Addis Ababa Institute of Technology (AAIT)
HIGHWAY ENGINEERING I
(CENG 3302)
CHAPTER THREE
GE O M E T R I C DE SI GN O F H I GH WA Y S
Emnete. T
2015
emnete.tadesse@aait.edu.et
Introduction
Geometric design: is the process whereby the
layout of the road in the terrain is designed to
meet the needs of the road users
 Emphasizes on the requirement of
 Drivers
 Vehicles and
 Pedestrians
 Appropriate geometric standard
 Stage 1 – Provision of access
 Stage 2 - Provision of additional capacity
 Stage 3 – Increase of operational efficiency
 Main features to consider in geometric design
 Cross sectional element
 Sight distance consideration
 Horizontal curvature
 Vertical curvature/ gradient
 Intersections
 Etc.
 Good design
 Maximum safety
 Reasonable operating cost
 Reasonable Construction cost
 Design control and criteria
Functional classification
II. Terrain/ topography
III. Design speed
IV. Design vehicle
V.
Traffic volume
VI. Human factor
VII. Environmental and adjacent land use
VIII. Traffic safety
IX. Economy and other factors
I.
Functional classification
 Functional classification is the process by which
streets and highways are grouped into classes, or
systems, according to the character of service
they are intended to provide
 The two major considerations in classifying
highway & street networks functionally are:
 Access
 Mobility
 Access is a fixed
requirement, necessary at
both ends of any trip.
 Mobility, along the path
of such trips, can be
provided at varying levels,
usually referred to as
"level of service."
 It can incorporate a wide
range of elements (e.g.,
riding comfort and
freedom from speed
changes) but the most
basic is
operating speed
or trip travel time
AASHTO classification
Rural areas
Principal arterials
 freeways Long through traffic, interstate, most heavily
traveled route
Minor arterials
 Linkage of cities, large towns, long distances, interstate/
inter-county (district)
Collectors
 Primarily inter-district, shorter travel than arterials,
moderate speed
Local roads
 Provides primarily access, short travel distance
Urbanized areas
Minor arterials
Principal arterials
 Interconnects with & augments
the principal arterial, moderate
 freeways Highest traffic volume
length, lower level of mobility
corridors, longest trip desired,
than principal
trips entering and leaving the
urban area,
 highest proportion of trip though Collector streets
small percentage
 Both land access and traffic
 Fully or partially controlled access circulation, may penetrate
residential areas,
 Spacing 1.6 km in highly developed
distributes/collects trips from
to 8 km I sparsely developed
arterials/local streets
Local streets
 primarily permits direct access to
abutting lands, lowest mobility
 Trunk Roads (Class I): Centers of international
importance and roads terminating at international
boundaries are linked with Addis Ababa by trunk roads.
Trunk roads have a present AADT 1000, although they can
have volumes as low as 100 AADT .
 Link Roads (Class II):Centers of national or international
importance, such as principal towns and urban centers,
must be linked between each other by link roads. A typical
link road has over 400 - 1000 first year AADT, although
values can range between 50-10,000 AADT.
 Main Access Roads (Class III):Centers of provincial
importance must be linked between each other by main
access roads. First year AADTs are between 30-1,000.
 Collector Roads (Class IV):Roads linking locally important
centers to each other, to a more important center, or to
higher class roads must be linked by a collector road. First
year AADTs are between 25-400.
 Feeder Roads (Class V):Any road link to a minor center
such as market & local locations is served by a feeder
road. First year AADTs are b/n 0-100
 ERA Method
 Terrain
 Design Speed - the maximum safe speed selected to establish
appropriate geometric design elements for a particular section of road
taking into account topography, anticipated operating speed, the
adjacent land use and the functional classification of the road
Desired Speed - the speed at which a driver wishes to travel,
determined by a combination of motivation and comfort.
 Operating Speed - observed speeds during free flow conditions.

operating speed is generally lower than desired speed since operating conditions
are not usually ideal.
 Running Speed – the average speed maintained over a given route
while a vehicle is in motion.

generally used in road planning and capacity and service level analyses.
 Posted Speed - is a speed limitation set for reasons of safe traffic
operations rather than for geometric design considerations and is
aimed at encouraging drivers to travel at appropriate speeds for all
prevailing conditions.
 Ss
Design Vehicle
characteristics of vehicle
 weight
 Height
 Width
 Acceleration and deceleration capability
 dimensions of features: intersections, ramps, climbing or passing
lanes, bus bays
 Turning capability
 Design Vehicle: is a representative vehicle having a standard
weight, height width and operating characteristics that uses to
established highway design elements
 Mostly used a largest vehicle that uses the road frequently
ERA Vehicle characteristics
Traffic Characteristics
 Volume
 Composition
 Directional Distribution
 Traffic Volume
Number of vehicles passing a point on a roadway a specified time
 Expressed as
 AADT
 ADT
 DHV
 PHT
 Directional Distribution
percentage of traffic volume flowing each direction
 Traffic Composition
Percentage of heavy vehicles in traffic mix
 Environmental,
 Road side population and adjoining land use
Human Factor
 When a design is incompatible with capabilities of drivers, the chance
for driver errors increases, and crushes or inefficient operation may
result


A roadway should confirm what drivers expect based on previous experience; and
Drivers should be presented with clear clues about what is expected of them
 Driver Reaction:
It takes time to process information After a person's eyes detect and
recognize a given situation, a period of time elapses before muscular
reaction occurs.
 Reaction time is appreciable and differs between persons. It also
varies for the same individual, being increased by fatigue, drinking,
age or other causes.
 Speed reduces the visual field, restricts peripheral vision, and limits
the time available for drivers to receive and processes information
Cross- sectional elements
 Consists of
 Carriage way
Part of road constructed for moving vehicle use
 Traffic lane,
 Auxiliary lane (accelerating and decelerating lanes)
 climbing lane
 passing lane and
 bus bays and lay bays
 Road way
Consists of
 Carriage way
 Shoulder
 Parking lane
 Pedestrian and cyclist way
 Viewing area
 Earth work profiles
 Side slopes and back slopes
Lane width
Directly influence
 Cost safety
 Maintenance cost
 Usually is ranges from 3-6m
 For different class of road different lane width
Shoulder
 Portion of highway adjacent to carriage way
 Provided for
 Accommodation of stopped vehicle, traditional and non motorized
traffic
 Animals and pedestrians
 Emergency use
 Recovery of errant vehicle
 Lateral support for pavement layers
 Width of shoulders depend on type of terrain and
design class of road
Normal cross fall
 Provided for
 Drainage
below poor drainage resulting
pavement deterioration
 Above maximum erosion of
material

 ERA 2013, for paved road
3%
 Generally 2-6%
Side slope and back slope
 Designed to
 insure stability of roadway and
 provide reasonable opportunity
for out of control vehicle
 Recoverable
-1:4 or flatter
 Non recoverable -1:3 to 1:4
 Critical –steeper than 1:3
 Height of cut and fill
 Type of soil
 economy
Medians
Median is the portion of a highway separating opposing
directions of the traveled way and the principal
functions are to:
Separate opposing traffic –reduce the probability of head-on accidents
 Provide recovery area during emergency
 Provide stopping area for left and U-turning vehicles (the width of
median should be sufficient enough to accommodate left turning
vehicles)
 Provide refuge for pedestrians
 Reduce headlight glare

 1.5 to 5 m width
Median end treatment
 Geometrical components of road
 Sight distance
 Horizontal alignment
 Vertical alignment
Sight Distance
 is defined as the continuous length of highway ahead visible to the
driver.
 A driver‘s ability to see ahead is of the utmost importance in the
 safe and
 efficient operation of a vehicle on a highway
 There are three different sight distances that are of interest in
geometric design:
 Stopping sight distance (SSD)
 Meeting sight distance (MSD)
 Passing sight distance (PSD)
Stopping Sight Distance
 is the minimum sight distance required by the
driver in order to be able to stop the car before
it hits an object on the highway
 It is applicable on all roads;

on sag vertical curves it is equivalent to the headlight sight
distance.
 It is the sum of two distances:
 brake reaction distance
 brake distance
SSD = BRD + BD
SSD - Stopping Sight Distance (m)
BRD - Brake Reaction Distance (m)
BD - Braking Distance (m)
 The perception-reaction distance (Brake Reaction Distance)
BRD = 0. 278*V*t
V= design speed, km/h;
t = AASHTO assumes a brake reaction time
of 2.5 seconds
 braking distance of a vehicle on a level roadway traveling at the
design speed of the roadway
V= design speed, km/h;
a = deceleration rate (3.4 m / s2)
 when a highway is on a grade, the equation for breaking distance
should be modified
G is the percent of grade divided by 100,
•positive for vehicle on up grade
•negative for down grade
 When adjusted to surface condition
𝐵𝐷 = 𝑉2
254(𝑓+𝑔/100)
where g/100 = G
f = coefficient of friction between tire and
roadway
Passing or Overtaking Sight Distance
the sight distance required by a driver to successfully overtake a
slower moving vehicle on its path on a two-lane road.
PSD = d1 + d2 + d3 + d4
 d1—Distance traversed during perception and reaction time and during the
initial acceleration to the point of encroachment on the left lane.
 d2—Distance traveled while the passing vehicle occupies the left lane.
 d3—Distance between the passing vehicle at the end of its maneuver and
the opposing vehicle.
 d4—Distance traversed by an opposing vehicle for two-thirds of the time
the passing vehicle occupies the left lane, or 2/3 of d2 above
d1=0.278*t1(v-m + a*t1/2)
d2=0.278*V*t2
d3=
d4=2d2/3
t1=time of initial maneuver
a= average acceleration km/h/s
V= average speed of passing vehicle, km/h
m= difference in speed of passed vehicle
and passing vehicle km/h
t2=time passing vehicle occupies left lane. s
V= average speed of passing vehicle, km/h
Meeting Sight Distance
 is the distance required to enable the drivers of
two vehicles traveling in opposite directions to
bring their vehicles to a safe stop after becoming
visible to each other.
 Meeting sight distance is normally calculated as
twice the minimum stopping sight distance
HORIZONTAL ALIGNMENT
 The horizontal alignment consists of straight sections of
the road (known as tangents) connected by curves.
 The curves are usually segments of circles, which have
radii that will provide for a smooth flow of traffic
 The design of the horizontal alignment entails
 the determination of the minimum radius,
 determination of the length of the curve,
 the computation of the horizontal offsets from the tangents to the curve
to facilitate locating the curve in the field.
 to avoid a sudden change from a tangent with infinite
radius to a curve of finite radius, a curve with radii
varying from infinite to the radius of the circular curve is
placed between the circular curve and the tangent
spiral or transition curve
 There are four types of horizontal curves:
 simple,
 compound,
 reversed, and
 spiral
Determination of minimum curve radius
 In order a vehicle to move
in a circular path an inward
radial force is required to
provide the necessary
centripetal acceleration
 Side way friction and
 Super elevation
 Rmin is the minimum curve
radius that insure smooth
and safe maneuver through
the horizontal curve
e + f = V2 / (127*R)
where:
e = rate of superelevation(m per m)
f = side friction factor (or coefficient of
lateral friction)
V = speed (Km/hr)
R = radius of curvature (m)
Determination of length of curve
SIMPLE HORIZONTAL CURVE
PC – point of curvature
PI – point of intersection
PT – point of tangency
T – Tangent length
Δ – central angle
R – radius of curve
D – Degree of curve
E - External distance
M - Middle ordinate
C - Long chord
D – degree of curve that
defines,
 Central angle which subtends
20m arc (arc definition),
 Central angle which subtends
20m chord (Chord definition)
 From arc definition,
R = 1145.916 / D
 From chord definition,
R = 10 / Sin(D/2)
 Tangent (T)
T = R*tan(Δ/2)
 External distance (E)
E = R*(Sec(Δ/2) – 1 Or
E = T*tan(Δ/4)
 Middle ordinate (M)
M = R*(1- Cos(Δ/2))
 Long chord(C):
C = 2R*Sin(Δ/2)
 Length of Curve (Lc)
Lc = 20* Δ/D Or
Lc = R*π* Δ/180
 Stations of PC, PI, and PT:
PC = PI – T
P T = PC + Lc or
PT = PI + T
Field Location of a Simple Horizontal Curve
 Simple horizontal curves are usually located in the
field by staking out points on the curve using
angles measured from the tangent at the point of
curve (PC) and the lengths of the chords joining
consecutive whole stations
 The first deflection angle VAp to
the first whole station on the curve,
which is usually less than a station
away from the PC, is equal to d1/2
based on the properties of a circle
 The next deflection angle VAq is
 and the next deflection angle VAv is
 The next deflection angle VAs is
 and the last deflection angle VAB is
 Lengths l1 and l2 are the actual distance along the curve.
Thus, to locate end points of these curves, chord lengths
corresponding to the arc length must be computed.
 The expression relating chord lengths to the corresponding
arc length l1 and l2 and 20m station length
COMPOUND CURVE
REVERSE CURVE
TRANSITION CURVE
θs = spiral angle
Δ = total central angle
Δc = central angle of the circular arc extending
from BC to EC = Δ - 2 θs
Rc = radius of circular curve
L = length of spiral from starting point to any
point
R = radius of curvature of the spiral at a point L
distant from starting point.
Ts = tangent distance
Es = external distance
S = shift
HIP = horizontal intersection point
BS = beginning of spiral
BC = beginning of circular curve
EC = end of circular curve
ES = end of spiral curve