1. Classification of the road in accordance with its major function,
ROAD CLASSIFICATION
The roads in Kenya are divided into the following 5 classes according to their major
function in the road network:
Class A - International Trunk Roads:
Roads linking centers of international importance and crossing international
boundaries or terminating at international ports.
Class B - National Trunk Roads:
roads linking nationally important centers. (Principal Towns/ Urban centers).
Class C - Primary Roads
Roads linking provincially important centers to each other or to higher class roads.
(Urban/Rural centers)
Class D - Secondary Roads:
Roads linking locally important centers to each other, to a more important contra,
or to higher class roads, (Rural/Market centers)
CONTROL OF ACCESS
If access is not controlled along roads whose function is mainly mobility there will
be
Increase in accidents along the road.
Reduced capacity for the road
Early obsolescence of the roads
To ensure roads maintain high standard traffic facilities it is important to control
access. A highway authority normally ensures access control by occupants of land.
The level of access control are as follows
1)Full access
This means that the authority to control access is exercised to give preference to
through traffic by providing access connections with selected public roads only and
by prohibiting direct private access connections
2)Partial access
This means that the authority to control access is exercised to give preference to
through traffic to a degree in that in addition to access connections with selected
public roads there may be some private access connections
3)Unrestricted access
This means that the authority to control access is exercised to give preference to
local traffic with the road serving adjoining areas and direct access connections
Road function determines the level of access control. Motorways should always
have full control of access. For all purpose roads the following general guidelines
are given for the level of access control in relation to the functional road
classification.
Reduced levels of access control may be applied for some road projects because of
practical and financial constraints.
The location and layout of accesses should be carefully considered at the design
stage and be included in the final design stage of the project.
ROAD RESERVES
Road reserves are provided in order to
accommodate future road connections
changes in alignment, road width or junction layout for existing roads
to enhance the safety, operation and appearance of the roads.
The road reserve should always be determined and shown on the final design plans
for road projects.
The following road reserve widths are applicable for the different road classes:
The reduced widths should be adopted only when this is found necessary for
economic, financial or environmental reasons in order to preserve valuable land,
resources or existing development or when provision of the desirable width would
incur unreasonably high costs because of. physical constraints.
For dual carriageway roads it may be necessary to increase the road reserve width above
the given values as a general rule the’ road reserve boundary should be at a distance
from the centerline of the nearest carriageway equal to half the road reserve width
for single carriageway roads.
Assignment 1
HIGHWAY GEOMETRIC DESIGN EECQ 4161
1.Draw and neatly label a typical cross section of a road. Use A3 Paper.
Use the following to guide you
The road manual part 1 page 1.4 for rural roads cross section. Draw both the single
and rural carriageway elements.
Draw the following for the urban single carriageway cross section detail
2.Describe the various cross section elements in detail.