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CEG 307TRANSPORTATION ENGINEERING I

Course Contents:

Introduction to Transportation Engineering;

Design controls and criteria;

Elements of design;

Fundamentals of traffic engineering;

Airport plan and layout;

Aircraft data related to airport classification and design;

Design standards.

Textbooks:

Fundamentals of Transportation EngineeringRobert G. Hennes & Martin Ekse

Transportation Engineering (Planning and Design)R. J. Paquette, N. Ashford & P. H.

Wright

FMWH Highway Manual Part IDesign

Highway EngineeringPaul H. Wright

Policies on Geometric Design of Rural Highways - AASHTO

INTRODUCTION TO TRANSPORTATION ENGINEERING

Definition of transportation

Definition of Transportation Engineering

Classification of transportation engineering namely Transportation Planning and Transportation

Development

Transportation medium includes Land, Air and Water.

Transportation means includes Vehicles, Trains, Ships, Pipelines, Belt Conveyors and Aircrafts.

Transportation systems provide the means of moving from one location to another.

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As shown in the Fig. 1 above, transportation planning is primarily a process of producing

information that can be used by decision makers to better understand the consequences of different

courses of action. The tasks that are part of identifying and assessing these consequences include

the following:

Inventory of Facilities

Transportation engineers and planners must know what the transportation network consists

of and the condition and performance of these facilities. In a state or urban area, much of the

transportation investment is aimed at upgrading the physical conditionof a facility (e.g., repaving a

road or building a new bridge) or improving its performance (e.g., building a new road to serve

existing demand).

Transportation agencies are expected to have a very extensive inventory of road system in

their jurisdiction including number of lanes, type of pavement, the last time the pavement was

replaced, the capacity of the road, accident record, etc. Transit agencies are also expected to have

an inventory of the different assets that constitute a transit system (e.g., buses, stations, shelters, rail

cars, etc.).

Collect and Maintain Socioeconomic and Land Use Data

Land use maps and other sources can be used to collect information such as the number of

trips to schools, shopping centers, residential units, office complexes, etc which can then be used in

transportation planning. Special surveys and census can be used to collect data on different

socioeconomic characteristics of residents living in a community. Such socioeconomic data include

level of income, number of members in the household, number of autos in the household, number of

children, age of head of household, and highest level of education achieved.

Define Goals and Objectives

Goals are generalized statements that indicate the desired ultimate achievement of a

transportation plan. Examples of goals statements might be, The transportation system should

meet the mobility needs of the population or The transportation system should provide enhanced

economic development opportunities

Objectives are more specific statements that indicate the means by which these goals will be

achieved. For example, the goal of meeting the mobility needs of the population could have the

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following objectives associated with it: Provide transit service to major markets in the region,

Reduce congestion on major highways, and Promote bicycle and pedestrian transportation.

Goals and objectives define the evaluation criteria that will be used later in the planning

process to assess the relative impacts of alternative projects and strategies. They also provide an

important linkage to the desires and values of the public that the transportation plan is serving.

Identify System Deficiencies or Opportunities

Transportation planning identifies and prioritizes those elements of the transportation system

where problems exist today or where problems will exist in the future given growth in travel.

Additionally, transportation planning can also identify areas where significant problems do not exist

today, but where changes to the system can provide opportunities for enhanced efficiency of

operation.

Develop and Analyze Alternatives

Once the planning process has identified areas where improvements are needed,

transportation planners define different strategies that could solve the problem. In the past, these

strategies have focused on improvements to highways, such as adding new lanes, improving traffic

control through signals or signing, or improving traffic flow through channelization.

However, other modern strategies that can be used to solve the transportation problem

include reducing the demand for transportation through flexible working hours, and application of

advanced transportation technologies to the operation of a road system, known as intelligent

transportation systems. Such systems might include network surveillance through video cameras,

centralized control centers that can re-route traffic around incidents, and dynamic traffic control

devices that provide coordinated traffic signal timings to maximize the amount of traffic that can

flow through a set of intersections.

Evaluate Alternatives

Evaluation brings together all the information gathered on individual alternatives and

provides a framework to compare the relative worth of the alternatives. In addition, evaluation

includes methods for comparing in an analytical way the relative value of the alternatives. One of

the most used approaches is the benefit/cost ratio, which compares the alternatives on the basis of

discounted benefits and costs.

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DESIGN AND LOCATION OF HIGHWAY

Introduction

The earliest forms of roads (highways) consisted mainly of hard tracks cleared of vegetation

and compacted by human and animal traffic. These were later widened because of heavier trafficand gravels and broken cobblestones were poured onto the tracks to take vehicular traffic. Even

these types of roads were found to be non-satisfactory in performance to ever increasing traffic

volume.

In the face of modern development therefore, the need for more resistant highway brought

about the idea of all-weather roads. These roads not only facilitate movement but also provide more

resistant and durable tracks for comfortable ride.

In Nigeria today, unpaved roads (earth and gravels) are mainly in the rural areas and farm

settlements while paved roads (flexible and rigid) are mainly in urban areas and between towns and

cities.

Classification of Roads

Roads are classified into three groups in Nigeria:

1. Trunk A roads

These are federally maintained roads and usually link the state capitals to the central

administration.

2. Trunk B roads

These are maintained by individual state governments and include all the roads linking the

towns within the state to the state headquarters.

3. Local roads

These are roads under the care of the local government authorities.

The highway types as defined in Highway Manual include the following:

Arterial Highway a general term denoting a highway primarily for through traffic,

usually on a continuous route.

ExpresswayA divided arterial highway for through traffic with full or partial control

of access.

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FreewayAn expressway with full control of access and all grade crossings eliminated.

Major Street or Major Highway An arterial highway with intersections at grade and

direct access to abutting property, and on which geometric design and traffic control

measures are sued to expedite the safe movement of through traffic.

Through Street or Through HighwayEvery highway or portion thereof at the entranceto which vehicular traffic from intersecting highways is required by law to stop before

entering or crossing the same when stop signs are erected.

Local Street or Local Road A street or road primarily for access to residence, business

or other abutting property.

Divided Highway A highway with separated roadways for traffic in opposite

directions.

Toll Road, Bridge, or Tunnel A highway, bridge, or tunnel open to traffic only upon

payment of a direct toll or fee.

Cul-De-Sac StreetA local street open at one end only, and with special provisions for

turning around.

Dead-End StreetA local street open at one end only, without special provisions for

turning around.

Principles of Highway Location

Some detailed guiding principles should be kept in mind in selecting the location for a

highway. The following outline is not in any particular order nor complete. In addition, some of

the elements tend to contradict one another; in practice, the location is selected which represents the

best compromise solution.

1. For the highway to serve its function of allowing convenient, continuous, free-flowing

traffic operation, it should be located where it can best meet the major traffic desire lines

and be as direct as possible.

2.

Keep grades and curvature to the minimum necessary to satisfy the service requirements

of the highway.

3. Avoid sudden changes in sight distance, especially near junctions.

4. Avoid having a sharp horizontal curve on or adjacent to a pronounced vertical curve.

5. In urban areas, site the highway through undeveloped or blighted areas, along the edges

of large parklands, and in general, away from highly-developed, expensive land areas.

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6. In urban areas, locate the highway as closed as possible to the principal parking

terminals.

7. In rural areas, locate as much as possible of the new highway on existing ones, so as to

minimize the use of farmland and reduce total initial and maintenance costs.

8.

Locate along the edges of properties rather than through the middle, so as to cause theminimum interference to cultivation and avoid the need for subway construction.

9. Avoid the destruction or removal of man-made culture.

10. Keep the highway away from cemeteries, places of worship, hospitals, schools and

playgrounds.

11. The effect of the proposed highway on existing ro future utilities above, on or under the

ground should be considered. It may be such as to warrant changes in order to avoid

expensive relocation of these utilities.

12.

Never have two roads intersecting near a bend or at the top or bottom of a hill.

13. In the case of a motorway, the need for an interchange with another road may dictate an

alignment that will intersect the other highway at a place, at an angle and in terrain that

will best permit the interchange to be constructed.

14. Avoid intersections at-grade with railway lines. If possible have the highway pass over

the railway where it goes into a cutting.

15. Seek favourable sites for river crossing. Preferably these should be at right angles to the

stream center line.

16. Do not have a bridge located on or adjacent to a highway curve.

17. Avoid the need for deep cuttings and expensive tunnel construction.

18. Avoid locations where rock is close to the surface, as this will usually require at least

some expensive excavation.

19. In hilly terrain, be aware of the possibilities of landslides.

20. To minimize drainage problems, select a location on high ground in contrast to one in a

valley.

21. Avoid bogs, marshes and other low-lying lands subject to flooding.

22. Locate the highway on soil, which will require the least pavement thickness above it.

23. Locate the highway adjacent to sources of pavement materials.

24. When the needs of all other factors have been satisfied, the best location is the one,

which results in the minimum total cost of earthworks. This means that the minimum

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quantities of excavation should be so balanced with the quantities of embankment as to

require a minimum of haulage with little need for overhaul.

25. In hilly terrain, the highway should cross ridges at their lowest points.

26. Avoid the unnecessary and expensive destruction of wooded areas.

27.

Avoid placing the highway at right angles to the natural drainage channels.28. To relieve the monotony of driving on a long straight road, it is an advantage to site it so

as to give a view of some prominent feature ahead.

Having these guiding principles, the highway engineer will then embark on route location,

which includes Reconnaissance Survey, Preliminary Survey and Final Survey.

Highway Surveys and Location

In the relocation or reconstruction of existing highways and the establishment of new ones,

highway surveys are required for the development of project plans and the estimation of costs.

Highway surveys usually involve measuring and computing horizontal and vertical angles, vertical

heights (elevations), and horizontal distances. The surveys can also be used to prepare base maps

with contour lines and longitudinal cross sections, as required.

Highway surveying techniques have been revolutionized during the past decade due to the

rapid development of electronic equipment and computers. These techniques can be grouped into

three general categories:

a. Conventional and traditional ground survey methods

b. Digital ground survey methods

c. Remote sensing techniques.

The performance of good surveys requires well-trained engineers who have an

understanding of the planning, design, and economic aspects of highway location and who are

sensitive to the social and environmental impacts of highway development.

The tasks involved in the highway location include:

(i) Desk studies

(ii) Reconnaissance survey

(iii) Preliminary survey

(iv) Final location survey

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(i) Desk Studies

A desk study includes the preliminary steps of evaluation of all available data procured in

the form of maps, aerial photographs, mosaic, or charts and may require the application of a large

variety of engineering, environmental, social, and economic knowledge. The type and amount of

data collected during this initial phase will vary with the functional classification of the road and thenature and size of the project.

The categories of desirable data are as follows:

1. Engineering data

i) Topographic and geological maps

ii) Stream and drainage basin maps

iii) Climatic records

iv) Preliminary survey maps of previous projects

v)

Traffic surveys and capacity studies

2. Environmental data

i) Agricultural soil surveys indicating soil erodibility

ii) Water quality studies

iii) Air pollution studies

iv) Noise and noise attenuation studies

v)

Fish and wildlife inventories

vi) Historical studies

3. Social data

i) Demographic and land-use information

ii) Census data, etc.

4. Economic data

i) Overall costs of previous projects

ii) Unit construction cost data

iii) Agricultural, industrial and commercial activities and trends

iv) Property values.

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When all the available data have been assembled, a detailed analysis should reveal

information pertinent to the proposed project. For example, analysis of the available information

may allow the engineer to determine the advisability of selecting an entirely new location or

improving the existing one.

After an exhaustive study of topographic maps, drainage maps, soil maps, and other data ismade in the office, a series of proposed locations may be selected for a field investigation.

(ii) Reconnaissance Survey

The reconnaissance survey consists of a field investigation that usually provides a means of

verification of conditions as determined from the preliminary desk study. For example, building

symbols on maps do not indicate the true values of property under consideration, and this

information can usually be secured by field investigation. A study is made of the profiles and

grades of all alternative routes and cost estimates made for grading, surfacing, structures, and right-

of-way. A comparison of alternative routes in this fashion will aid the final selection of the most

likely location.

(iii) Preliminary Survey

A preliminary survey is made to gather information about all the physical factors that affect

the tentatively accepted route. In general, a regular survey party carries out the work. The raw data

is normally acquired using some of the conventional surveying equipment including:

a) Tapes, Theodolite and level

b) Theodolite and Electromagnetic Distance Measurement

c) Combined Theodolite and EDM system

d) Total Station

Field sheets or field books are required to record all observations by hand. Data loggers are

available for automatic recording of observations when Total Station is used.

A primary traverse or baseline is established as an open traverse consisting of tangent

distances and deflection angles following approximately the line recommended in the

reconnaissance report. Traditionally, conventional ground surveys are carried out by the use of

Theodolite to measure angles in both vertical and horizontal planes, the Levelling instrument for

measuring changes in elevations (heights), and the tape for measuring horizontal distances.

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However, with developments in electronics, EDM mounted on Theodolite or total station can now

be used more effectively for most surveying projects.

When the preliminary line has been established, the topographic features are recorded. The

extent to the right and left of the traverse line to which the topography should be determined will

vary but should not be less than the proposed width of right-of-way.Using the preliminary survey as a basis, a preliminary survey map is drawn. The

preliminary map should show all tangents with their bearings and distances, all deflection angles,

ties to property corners, etc. Certain topographic features such as streams, watercourses, lakes,

hills, and ravines, and man-made features such as buildings, drainage structures, power lines, and

other public facilities are shown on the map.

(iv) Final Location Survey

The final location survey is the detailed layout of the selected route, during which time the

final horizontal and vertical alignments are determined and the final positions of structures and

drainage channels are also determined. The final location survey serves the dual purpose of

permanently establishing the centerline and collecting the information necessary for the preparation

of plans for construction. The line to be established should follow as closely as is practical, the line

drawn on the preliminary map, conforming to the major and minor control points and the alignment

that was previously determined.

The first step in the final location survey requires the establishment of the centerline, which

is used as a survey reference line, upon which property descriptions are based for the purpose of

purchasing right-of-way. Level work is of the utmost importance, because the grade line,

earthwork, and drainage are designed from the level notes. Finally, cross-sections levels are taken

at intervals ranging from 1 to 5m in the transverse direction and longitudinally at regular interval of

25m stations and at any intermediate points with abrupt slope changes.

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ELEMENTS OF GEOMETRIC DESIGN

The essential design features of a roadway or railway are its location and its cross-section.

In the horizontal plane, the locations of points are referenced to a coordinate system in which the

positivey-axis is north and the positive x-axis is east. Positions along they-axis are called latitudes

and those along thex-axis are called longitudes (departures).

Customarily, points along the route are identified by chainages (stations), the distance in

metres from some reference point, commonly the beginning point for the project. The location of

points in the vertical plane (or along thez-axis) is given as the elevation above mean sea level.

The cross section of a roadway is described by its dimensions at a right angle to the direction

of the alignment, including widths, clearances, slopes, and so on.

A CIRCULAR CURVES - GENERAL

A highway route survey is initially laid out as a series of straight lines (tangents). Once the

centerline location alignment has been confirmed, the tangents are joined by circular curves that

allow for smooth vehicle operation at the speeds for which the highway was designed.

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Figure 1Circular curve terminology

Figure 1 illustrates how two tangents are joined by a circular curve and shows some related

circular curve terminology. The point at which the alignment changes from straight to circular is

known as the BC (beginning of curve). The BC is located distance T (subtangent) from the PI

(point of tangent intersection). The length of circular curve (L) is dependent on the central angleand the value of R (radius).

The point at which the alignment changes from circular back to tangent is known as the EC

(end of curve). Since the curve is symmetrical about the PI, the EC is also located distance T from

the PI. From geometry, the radius of a circle is perpendicular to the tangent at the point of

tangency. Therefore, the radius is perpendicular to the back tangent at the BC and the forward

tangent at the EC.

The terms BC and EC are also referred to by some agencies as PC (point of curve or

curvature) and PT (point of tangency), and by others as TC (tangent to curve) and CT (curve to

tangent).

B CIRCULAR CURVE GEOMETRY

Most curve problems are calculated from field measurements (and the chainage of PI) and

from design parameters (R). Given R (which is dependent on the design speed) and , all other

curve components can be computed.

Analysis of Figure 2 will show that the curve deflection angle (PI, BC, EC) is2

and that

the central angle at 0 is equal to , the tangent deflection.

The line (0PI), joining the center of the curve to the PI, effectively bisects all related lines

and angles. For the following derivation of equations, refer to Figure 2.

Tangent: In triangle BC, O, PI,R

T= tan

2

T = R tan2

(1)

Chord: In triangle BC, O, B,R

C2/1= sin

2

C = 2R sin2

(2)

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Figure 2Geometry of the circle

Mid-ordinate:R

OB= cos

2

OB = Rcos2

But OB = R M

R - M = Rcos2

M = R

2cos1 (3)

External: In triangle BC, O, PI, O to PI = R + E

ER

R

= cos

2

E = R

1

2/cos

1 (4)

= R

1

2sec (alternate)

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Figure 3Relationship between the degree of curve (D) and the circle

From Figure 3,

Arc:R

L

2=

360

,

3602

RL (5)

where is expressed in degrees and decimals of a degree.

From Figure 3,

D and R:R

D

2

100

360 ,

RD

58.5729 (6)

Arc:R

D

2

100

360 ,

RD

58.5729 (7)

where D (degree of curve) is defined as the central angle subtended by 100ft of arc.

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C. VERTICAL ALIGNMENT

(a) General

The longitudinal profile of the proposed highway is first drawn as a series of intersecting

gradients. The straights subsequently form tangents to vertical curves, which are fitted to them.The geometric proportions encountered in most cases point to the fact that the simple

parabola is the most convenient to use.

The vertical alignment affects:

a. The construction cost of the project.

b. The operating cost of vehicles using the road.

The vertical alignment must have good correlation with the horizontal alignment and ensure

good sight distances over crests. Care must be taken to avoid:

i. very short sag vertical curves

ii. sharp drop immediately after a long up-grade

iii. short grade between crest and sag curves

iv. combination of two vertical curves in the same directionthey must be replaced by

a single vertical curve.

Grades and Grade Control

The vertical alignment of the roadway and its effect on the safe and economical operation of

the vehicle constitutes one of the most important features of highway and railway design. The

vertical alignment, which consists of a series of straight lines connected by vertical parabolic or

circular curves, is known as the grade line. When the grade line is increasing from the horizontal, it

is known as a plus grade, and when it is decreasing from the horizontal it is known as a minus

grade.

Establishment of vertical alignment

1. An ideal situation is one in which the cut is balanced against the fill without a great deal

of borrow or an excess of cut to be wasted.

2. All hauls should be downhill if possible, and not too long.

3. Ideal grades should have long distances between points of intersection, with long vertical

curves between grade tangents to provide smooth riding qualities and good visibility.

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4. The grade should follow the general terrain and rise and fall in the direction of the

existing drainage.

5. In rock cuts and in flat, swampy areas, it is necessary to maintain higher grades.

6. The presence of grade separations and bridge structures also control grades.

7.

Change of grade from plus to minus (summit curves) should be placed in cuts, andchanges from a minus grade to a plus grade (sag curves) should be placed in fills.

8. Urban projects will usually require a more detailed study of grade controls and a fine

adjustment of elevations than do rural projects.

9. In urban projects, it is best to adjust the grade to meet existing conditions because of

additional expense when doing otherwise.

10. Grades are normally dependent on design speed and topography.

D. VERTICAL CURVES - GENERAL

Vertical curves are used in highway and street vertical alignment to provide a gradual

change between two adjacent grade lines.

Figure 4Vertical curve terminology (profile view shown).

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From Figure 4,

g1 = slope (in percent) of the lower chainage grade line,

g2 = slope (in percent) of the higher chainage grade line,

BVC = beginning of the vertical curve,

EVC = end of the vertical curve,L = length of vertical curve. This is same as the projection of the curve onto a

horizontal surface and as such corresponds to plan distance.

A = algebraic change in slope direction i.e. A = g2g1

There are two types of vertical curves viz: Summit or Sag vertical curve (see Figure 5). The

parabolic curve is used almost exclusively in connecting grade tangents because (1) it has a constant

rate of change of slope, and (2) ease of computation of vertical offsets, which permits easily,

computed curve elevations.

(i) The rate of change of slope of a simple parabola is constant, i.e.

kdx

Yd

2

2

(8)

(ii) The offset (y) from the grade (see Figure 5) is proportional to the square of

distance from tangent point, i.e.,

2Axy (where A = constant) (9)

Integrating equation (8),

Bkxdx

dY (where B is constant) (10)

From Figure 6, it will be noted that:

when x = 0, %1gdx

dY , %1gB ,

when x = L, %2gdx

dY ,

Substitute %2gdx

dY , and x = L in equation (10)

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Figure 5Summit (Crest) and Sag Vertical Curves

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%% 12 gkLg ,L

ggk

)( 12

Equation (10) becomes:

%)%( 112 gxLgg

dxdY

, (11)

Integrating equation (11),

Cxgx

L

ggY

1

212

2

)%( (12)

x = 0 when y = 0, C = 0

From Figure 6,x

Yyg

%1

Equation (12) becomes:

xx

Yyx

L

ggY

2

)%( 212

YyxLggY

2)%(

2

12

L

Axx

L

ggy

2002100

)( 2212

where A = algebraic difference between the two grades measured in percent.

The vertical offset (y) from the grade at any point (x)from the tangent point is given by

L

Axy

200

2

(13)

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Computation of the High or Low Point on a Vertical Curve

The highest point on the vertical curve does not lie vertically below or above the point of

intersection, except in the case where the two grades are equal. The highest point occurs when

the gradient is zero.

This means that equation (11) may be equated to zero, i.e.,

%)%(

1

12 gxL

gg

dx

dY

= 0

%)%(

1

12 gxL

gg

,

A

Lg

x 1

(14)

Thus, the highest point occurs at a point (x) given by equation (14).

Substituting equation (14) into equation (13) to determine the vertical offset (y) at the

highest point gives:

A

Lg

A

Lg

L

Ay

200200

2

1

2

1

A

Lgy

200

2

1 (15)

N.B.- The above formulas apply only for the symmetrical curve, i.e., one in which the tangents

are of equal length. The unequal tangent or unsymmetrical vertical curve is a compound

parabolic curve. Its use generally is warranted only where a symmetrical curve cannot meetimposed alignment conditions.

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E. DESIGN CONTROLS AND CRITERIA

The geometric design of a road is the arrangement of the visible elements of a road, such as

alignment, grades, sight distances, widths, slopes, etc. These elements are influenced by the

following design controls and criteria:

i)

Functional classification of the roadway being designed.ii) Design speed.

iii) Topography.

iv) Cost and available funds.

v) Human sensory capacities of drivers, bikers and pedestrians.

vi) Size and performance characteristics of the vehicles that will use the facility.

vii) Safety considerations.

viii) Social and environmental concerns.

However, the principal design criteria for highways for which there are design standards and

procedures are:

a) Traffic volume

b) Design speed

c) Vehicle Characteristics

d) Highway Capacity.

1. Traffic Volume

The number of vehicles passing a particular section of the road per unit time at a specified

time is called traffic volume. This study can be carried out separately for vehicles and pedestrians

or combined.

The purposes of traffic volume study can be listed as follows:

a. It establishes the importance of any road and thus help in deciding the relative

priority for improvement and expansion.

b. The data are used for planning, designing and regulation phase of traffic engineering.

c. It helps in the design of road pavements, bridges and culverts.

d. It helps in the design of new routes and new facilities.

e. It helps analyse traffic pattern and trend.

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The general unit for reporting traffic using a particular facility is the average daily traffic

(ADT). Numerically, the ADT is the total annual volume of traffic divided by the number of days

in the year. The ADT is readily obtainable where continuous counts of traffic are available. ADT

volumes are useful in economic study of the highway and also in the design of the structural

elements of the road.Counting of traffic may be done mechanically or manually. Photo electric cells, magnetic

detectors, radar detectors and impulse actuated recorders are some of the mechanical or automatic

count devices.

Traffic Projection Factor

Normal increase in traffic volume for long term can be expected to be about 5 per cent

compounded. Traffic projection factor gives the ultimate volume at the end of design period.

According to the Highway Manual of the FMW&H,the Design Hourly Volume (DHV)

should be representative of the future year chosen for design. It should be predicted on current

traffic allowing for normal traffic growth, generated traffic or diverted traffic and development

traffic. A period of 20 years shall be used generally as the basis for design; but this period shall be

reduced to 10 years where stage construction is involved.

Definitions

a. Normal Traffic: - Normal traffic growth is the increase in traffic volume due to

increase in number of transport vehicles.

c. Generated Traffic: - This is the traffic created due to extra facility provided.

d. Development Traffic: - It is the traffic which is due to improvements carried out in

adjacent area.

e. Current Traffic: - It is that traffic which would immediately use a new road or an

improved one when opened to traffic.

The formula used for analyses as developed in the United Kingdom is:

A = 21 rP

Where

A = Number of vehicles per day for design

P = Number of vehicles per day at last census

r = Annual rate of increase in traffic and may be taken as 0.05 (i.e. 0.5%)

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n = Number of years between last census and year of consideration for widening.

2. Design Speed

The design speed is the speed selected for the purpose of correlating those features of the

highways such as curvature, super-elevation and visibility distances for safe operation of vehicles.The design speed is the highest continuous speed at which individual vehicles can travel

with safety upon the highway, when weather conditions are favourable, traffic density is low, and

the design features of the highway are the governing factors of safety.

The design speed is therefore dependent upon:

(ii) The terrain of the proposed route.

(iii) Type and volume of traffic anticipated.

(iv) Type of highway.

(v)

Environmental conditions.

Recommended values are given in Table 1 below:-

Table 1DESIGN SPEED

[Highway Design Manual, 1973]

Type of

Highway Terrain

Design Speed (km/hr)

AASHTO*Minimum Desirable

Limited Access All Terrain 96 112

Unlimited

Access

Level

Rolling

Hilly

96

80

64

112

96

80

112

96

80

*Not recommended

3. Vehicle CharacteristicsVehicle characteristic dimensions are of great importance in the design of parking facilities.

In such cases, where the economy permits, consideration should be given to the possibility of

making provision for one or two doors open.

The layout of roads, especially junctions, must be related to the vehicles using them. Hence,

vehicle sizes are essential in geometric design, especially for sharp radius turns. On the other hand,

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the size, weight and features of legally permitted vehicles govern the standards to be set for lane

width, vertical clearance, pavement thickness and bridge loadings.

Four (4) design vehicles have been adopted to represent the main vehicle types in use (U.S.

practice). The design vehicles are:

P = Passenger carsSU = Single unit or buses

WB 40 = Medium Semi-trailer combination

WB 50 = Large Semi-trailer combination

Typical dimensions of various design vehicles are given in Table 2 below:

Table 2DESIGN VEHICLE

[Highway Design Manual, FMWH]

Design

Vehicle Dimensions (metres)

Type Symbol Wheel Base Overall

Length

Overall Width Height

Passenger Car P 3.4

(11)

5.8

(19)

2.1

(7)

-

Single Unit

Truck (Buses)

SU 6.1

(20)

9.1

(30)

2.6

(8.5)

4.1

(13.5)Small semi-

trailer

combination

WB40 12.2

(40)

15.2

(50)

2.6

(8.5)

4.1

(13.5)

Large semi-

trailercombination

WB50 15.2(50)

16.8(55)

2.6(8.5)

4.1(13.5)

Note: Figures in brackets are in feet.

Weights (tons)

AASHTO Maximum

Single axle - 10 12

Tandem axles - 16 20

Max. Gross Weight - 43 69

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4. Highway Capacity

The Highway Capacity Manual defines the practical capacity (or design capacity) as the

number of vehicles that can pass over a given section of the roadway during one hour under

specified traffic conditions and operating at a level of service.

The level of service depends on:(i) Probability of traffic interruptions

(ii) Desired speed of operation

(iii) Location and type of highway facility.

(iv) Cost of vehicle operation.

(v) Building, operating, and maintenance of the highway.

As shown in Table 3(a) below, the manual recommends maximum practical capacities as follows:-

(i) Two lane road - 900 pcu/hr

(ii) Three lane road - 1500 pcu/hr

(iii) Multi-lane road - 1000 pcu/hr

* pcu = passenger car unit

Table 3(a)DESIGN CAPACITY FOR VARIOUS TYPES OF RURAL ROADS

(Two-Way Total) [British Standards]

Passenger Car Unit per hour (p.c.u./hr)

Type of Road Roads in Rural Areas Highway Capacity Manual

Two-Lane Carriage way 900 900

Three-Lane Carriage way 1500 1500

Dual Two-Lane Carriage way 3300 1000 per lane in the

direction of heavier flowDual Three-Lane Carriage way 5000

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Table 3(b)DESIGN CAPACITY FOR URBAN ROADS

[British Standards]

Type of Road Capacity (p.c.u./hr) Remarks

Four-lane urban motorway(with grade separation)

3000 Capacity for one directionof flow

Six-lane urban motorway(with grade separation)

4500 Capacity for one directionof flow (Highest distributor)

Two-lane all purpose road with

unlimited access

1500 Capacity for both direction

of flow

Three-lane all purpose road

with limited access

2200 Capacity for both

directions

Four-lane all purpose road with

limited access

2400 Capacity for both

directions

Two-lane all purpose road with

capacity restrictions

600 - 750 Two way capacity waiting

vehicles and junctions

F. SIGHT DISTANCE

Under ideal conditions, geometric design standards should ensure that vehicles are mutually

visible within eyesight distance of each other. Highway designs must ensure that the driver has

ample distance of clear vision ahead so that he can avoid hitting unexpected obstacles and can pass

slower vehicles safely. Recommended sight distances are given below in Tables 1 and 2 below.

1) STOPPING SIGHT DISTANCE (Horizontal Alignment)

Sight distance at every point on the highway should be as long as possible but never less

than the minimum stopping sight distance.

The safe stopping sight distance is the minimum distance required for stopping a vehicle

traveling with or near the design speed before reaching a stationary object or vehicle on the

highway. The safe stopping (or non-passing) sight distance can be considered as the sum of two

components viz:

(a) Perception - Reaction Distance

This is the distance covered within the period the stationary vehicle/object is sighted and

actual braking operation starts. The elements, which make up the reaction time, depend upon many

modifying factors and individual driving abilities.

AASHTO recommends P-R time of 2.5 seconds for Rural Roads and 1.5 seconds for Urban

Roads. Therefore, for a rural road, the distance covered during P-R time of 2.5 seconds is d1where

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d1 = vt (1)

where v = speed in (metre/sec)

t = perception-reaction time (seconds)

d1 = distance covered (metres)

If in expression (1), the speed is expressed in km/hr, then

d1 = 5.23600

1000

v

=44.1

v metres

ii) Braking distance

This is the distance, d2, covered during the actual braking operation. This is estimated by

utilizing the principle that the change in kinetic energy is equal to force multiplied by distance.

gf

vd

2

2

2 (2)

whered2 = braking distance (metres)

v = driving speed (metres/sec)

f = coefficient of friction between tyres and road surface

g = acceleration due to gravity = 9.81m/sec2

If the speed is expressed in V km/hr,

f

vd 2.254

2

2

Therefore, safe stopping (non-passing) sight distance

D = d1 + d2

=44.1

v +

f

v

2.254

2

When the vehicle is on a slope q%, the braking distance is modified to

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)(2.254

2

2qf

vd

Upward grades carry positive signs as against negative signs for downward grades. This means that

upward braking distances are shorter than those for downward grades.

Stopping (non-passing) sight distance over crests is the longest distance a driver whose eye

is 1.143m (3.75ft) above the pavement can see the top of an object 0.15m (o.50ft) high on the road.

See Figure 1 for approved AASHTO method of measuring stopping sight distance over crests.

Under the condition where the difference in grade is small, ease of riding and appearance may

demand longer curves than value allowed for sight distance.

Table 1DESIRABLE STOPPING SIGHT DISTANCES

*[AASHTO Recommended Sight Distances]

DESIGN

SPEED(km/hr)

P-R DISTANCE BRAKING DISTANCE STOPPING SIGHTDISTANCE (metres)

Time(sec)

Distance(m)

Coeff. offriction (Wet

surface)

Distance(on level

ground)

ComputedRounded

for Design

40

60

80100110

120

2.5

2.5

2.52.52.5

2.5

27.8

41.7

55.669.476.4

83.3

0.36

0.33

0.310.300.29

0.27

17.5

42.9

81.2131.1164.1

209.8

45.3

84.6

136.8200.5240.5

293.1

45

85

135200240

290

* Note: used mainly for horizontal sight distance

(2) PASSING SIGHT DISTANCE

In the design of the horizontal alignment of two/three lane carriageways, provision should

be made for adequate passing sight distance in order that faster vehicles may overtake slower-

moving vehicles without any fear of head-on collision.

The safe passing sight distance is the distance required to allow safe overtaking at or near

the design speed in the face of an on-coming vehicle.

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Table 2(a): SIGHT DISTANCERURAL ROADS (British Practice)

Carriageway

Design Speed

(Km/hr)

Stopping Sight Distance Overtaking

Sight Distance

(crests)

Minimum

Stopping

Sight Distancek value for

crestk value for

sags

Dual TwoLane 120

105 75 -- 300

Three Lane 100 50 50 450 210Two Lane 80 20 30 360 140

Table 2(b): SIGHT DISTANCEURBAN ROADS (British Practice)

Design Speed(km/hr)

Minimum OvertakingSight Distance (m)

Minimum StoppingDistance (m)

80 360 140

60 270 90

50 225 70

30 135 30

Table 2(c): SIGHT DISTANCES(metres)

DesignSpeed

(km/hr)

STOPPING PASSING

UK AUSTRALIA USA UK AUSTRALIA USA

60 90 80 84 270 300 457

80 140 120 107 360 450 549

100 210 170 145 450 750 640

120 300 250 183 -- -- --

Table 2(d): Highway Manual Design (Federal Republic of Nigeria)

Design Speed

(km/hr)

Passing Sight Distance

(metres)

48 244

64 39680 518

96 610

112 701

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