Geometric Design of Roads and Highways

Faculty of engineering - Shoubra
Benha University
Research Article
in fulfillment of the requirements of
Department Surveying Engineering Department
Division
Academic Year Second Year Surveying
Course name Highway, Railway and Tunnels
Course code SUR221
Topic 1: Geometric Design of Roads and Highways
By:
Name Edu mail B. N
Ahmed Yasser Ahmed Mohamed Nassar ahmed170165@feng.bu.edu.eg 210018
Approved by:
Examiners committee Signature
.‫م‬.‫أ‬
‫د‬
‫أبو‬ ‫يوسف‬ .
‫العباس‬
‫شهاب‬ .‫م‬.‫د‬
‫حسن‬
‫رشوان‬ ‫كريم‬ .‫م‬.‫د‬
‫حامد‬ ‫محمود‬ .‫د‬.‫أ‬

Benha University
Faculty of Engineering - Shoubra
Academic year 2019-2020
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Topic 1: Geometric Design of Roads and Highways

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Table of Contents
Abstract ............................................................................................................................................................................... 5
1. Introduction................................................................................................................................................................ 6
2. Highway Design Elements and Considerations ....................................................................................................... 7
2.1. Vehicle design and characteristics ................................................................................................................... 7
2.2. Design speed....................................................................................................................................................... 7
2.3. Topography and Nature of Terrain................................................................................................................. 7
2.4. Economic Considerations ................................................................................................................................. 7
2.5. Traffic capacity.................................................................................................................................................. 7
2.6. Traffic Data........................................................................................................................................................ 7
2.7. Safety.................................................................................................................................................................. 7
2.8. Environmental and Human factors ................................................................................................................. 7
2.9. Aesthetics ........................................................................................................................................................... 7
2.1. Vehicle Design and Characteristics ................................................................................................................. 7
2.2. Design speed....................................................................................................................................................... 8
2.3. Topography and Nature of Terrain................................................................................................................. 8
2.4. Economic Considerations ................................................................................................................................. 8
2.5. Traffic Capacity................................................................................................................................................. 9
2.5.1. Basic Capacity........................................................................................................................................... 9
2.5.2. Possible Capacity ...................................................................................................................................... 9
2.5.3. Practical Capacity..................................................................................................................................... 9
Design Capacity.......................................................................................................................................................... 9
2.6. Traffic Data...................................................................................................................................................... 10
2.6.1. Traffic Volume........................................................................................................................................ 10
2.6.2. Annual Average Daily Traffic (AADT) ................................................................................................ 10
2.6.3. Design Hourly Volume (DHV)............................................................................................................... 10
2.6.4. Average Annual Weekday Traffic (AAWT)......................................................................................... 10
2.6.5. Average Weekday Traffic (AWT)......................................................................................................... 10
Passenger Car Unit (PCU)....................................................................................................................................... 10
2.7. Safety................................................................................................................................................................ 11
2.8. Environmental and Human factors ............................................................................................................... 11
2.9. Aesthetics ......................................................................................................................................................... 11
Traffic Volume Forecasting......................................................................................................................................... 11
3. Traffic Engineering.................................................................................................................................................. 12
3.1. Level of Service (LOS) .................................................................................................................................... 12
3.2. Traffic Characteristics.................................................................................................................................... 12

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3.2.1. Static Characteristics ............................................................................................................................. 12
3.2.2. Dynamic Characteristics........................................................................................................................ 12
3.3. Factors Affecting Capacity, Level of Service and Volume........................................................................... 13
3.3.1. Traffic Conditions................................................................................................................................... 13
3.3.2. Roadway Conditions............................................................................................................................... 13
3.4. Traffic Signs..................................................................................................................................................... 14
4. Geometric Design..................................................................................................................................................... 15
4.1. Weaving Sections............................................................................................................................................. 15
4.2. Cross-Section Elements................................................................................................................................... 15
4.2.1. Lane Width (Pavement Width) ............................................................................................................. 15
4.2.2. Shoulder................................................................................................................................................... 16
4.2.3. Median..................................................................................................................................................... 16
4.2.4. Camber.................................................................................................................................................... 17
4.2.5. Curbs and Gutters.................................................................................................................................. 17
4.2.6. Side Walks............................................................................................................................................... 18
4.2.7. Side Slopes............................................................................................................................................... 18
4.2.8. Parking Lane........................................................................................................................................... 18
4.3. Sight Distance .................................................................................................................................................. 19
4.3.1. Stopping Sight Distance (SSD) .............................................................................................................. 19
4.3.2. Passing Sight Distance (PSD)................................................................................................................. 20
4.4. Horizontal Alignment...................................................................................................................................... 21
4.4.1. Design Elements of Simple Horizontal Curve...................................................................................... 21
4.4.2. Transition Curve..................................................................................................................................... 22
4.4.3. Superelevation......................................................................................................................................... 23
4.4.4. Extra Widening....................................................................................................................................... 23
4.5. Vertical Alignment .......................................................................................................................................... 24
4.5.1. Types of Vertical Alignments ................................................................................................................ 24
4.5.2. Elements of Vertical Alignment............................................................................................................. 25
4.5.3. Design of Vertical Alignment................................................................................................................. 25
4.6. Intersection ...................................................................................................................................................... 27
4.6.1. Acceleration/deceleration lanes ............................................................................................................. 28
4.6.2. Forms of At-grade Intersections............................................................................................................ 29
4.6.3. Rotary Intersection................................................................................................................................. 30

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List of Figures
Figure ID. Description Page
3-1 Pavement Markings 14
3-2 Traffic Signs 14
3-3 Traffic Signals 14
3-4 Traffic Markers 14
4-1 Basic & Dual-Purpose Weaving section 15
4-2 Single & Double Lane Width 15
4-3 Highway Cross-Section Elements 16
4-4 Car Maintenance at Shoulder 16
4-5 Median 16
4-6 Types of Camber 17
4-7 Types of Curbs 17
4-8 Curb and Gutter 17
4-9 Side Walks 18
4-10 Side Slope 18
4-11 Parallel Parking Lane 18
4-12 Reaction and Brake Distances of SSD 19
4-13 Passing Sight Distance 20
4-14 Simple, Compound, Broken Back and Reversed Curves 21
4-15 Simple Horizontal Curve 21
4-16 Degree of Curve 21
4-17 Transition Curve 22
4-18 Superelevation 23
4-19 Extra Widening of Horizontal Curve 23

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4-20 Vertical Alignment profile 24
4-21 Sag or Valley Curve 24
4-22 Crust or Summit Curve 24
4-23 Elements of Vertical Alignment 25
4-24 Design of Vertical Alignment 25
4-25 At-grade Intersection 27
4-26 Grade separation Intersection 27
4-27 Interchange 28
4-28 Types of Acceleration lanes 28
4-29 Types of Deceleration lanes 29
4-30 Forms of At-grade Intersections 29
4-31 Rotary Intersections 30

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Abstract
This research article mainly talks about highway geometric design, which includes pre-
design operations such as determining the number of lanes, determining road width,
horizontal alignments, vertical alignments, radius of curvature, intersections ... etc.
article also covers transportation engineering and some of its factors, the design
elements such as the speed of design, the nature of the terrain on which the highway is
located, and safety factors, and some other factors that achieve the highest levels of
service and safety for the user experience
1.Introduction
The Geometrical design of the highways deals with the nature of the visible dimensions
and the setting of the visible features of the highway. The focus of geometric design is
to meet the requirements of the driver and the vehicle such as safety, comfort, efficiency,
smooth-flowing, and crash-free facility…etc. The features generally considered are
elements of cross-section, sight distance, horizontal and vertical alignments, gradients,
intersections, bicycle, and pedestrian lanes. The design of these features depends greatly
on driver behavior, the characteristics of the vehicle, and traffic characteristics such as
speed and size. The right geometric design will help reduce the severity of accidents.
Hence, the goal of the geometric design is to ensure and provide optimum efficiency in
the operation of traffic and maximum safety at an economic cost. We mean here to
reduce costs and improve the quality and function of the highway. The main components
to be discussed are:
• The Design Elements
• Traffic Engineering
• Geometric Design

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2.Highway Design Elements and Considerations
geometric design should provide many considerations for safe and smooth-flowing
operation of vehicles at uniform design speed.
The geometric design elements considerations are:
2.1.Vehicle design and characteristics
2.2.Design speed
2.3.Topography and Nature of Terrain
2.4.Economic Considerations
2.5.Traffic capacity
2.6.Traffic Data
2.7.Safety
2.8.Environmental and Human factors
2.9.Aesthetics
2.1.Vehicle Design and Characteristics
There are different sizes, weights, and operation Characteristics of vehicles using the
highway, so the weight and size of vehicles are very important to determine the
components of the highway such as Radii of Curvature, Parking areas, Pavements…etc.
According to AASHTO (The American Association of State Highway Transportation
Officials), there are 4 general vehicles classes:
1. Passenger-cars 2. Trucks 3. Buses 4. Recreational vehicles
Vehicle(s) gross load should not exceed the weight 𝑾
𝑾 = 𝟏𝟓𝟐𝟓 (𝑳 + 𝟕. 𝟑) – 𝟏𝟒. 𝟕𝑳𝟐
where:
𝑾 = Vehicle Gross Weight (kg)
𝑳 = Distance between The Axles (m)

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TABLE 2-1: DESIGN VEHICLE DIMENSIONS (Egyptian Code of Practice for Roads)
2.2.Design speed
It is the Maximum continuous speed that allows the vehicles to travel safely with smooth
flow on the highway under conducive weather conditions, also it is the most crucial
factor that affects the geometric design of the highway, it affects on vertical and
horizontal alignment, sight distance. There is only one design speed adopted for all
conditions because speed is constantly varied with the variation of conditions (e.g.
driver). there is difference in used speed on the highway because of different speed used
by vehicle types and different drivers, so design speeds must suit all drivers, also higher
design speed affect in the geometric design, which means higher cost, so the design
speed is normally taken 85%.
2.3.Topography and Nature of Terrain
Topography is the second important factor affecting geometric design elements (Such
as vertical and horizontal alignments, sight distance, grades …etc.) after design speed.
The location of the highway depends on the topography of the road; it is easy to design
a road on plan terrain with required standards, but difficult to design a road on hilly or
mountainous terrain, which will increase the construction costs in an uneconomic way.
2.4.Economic Considerations
The geometric design should be economical, the budget should cover the costs as much
as possible. It is well known that efficiency lies in good quality and low cost.

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2.5.Traffic Capacity
The maximum number of vehicles can pass a given section of a lane or a highway in
one or both directions at a given period under the current conditions of road and traffic.
(vehicle/hour)
Traffic Capacity Types:
2.5.1. Basic Capacity
the maximum number of passenger cars that can pass a given point on a lane or a
roadway under ideal highway and traffic conditions that can be obtained, per hour.
2.5.2. Possible Capacity
roadway Under prevailing highway and traffic conditions, per hour.
2.5.3. Practical Capacity
roadway Under prevailing highway and traffic conditions with additional unreasonable
delay, per hour.
Design Capacity
The practical value of capacity of geometric design
𝑪 = 𝟏𝟎𝟎𝟎
𝑽
𝑺
where:
𝑪 = Capacity of a single lane
𝑽 = Design Speed (Kmph)
𝑺 = Average spacing of vehicles

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2.6.Traffic Data
2.6.1. Traffic Volume
The number of vehicles can pass a given section of a lane or a highway in one or both
directions at a given period. (Vehicle/hour)
2.6.2. Annual Average Daily Traffic (AADT)
traffic volume across all days for a year for a given location along a roadway.
Annual No. of Cars / 365 Days (Vehicle/day)
2.6.3. Design Hourly Volume (DHV)
traffic Design Hourly Volume and Peak Hourly Volume (PHV) for a given location
along a roadway.
Daily volume of Cars / 24 Hours (Vehicle/Hour)
2.6.4. Average Annual Weekday Traffic (AAWT)
the average of 24-hour traffic volume on a week over an entire year.
2.6.5. Average Weekday Traffic (AWT)
An average 24-hour traffic volume across weekdays for period less than one year (e.g.
month).
Passenger Car Unit (PCU)
Also known as Passenger Car Equivalent (PCE). used to
calculate the rate of traffic flow by converting different kinds
of vehicles to passenger cars.
Table 2-2: Values of PCU

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2.7.Safety
With an increasing number of accidents every year, attention should be paid to road
safety, which has been a very neglected area of design. it is the designer's responsibility
to ensure that the road environment is as safe as possible.
2.8.Environmental and Human factors
Human beings are complex, varied and influential characteristics. the physical, mental
and psychological properties of drivers and pedestrians, such as response time, are
important to influence geometric design. Also, the geometrical design of roads should
take account of environmental factors such as air pollution, hazards, noise pollution, etc.
2.9.Aesthetics
highway design aesthetics is the art of locating and integrating roads into the
environment for people to use and enjoy. it is a combination of function and beauty.
The product is necessary to match the visual appearance of existing facilities, so that the
aesthetics of the region are not affected.
Traffic Volume Forecasting
the design of new highways or improvement to an existing one should be focused on
the potential traffic expected to use the facilities, not on current traffic volumes. The
forecast year design traffic should not be so far ahead that estimates cannot be made
with reasonable accuracy (15:20 years).
𝑭𝒖𝒕𝒖𝒓𝒆 𝑨𝑫𝑻 = 𝑪𝒖𝒓𝒓𝒆𝒏𝒕 𝑨𝑫𝑻 𝒙 𝑻𝒓𝒂𝒇𝒇𝒊𝒄 𝒑𝒓𝒐𝒋𝒆𝒄𝒕𝒊𝒐𝒏 𝒇𝒂𝒄𝒕𝒐𝒓
𝑻𝒓𝒂𝒇𝒇𝒊𝒄 𝒑𝒓𝒐𝒋𝒆𝒄𝒕𝒊𝒐𝒏 𝒇𝒂𝒄𝒕𝒐𝒓 = (𝟏 + 𝒓)𝒏+𝒙
where:
𝒓 =annual traffic growth rate
𝒏 = traffic analysis period , 𝒙 = required years for construction

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3.Traffic Engineering
3.1.Level of Service (LOS)
a concept to associate traffic quality
with a certain rate of flow.
LOS is based on factors, including:
• Travel time and distance
• Traffic restrictions
• Freedom to maneuver
• Traffic volume to capacity rates.
• Safety and potential accident rate
Table 3-1
3.2.Traffic Characteristics
Traffic characteristics knowledge is necessary for proper geometric design and traffic
management systems. It’s divided into Static and Dynamic characteristics.
3.2.1. Static Characteristics
Vehicle length, width, and height are important to determine design standards for many
physical components of the highway, such as lane width, parking area, clearance,
shoulder width, gradient, and alignments standards.
Vehicle weight is Important for the determination of pavement depth and max grades.
3.2.2. Dynamic Characteristics
Speed, acceleration, braking, and resistance are important in all design elements.

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3.3.Factors Affecting Capacity, Level of Service and Volume
3.3.1. Traffic Conditions
They affected by the composition of different types of vehicles in the traffic flow
variation, stream, interruptions, and lane distribution. The number of lanes, vehicular
and driver characteristics, and one- or two-way direction related the highway capacity.
3.3.2. Roadway Conditions
• Lane Width
A width of 3.65 m for smooth flow is considered ideal. The capacity will be reduced to
25% by smaller widths than this.
• Lateral Clearance
Lateral Obstructions such as road signs, parked cars and light poles works as abutments,
but if they are located nearer than 1.83 meters from the edge of a traffic lane, capacity
decreases.
• Shoulders
It facilitates the flow of traffic.
• Horizontal Alignment
Sharp curves and restrictive sight distances also reduce the lane efficiency.
• Gradient
Gradients affect vehicles' speed, specially trucks, and have a negative effect on capacity.
• Traffic Composition
Traffic composition calculated by converting different kinds of vehicles in the traffic to
passenger cars, to avoid any reduction in speeds or volumes

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3.4.Traffic Signs
Pavement Markings
Figure 3-1
Traffic Signs Traffic Signals
Traffic Markers
50
Multilane, two-direction roads
Stay on your own side of the double
yellow line that's down the center
of the road. If you need to pass
another vehicle, do so using lanes
on your own side of the yellow.
Two-lane, two-direction roads
with some passing
When the solid yellow line is on
your side of the road, you can’t
cross it to pass. On the other side,
you can if it’s safe to do so.
Two-direction roads with a center
turn lane
You can cross a solid yellow line on
your side of the road to get into the
center left-turn lane.
Figure 3-2
Figure 3-3
Figure 3-4

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4.Geometric Design
Geometric Design of Highways is the design of the visible layout of Highway features
as horizontal and vertical alignment, cross-sections, intersections, bike and pedestrian
lanes. The main purpose of geometric design is to produce a road that operates safely,
effectively and economically while preserving its aesthetics and environment.
4.1.Weaving Sections
vehicles entering and leaving the highway at common points without traffic control
devices create weaving.
Basic weaving section serves all traffic weaves.
Dual-purpose weaving section serves weaving and non-weaving traffic.
4.2.Cross-Section Elements
4.2.1. Lane Width (Pavement Width)
The driving safety and comfort are strongly influenced by lane width. require minimum
lane width is 3.75 m for a single lane road. Two lane road require minimum lane width
is 3.5 m for each lane.
Figure 4-1: Basic and Dual-Purpose Weaving section
Figure 4-2: Single & Double Lane Width

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4.2.2. Shoulder
the part of highway between the pavement outer edge and the borrow pit inner edge.
Uses of Shoulders:
• parking area.
• Suitable space for fixing signs.
• Reducing accident.
4.2.3. Median
It is a physical separator between two opposite directions of traffic roads.
Usage of Medians:
• To reduce the Collision lights.
• To eliminate the point of connect, so to
reduce accidents.
Figure 4-3: Highway Cross-Section Elements
Figure 4-4: Car Maintenance at Shoulder
Figure 4-5: Median

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4.2.4. Camber
Camber is the gradient provided to raise center of the road surface in the width direction
to drain the rainwater from the road. the Too steep slope will erode the surface, so it is
unwanted. The common types of camber are parabolic, straight, or combination of them.
Usage of Camber:
Quick drying of pavement which in turn increases safety.
Protection of the subgrade through proper drainage.
Road Surface protection.
4.2.5. Curbs and Gutters
Curb is a short wall built between the roadway and the shoulder to hold the edges
of pavement and act as a barrier between yard and street. It is adjacent to a gutter
which is a flat concrete slab that drains water away from the yard. Usually, curb
and gutter are built together.
Figure 4-6: Types of Camber
Table 4-1: Camber Values of Various Pavements
Figure 4-7: Types of Curbs Figure 4-8: Curb and Gutter

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4.2.6. Side Walks
“Sidewalk” means that portion of a road that is plotted adjacent to property lines and
that is intended for the use by pedestrians and used in urban areas.
4.2.7. Side Slopes
Side slope is defined as the cut or fill slope expressed by the horizontal distance to the
vertical distance ratio. It should not exceed 3:1, but it should be 2:1 on clay slopes.
4.2.8. Parking Lane
Parking lanes for side parking are available on urban roadways. It is recommended to
parallel parking because it is safe for road moving vehicles. The parking lane should
be at least 3.0 m wide in parallel parking.
Figure 4-11: Parallel Parking Lane
Figure 4-9: Side Walks
Figure 4-10: Side Slope

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4.3.Sight Distance
The safe and smooth operating of vehicles on the road significantly depends on the
road visibility in front of driver. Sight distance is the length of road ahead visible to
drivers with an acceptable clarity level. The geometric design must provide a sufficient
distance for drivers to control vehicle speed so that the driver can see any obstacles to
the road's length from a distance ahead or pass a slower vehicle without conflicting
with the opposite traffic.
Sight distance Situations are:
4.3.1. Stopping Sight Distance (SSD)
Stopping sight distance is the minimum sight distance available on a highway at any
position having enough length to intend the driver to stop the vehicle traveling at design
speed, to safely avoid collision with any moving or stationary obstruction in vehicle
path. Minimum SSD is the sum of Reaction Distance (𝒅𝟏) and Brake Distance (𝒅𝟐)
.
(a)Reaction Distance
𝑑1 = 0.278 × 𝑉 × 𝑇
𝑤ℎ𝑒𝑟𝑒
V= vehicle Speed.
T= reaction time.
(b)Brake Distance
𝑑2 =
𝑉2
254 × 𝑓
𝑤ℎ𝑒𝑟𝑒 𝑓= fraction coefficient between vehicle and road.
Speed(v) 50 65 80 100
Reaction Time (T) 3 2.75 2.5 2
Figure 4-12: Reaction and Brake Distances of SSD
Table 4-2: Vehicle Speed and Reaction Time of SSD

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∴ 𝑆𝑆𝐷 = 𝑑1 + 𝑑2 = 0.278 × 𝑉 × 𝑇 +
𝑉2
254 × (𝑓 ± 𝑔)
𝑤ℎ𝑒𝑟𝑒 𝑔= Road grade (+ for Upward, – for Downward)
𝑆𝑆𝐷2 (𝑖𝑛 𝑡𝑤𝑜 𝑤𝑎𝑦 𝑤𝑖𝑡ℎ 𝑠𝑖𝑛𝑔𝑙𝑒 𝑙𝑎𝑛𝑒) = 2 × 𝑆𝑆𝐷
4.3.2. Passing Sight Distance (PSD)
Most rural roads are two-lane two way on which vehicles frequently overtaking slower
vehicles. The PSD is the minimum distance required for the driver vision of a vehicle
to pass-safely a slower vehicle ahead safely against the traffic in the opposite direction
without causing collision with an opposite vehicle. it is determined along the center line
of the road which a driver with his 1.2 m above the roadway eye level can see the top
of a 1.2 m vehicle above the roadway surface. The minimum PSD is the sum of:
• Reaction time distance and during initial acceleration to the passing lane.
𝑑1 = 0.278(𝑉 − 𝑚) × 𝑡° 𝑤ℎ𝑒𝑟𝑒 𝑡° = Reaction time
• Overtaking distance travelled while the passing vehicle at the passing lane.
𝑑2 = 2𝑆 + 0.278(𝑉 − 𝑚) × 𝑡 𝑤ℎ𝑒𝑟𝑒 𝑡 = Overtaking time
𝑆 = 0.2(𝑉 − 𝑚) + 6 𝑚 = difference between Velocities.
• Traversed distance by an opposite vehicle for 𝒕 seconds.
𝑑3 = 0.278 𝑉 × 𝑡
∴ 𝑃𝑆𝐷 = 𝑑1 + 𝑑2 + 𝑑3
𝑚𝑖𝑛 𝑃𝑆𝐷 = 3 × 𝑃𝑆𝐷
𝐷𝑒𝑠𝑖𝑟𝑎𝑏𝑙𝑒 𝑃𝑆𝐷 = 5 × 𝑃𝑆𝐷
Figure 4-13: Passing Sight Distance

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4.4.Horizontal Alignment
4.4.1. Design Elements of Simple Horizontal Curve
Symbol Name Equation
PC Point of curvature
PT Point of tangency
PI Point of intersection
R Radius of curve
L Length of curve 𝐿 = 𝐸𝐶– 𝐵𝐶
𝜟 Deflection angle 𝛥 = 180 − 𝑃𝐼
T Tangent length 𝑇 = 𝑅 × 𝑡𝑎𝑛(𝛥/2)
M Middle ordinate 𝑀 = 𝑅[1 − cos(𝛥/2)]
C Length of long chord 𝐶 = 2𝑅 × 𝑠𝑖𝑛(𝛥/2)
E External Distance 𝐸 = 𝑅[𝑠𝑒𝑐(𝛥/2) − 1]
2𝜋𝑅
360
=
30
𝐷
∴ 𝑅 =
1720
𝐷
where
𝐷: Degree of Curve
𝑅: Radius of Curve
Figure 4-14: Simple, Compound, Broken Back and Reversed Curves.
Figure 4-15: Simple Horizontal Curve
Table 4-2: Elements of Simple Horizontal Curve
Figure 4-16: Degree of Curve

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Elements Effects on the design of the curve:
• Super-elevation ratio.
• Design speed.
• Stopping sight distance.
• Distance between curves.
• Pavement friction coefficient.
• Centrifugal ratio.
4.4.2. Transition Curve
A curve of gradually increasing radius that allows the vehicle to transition easily
between a horizontal curve and line. the radius changes from tangent (R=infinity) to
horizontal curve radius.
𝑪 =
𝟕𝟑
𝑽 + 𝟔𝟒
𝑳𝒔 =
𝑽𝟑
𝟒𝟔. 𝟓 𝑪𝑹
𝑤ℎ𝑒𝑟𝑒
𝑳𝒔: Spiral length
𝑪: Centrifugal acceleration ratio
𝑽: Vehicle speed (km/hr.)
Figure 4-17: Transition Curve

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Figure 4-19: Extra Widening of Horizontal Curve
4.4.3. Superelevation
centrifugal force (the outward pull) of curves which may cause vehicle turning.
Superelevation and side friction are making the vehicle stable on the curve.
Superelvation is the Elevation on the surface of the road when moving from the inside
of the road to the outside.
𝒆 + 𝒇 =
𝑽𝟐
𝟏𝟐𝟕 × 𝑹
𝑤ℎ𝑒𝑟𝑒
𝒇: friction force factor (neglected).
𝑽:Vehicle speed.
𝑹: Curve Radius.
4.4.4. Extra Widening
additional width given to highway curve during construction.
𝑊
𝑒 =
𝑛𝐿2
2𝑅
+
𝑉
9.5√𝑅
Where
𝑛= number of lanes.
𝐿=length of the longer vehicle.
R=radius of the curve.
V= design speed.
Figure 4-18: Superelevation

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4.5.Vertical Alignment
The highway vertical alignment is a combination of grades (straight lines in a vertical
plane) and vertical curves. The vertical alignment is a profile which the horizontal axis
is the horizontal distance or station along the highway centerline and the vertical axis is
the elevation. When these two curves combined, a Convex vertical curve (summit or
crest) or concave vertical curve (valley or sag) formed.
4.5.1. Types of Vertical Alignments
• Crust or Summit Curve
• Sag or Valley Curve
Figure 4-20: Vertical Alignment profile
Figure 4-21: Sag or Valley Curve Figure 4-22: Crust or Summit Curve

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4.5.2. Elements of Vertical Alignment
4.5.3. Design of Vertical Alignment
Symbol Name
PVC Point of vertical curvature
PVT Point of vertical tangency
PVI Point of vertical intersection
L Horizontal length of vertical curve
G1 Tangent grade (location of PVC)
G2 Tangent grade (location of PVT)
G Algebraic difference between G1 and G2
M Mid-point between PVC and PVT
C Mid-point of Vertical Curve
Figure 4-23: Elements of Vertical Alignment
Table 4-3: Elements of Vertical Alignment
Figure 4-24: Design of Vertical Alignment

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𝑪𝒉𝒂𝒊𝒏𝒂𝒈𝒆(𝑨) = 𝒄𝒉𝒂𝒊𝒏𝒂𝒈𝒆(𝑽)–
𝑳
𝟐
𝑪𝒉𝒂𝒊𝒏𝒂𝒈𝒆(𝑩) = 𝒄𝒉𝒂𝒊𝒏𝒂𝒈𝒆(𝑨) + 𝑳
𝑯𝑨 = 𝑯𝑽 + 𝒈𝟏 ×
𝑳
𝟐
𝑯𝑩 = 𝑯𝑽 − 𝒈𝟐 ×
𝑳
𝟐
𝑯𝑬 =
𝑯𝑨 + 𝑯𝑩
𝟐
𝑯𝑪 =
𝑯𝑬 + 𝑯𝑽
𝟐
𝑪𝑽 = 𝑯𝑪 − 𝑯𝑽
𝑯𝑻 = 𝑯𝑨 ± 𝒈𝟏𝑿𝒑
𝒉𝒑 =
𝑮
𝟐𝑳
× 𝑿𝒑
𝟐
𝑮𝒔𝒖𝒎𝒎𝒊𝒕 = 𝒈𝟐 − 𝒈𝟏
𝑮𝒔𝒂𝒈 = 𝒈𝟐 − 𝒈𝟏
Highest/Lowest point on the curve:
𝑿𝒑 =
𝑳𝒈𝟏
𝑮
𝒀𝒑 = 𝑯𝑨 ± 𝒈𝟏𝑿𝒑 ± 𝒉𝒑

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4.6.Intersection
Intersection is an area where two or more highways join or separate, so the vehicles
move to various directions to reach their desired directions. highway intersections are
complex locations. The flow of traffic relies on intersection efficiency. it also influences
the highway's capability. efficiency, safety, speed, cost of operation, and capacity of the
facility depend on Intersections design which are an important part of a highway system.
The three main types of Intersections are:
• at-grade intersections
Same and connected vertical level joining or separating the highway.
• grade separation intersections
different and not connected vertical level.
Figure 4-25: At-grade Intersection
Figure 4-26: Grade separation Intersection

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• interchanges.
Different but connect vertical level joining or separating the highway.
4.6.1. Acceleration/deceleration lanes
Also Known as speed change lanes, Acceleration/deceleration lanes allow drivers to
speed up or slowdown in unused area by high-speed traffic. On highways and several
main roads, the speed change can cause stop-and-go traffic, traffic paralysis, a greater
number of crashes. The merging of speed change lanes in road design can reduce these
problems.
• Acceleration Lanes
acceleration lanes allow cars entering the main road to speed up to suit the traffic flow.
Figure 4-27: Interchange
Figure 4-28: Types of Acceleration lanes

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• Deceleration lanes
Deceleration lanes allow traffic leaving the main road to slow down to a safer speed to
turn at the intersection without impacting with the main traffic flow.
4.6.2. Forms of At-grade Intersections
Figure 4-29: Types of Deceleration lanes
Figure 4-30: Forms of At-grade Intersections

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4.6.3. Rotary Intersection
Figure 4-31: Rotary Intersections

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References
1. “Civil Engineering - Transportation Engineering I.” NPTEL, National Program on Technology Enhanced
Learning, nptel.ac.in/courses/105101087/ .
2. New York State Department of Transportation. “Highway Design Manual.” Department of
Transportation, www.dot.ny.gov/divisions/engineering/design/dqab/hdm.
3. “Roadway Geometric Design II: Cross-Sections and Road Types.” Www.cedengineering.com,
4. AASHTO, 2018. “Policy on Geometric Design of Highways and Streets.” 7th ed.
5. “A Guide on Geometric Design of Road.” Road Engineering Association of Malaysia (REAM): JKR.
6. Purdue University. n.d. “Aesthetics of Highway Design.” [online]
7. James H Banks. Introduction to transportation engineering. Tata Mc-Graw Hill, 2004.
8. Brockenbrough, R. L. Highway Engineering Handbook. Mcgraw-Hill, 2012.
9. Fajardo, Max B. Elements Of Roads And Highways. 5138 Merchandising, 1993.
10. Fwa, T. F. The Handbook Of Highway Engineering. Taylor & Francis, 2006.
11. Garber, Nicholas J, and Lester A Hoel. Traffic And Highway Engineering.
12. Uren, J, and W. F Price. Surveying For Engineers. Macmillan Education, Limited, 2016.
13. Wolhuter, K. M. Geometric Design Of Roads Handbook.
14. Engineering, Highway. "Curves In Alignment Of Highways - Types Of Curves". The Constructor, 2020,
https://theconstructor.org/transportation/types-curves-highway-alignment/21242/.

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