Road Margins

The portion of the road beyond the carriageway and on the roadway can be generally called road margin. Various elements that form the road margins are given below.

1) Shoulders

Shoulders are provided along the road edge and are intended for accommodation of stopped vehicles, serve as an emergency lane for vehicles and provide lateral support for base and surface courses. The shoulder should be strong enough to bear the weight of a fully loaded truck even in wet conditions. The shoulder width should be adequate for giving working space around a stopped vehicle. It is desirable to have a width of 4.6 m for the shoulders. A minimum width of 2.5 m is recommended for 2- lane rural highways in India.

2) Parking Lanes

Parking lanes are provided in urban lanes for side parking. Parallel parking is preferred because it is safe for the vehicles moving on the road. The parking lane should have a minimum of 3.0 m width in the case of parallel parking.

3) Bus-bays

Bus bays are provided by recessing the kerbs for bus stops. They are provided so that they do not obstruct the movement of vehicles in the carriage way. They should be at least 75 meters away from the intersection so that the traffic near the intersections is not affected by the bus-bay.

4) Service Roads

Service roads or frontage roads give access to access controlled highways like freeways and expressways. They run parallel to the highway and will be usually isolated by a separator and access to the highway will be provided only at selected points. These roads are provided to avoid congestion in the expressways and also the speed of the traffic in those lanes is not reduced.

5) Cycle Track

Cycle tracks are provided in urban areas when the volume of cycle traffic is high. Minimum width of 2 m is required, which may be increased by 1 m for every additional track.

6) Footpath

Footpaths are exclusive right of way to pedestrians, especially in urban areas. They are provided for the safety of the pedestrians when both the pedestrian traffic and vehicular traffic is high. Minimum width is 1.5 m and may be increased based on the traffic. The footpath should be either as smooth as the pavement or smoother than that to induce the pedestrian to use the footpath.

7) Guard rails

They are provided at the edge of the shoulder usually when the road is on an embankment. They serve to prevent the vehicles from running off the embankment, especially when the height of the fill exceeds 3 m. Various designs of guard rails are there. Guard stones painted in alternate black and white are usually used. They also give better visibility of curves at night under headlights of vehicles.

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Kerbs in Pavement

Kerbs indicate the boundary between the carriage way and the shoulder or islands or footpaths. The different types of kerbs are given below.

1) Low or Mountable Kerbs

This type of kerb is provided such that they encourage the traffic to remain in the through traffic lanes and also allow the driver to enter the shoulder area with little difficulty. These kerbs are indicator between the boundary of a road and shoulder. The height of this kerb is about 10 cm above the pavement edge with a slope which allows the vehicle to climb easily. This is usually provided at medians and channelization schemes and also helps in longitudinal drainage.

Fig. 1 Low or Mountable Kerb

2) Semi-Barrier Type Kerbs

When the pedestrian traffic is high, these kerbs are provided. Their height is 15 cm above the pavement edge. This type of kerb prevents encroachment of parking vehicles, but at acute emergency it is possible to drive over this kerb with some difficulty.

Fig. 2 Semi-Barrier Type Kerb

3) Barrier Type Kerbs

They are designed to discourage vehicles from leaving the pavement. They are provided when there is considerable amount of pedestrian traffic. They are placed at a height of 20 cm above the pavement edge with a steep batter. Generally, its height is 23 to 45cm and such kerbs are provided on hills, bridges etc.

Fig. 3 Barrier Type Kerb

4) Submerged Kerbs

They are used in rural roads. The kerbs are provided at pavement edges between the pavement edge and shoulders. They provide lateral confinement and stability to the pavement.

Fig. 4 Submerged Kerb 

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Width of Carriage Way, Width of Formation and Right of Way

Width of Carriage Way

Width of the carriage way or the width of the pavement depends on the width of the traffic lane and number of lanes. Width of a traffic lane depends on the width of the vehicle and the clearance. Side clearance improves operating speed and safety. The maximum permissible width of a vehicle is 2.44 m and the desirable side clearance for single lane traffic is 0.68 m. This require minimum of lane width of 3.75 m for a single lane road (Fig.1). However, the side clearance required is about 0.53 m, on either side and 1.06 m in the center. Therefore, a two lane road require minimum of 3.5 meter for each lane (Fig.1). The desirable carriage way width recommended by IRC is given in Table 1.

Fig. 1 Lane Width of Single and Two Lane Roads

Table 1 IRC Specification for Carriage Way Width

Type of Carriage Way	Width (m)
Single lane	3.75
Two lane, no kerbs	7.0
Two lane, raised kerbs	7.5
Intermediate carriage	5.5
Multi-lane	3.5

Fig. 2 Representation of Various Road Width

Width of Formation/Roadway Width

Width of formation or roadway width is the sum of the widths of pavements or carriage way including separators and shoulders. This does not include the extra land in formation/cutting. The values suggested by IRC are given in Table 2.

Table 2 Width of Formation of Various Classification of Roads

Road Classification	Roadway width in m
	Plain and Rolling Terrain	Mountainous and Steep Terrain
NH/SH	12	6.25 - 8.8
MDR	9	4.75
ODR	7.5 - 9.0	4.75
VR	7.5	4.0

Right of Way/ Land Width

Right of way (RoW) or land width is the width of land acquired for the road, along its alignment. It should be adequate to accommodate all the cross-sectional elements of the highway and may reasonably provide for future development. To prevent ribbon development along highways, control lines and building lines may be provided. Control line is a line which represents the nearest limits of future uncontrolled building activity in relation to a road. Building line represents a line on either side of the road, between which and the road no building activity is permitted at all. The right of way width is governed by:

• Width of formation : It depends on the category of the highway and width of roadway and road margins.
• Height of embankment or depth of cutting : It is governed by the topography and the vertical alignment.
• Side slopes of embankment or cutting : It depends on the height of the slope, soil type etc.
• Drainage system and their size which depends on rainfall, topography etc.
• Sight distance considerations : On curves etc. there is restriction to the visibility on the inner side of the curve due to the presence of some obstructions like building structures etc.
• Reserve land for future widening : Some land has to be acquired in advance anticipating future developments like widening of the road.

The importance of reserved land is emphasized by the following. Extra width of land is available for the construction of roadside facilities. Land acquisition is not possible later, because the land may be occupied for various other purposes (buildings, business etc.) The normal RoW requirements for built up and open areas as specified by IRC is given in Table 3.

Table 3 Normal Right of Way for Open Areas

Road Classification	Roadway width in m
	Plain and Rolling Terrain	Mountainous and Steep Terrain
Open Areas
NH/SH	45	24
MDR	25	18
ODR	15	15
VR	12	9
Built-up Areas
NH/SH	30	20
MDR	20	15
ODR	15	12
VR	10	9

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Camber in Road

Camber or cant is the cross slope provided to raise middle of the road surface in the transverse direction to drain off rain water from road surface and keeps cars from sliding and causing damage to the pavement. It has a lot of benefits that make the road surface last longer. Generally, a camber is a crown-shaped part of the road surface that is made by raising the middle of the road compared to its edges. Most of the time, the rate of camber or cross slope of a road is written as “1 in n,” which means that the transverse slope is in the ratio of 1 vertical to n horizontal or it can be written as a percentage. For example, a road with 2.5% camber has a camber of 1 in 40. Too steep slope is undesirable because it will erode the surface. The common types of camber are parabolic, straight or combination of them. The objectives of providing camber are given below.

• Surface protection especially for gravel and bituminous roads
• Sub-grade protection by proper drainage
• Quick drying of pavement which in turn increases safety

Required camber depends on

• Type of pavement
• Amount of rainfall

The values suggested by IRC for various categories of pavement are given in Table 1. 

Table 1 - IRC Recommended Values of Camber in Road for Different Types of Road Surfaces 

Type of Road Surface	Range of Camber in Areas of
	Low Rainfall	Heavy Rainfall
Cement concrete and thick bituminous surface	1 in 60 or 1.7%	1 in 50 or 2.0%
Water bound macadam and gravel pavement	1 in 40 or 2.5%	1 in 33 or 3.0%
Thin bituminous surface	1 in 50 or 2.0%	1 in 40 or 2.5%
Earth Road	1 in 33 or 3.0%	1 in 25 or 4.0%

Types of Camber in Road 

1) Sloped or Straight Camber 

Straight line camber comprises two slopes that come from the edges and meet in the middle of the carriageway. It is the simplest type of camber. It is easy to build and easy to keep in good shape. 

Fig.1 Sloped or Straight Camber 

2) Parabolic or Barrel Camber 

It is a continuous elliptical or parabolic curve. It provides a level roadway in the centre and gradually rises to a steeper grade on the road’s periphery. Greater drainage efficiency results from the sharper edges of this camber type. Faster vehicles prefer this camber, so it is recommended for city streets. Maintaining and building a camber like this is challenging. Barrel camber is less user-friendly due to its sharper corners. Also, extra curbs are needed because the sharper the edge, the faster it will be damaged if it is not protected.

Fig. 2 Parabolic Camber

3) Composite Camber

Composite camber could be part parabola and part straight line or it could be made up of two straight lines with different slopes. Most of the time, the middle of the road is made to be parabolic and the edges are given straight slopes. It helps to lessen the force of the pressure by making the wheel’s contact area bigger.

Fig. 3 Composite Camber

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Pavement Surface Characteristics – Friction, Unevenness, Light Reflection & Drainage

The features of the cross-section of the pavement influences the life of the pavement as well as the riding comfort and safety. Of these, pavement surface characteristics affect both of these. Camber, kerbs and geometry of various cross-sectional elements are important aspects to be considered in this regard. For safe and comfortable driving four aspects of the pavement surface are important; the friction between the wheels and the pavement surface, smoothness of the road surface, light reflection characteristics of the top of pavement surface and drainage to water.

1) Friction

Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also affects the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles. Skidding happens when the path travelled along the road surface is more than the circumferential movement of the wheels due to friction. Slip occurs when the wheel revolves more than the corresponding longitudinal movement along the road. Various factors that affect friction are given below.

• Type of the pavement (like bituminous, concrete or gravel)
• Condition of the pavement (dry or wet, hot or cold etc.)
• Condition of the tyre (new or old)
• Speed and load of the vehicle

The frictional force that develops between the wheel and the pavement is the load acting multiplied by a factor called the coefficient of friction and denoted as “f”. The choice of the value of this is a very complicated issue since it depends on many variables. IRC suggests the coefficient of longitudinal friction as 0.35 - 0.4 depending on the speed and coefficient of lateral friction as 0.15. The former is useful in sight distance calculation and the latter in horizontal curve design.

2) Unevenness

It is always desirable to have an even surface, but it is seldom possible to have such a one. Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to pavement failures. Unevenness affect the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tyres.

Unevenness index

It is a measure of unevenness which is the cumulative measure of vertical undulations of the pavement surface recorded per unit horizontal length of the road. An unevenness index value less than 1500 mm/km is considered as good, a value less than 2500 mm/km is satisfactory up to speed of 100 kmph and values greater than 3200 mm/km is considered as uncomfortable even for 55 kmph.

3) Light Reflection

White roads have good visibility at night, but caused glare during day time. Black roads have no glare during day, but has poor visibility at night. Concrete roads has better visibility and less glare. It is necessary that the road surface should be visible at night.

4) Drainage

The pavement surface should be absolutely impermeable to prevent seepage of water into the pavement layers. Further, both the geometry and texture of pavement surface should help in draining out the water from the surface in less time.

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Engineering Surveys for Highway Alignment

Before a highway alignment is finalized in a new highway project, engineering surveys are to be carried out. These engineering surveys may be completed in the following four stages.

i) Map Study

ii) Reconnaissance Survey

iii) Preliminary Surveys

iv) Final Location and Detailed Surveys

1) Map Study

If the topographic map of the area is available, it is possible to suggest the likely routes of the road. In India, topographic maps are available from the Survey of India with 15 or 30 meter contour intervals. The main features like rivers, hills valleys, etc. are also shown on these maps. By careful study of such maps, it is possible to have an idea of several possible alternate routes so that further details of these may be studied later at the site. The probable alignment can be located on the map from the following details available on the map.

1. Alignment avoiding valleys, ponds or lakes
2. When the road has to cross a row of hills or mountains, possibility of crossing through a mountain pass
3. Approximate location of bridge site for crossing rivers, avoiding bend of the river, if any
4. When a road is to be connected between two stations, one of the top and the other on the foot of the hill, then alternate routes can be suggested by keeping in view that the design or ruling gradient and the maximum permissible gradient.

2) Reconnaissance

The second stage of engineering surveys for highway alignment is the reconnaissance survey. During the reconnaissance, the engineer visits the site and examines the general characteristics of the area before deciding the most feasible routes for detailed studies. A field survey party may inspect a fairly broad stretch of land along the proposed alternative routes of the map in the field. Only very simple survey instruments are used by the reconnaissance party to collect additional details rapidly, but not accurately. All relevant details which are not available in the map are collected and noted down. Some of the details to be collected during reconnaissance are given below.

1. Valleys, ponds, lakes, marshy land, ridge, hills, permanent structures and other obstructions along the route which are not available in the map.
2. Approximate values of gradient, length of gradients and radius of curves of alternate alignments.
3. Number and type of cross drainage structures, maximum flood level and natural ground water level along the probable routes.
4. Soil type along the routes from field identification tests and observation of geological features
5. Sources of construction materials, water and location of stone quarries
6. When the road passes through hilly or mountainous terrain, additional data regarding the geological formation, type of rocks, dip of strata, seepage flow etc. may be observed so as to decide the stable and unstable sides of the hill for highway alignment.

3) Preliminary Survey

The main objectives of the preliminary survey are:

1. To survey the various alternate alignments proposed after the reconnaissance and to collect all the necessary physical information and details of topography, drainage and soil.
2. To compare the different proposals in view of the requirements of a good alignment
3. To estimate quantity of earth work materials and other construction aspects and to work out the cost of alternate proposals
4. To finalize the best alignment from all considerations the preliminary survey is carried out to collect all the physical information which are necessary in connection with the proposed highway alignment.

The preliminary survey may be carried out by any one of the following methods.

i) Soil Survey

Soil survey is an essential part of the preliminary survey as the suitability of the proposed location is to be finally decided based on the soil survey data. The soil survey conducted at this stage also helps in working out details of earth work, slopes, suitability of materials, subsoil and surface drainage requirements and pavement type and the approximate thickness requirements. All these details are required to make a comparative study of alternate proposals.

ii) Material Survey

The survey for naturally occurring materials likes stone aggregates, soft aggregates, etc. and identification of suitable quarries should be made. Also, availability of manufactured materials like cement, lime, brick, etc. and their locations may be ascertained.

iii) Traffic Survey

Traffic surveys conducted in the region form the basis for deciding the number of traffic lanes and roadway width, pavement design and economic analysis of the highway project. Traffic volume counts of the classified vehicles are to be carried out on all the existing roads in the region, preferably for 24 hours per day for seven days. Origin and destination surveys are very useful for deciding the alignment of the roads. This study may be carried out on a suitable sample of vehicle users or drivers. In addition, the required traffic data may also be collected so that the traffic forecast could be made for 10 to 20 year periods.

iv) Determination of Final Centre Line

After completing the preliminary surveys and conducting the comparative studies of alternative alignments, the final centre line of the road is to be decided in the office before the final location survey. For this, the preliminary survey maps consisting of contour plans, longitudinal profile and cross sections of the alternate alignments should be prepared and carefully studied to decide the best alignment satisfying engineering, aesthetic and economical requirements. After selecting the final alignment, the grade lines are drawn and the geometric elements of the horizontal and vertical alignments of the road are designed.

v) Rapid Method Using Aerial Survey and Modern Technique Using GPS

Aerial photographic surveys and photogrammetric methods are very much suited for preliminary surveys, especially when the distance and area to be covered are vast. The survey may be divided into the following steps.

Taking aerial photographs of the strips of land to be surveyed with the required longitudinal and lateral overlaps. Vertical photographs are necessary for the preparation of mosaics.

1. The photographs are examined under stereoscopes and control points are selected for establishing the traverses of the alternate proposals. The control points are located on the maps.
2. Using stereo-pair observations, the spot levels and subsequently contour details may be noted down on the maps
3. Photo-interpretation methods are used to assess the geological features, soil conditions, drainage requirements etc.

4) Final Location and Detailed Survey

The alignment finalized at the design office after the preliminary survey is to be first located on the field by establishing the centre line. Next detailed survey should be carried out for collecting the information necessary for the preparation of plans and construction details for the highway project.

i) Location

The centre line of the road finalized in drawings is to be transferred on the ground during the location survey. This is done using a transit theodolite and by staking of the centre line. The location of the centre line should follow, as closely as practicable, the alignment finalized after the preliminary surveys. Major and minor control points are established on the ground and centre pegs are driven, checking the geometric design requirements. However, modifications in the final location may be made in the field, if found essential. The centre line stakes are driven at suitable intervals, say at 50 metre intervals in plain and rolling terrains and at 20 metre in hilly terrain.

ii) Detailed Survey

Temporary bench marks are fixed at intervals of about 250 m and at all drainage and under pass structures. Levels along the final centre line should be taken at all staked points. Levelling work is of great importance as the vertical alignment, earth work calculations and drainage details are to be worked out from the level notes. The cross-section levels are taken up to the desired width, at intervals of 50 to 100 m in plain terrain, 50 to 75 m in rolling terrain, 50 m in built up areas and 20 m in hilly terrain. The cross sections may be taken at closer intervals at horizontal curves and where there is abrupt change in cross slopes. All river crossing, valleys etc. should be surveyed in detail up to considerable distances on either side. All topographical details are noted down and also plotted using conventional signs. Adequate hydrological detail is also collected and recorded.

Drawings and Report

Drawings

The following drawings are usually prepared in a highway project.

i) Key Map

Key map should show the proposed and existing roads, and important places to be connected. The size of the plan generally should not exceed 22 x 20 cm. The scale of the map is chosen suitably depending upon the length of road.

ii) Index Map

Index map should show the general topography of the area. The details are symbolically represented. The index map should also be of suitable scale, the size being 32 x 20 cm.

iii) Preliminary Survey Plans

Preliminary survey plans showing details of the various alternate alignments and all information collected should be normally drawn to scale of 10 cm = 1 km to 25 cm =1 km.

iv) Detailed Plan and Longitudinal Section

Detailed plans show the ground plan with alignment and the boundaries, contours at intervals of 1 to 2 m in plain terrain and 3 to 6 m in hills, showing all details including existing structures. A scale of 1/2400 in close country and a scale of 1/1200 may be adopted for detailed plans. The size of the drawing may be A2 size or 60 x 42 cm approximately.

Longitudinal sections should be drawn to the same horizontal scale of the ground as in detailed plan. Vertical scale may be enlarged 10 times of the longitudinal scale. The longitudinal section should show the details such as datum line, existing ground surface, vertical profile of the proposed road and position of drainage crossings.

v) Detailed Cross Section

Detailed cross sections are generally drawn to natural scale of 1 cm = 2.0 to 2.5 m. Cross section should be drawn every 100 m or where there are abrupt changes in level. In hill roads the cross sections should be drawn at closer intervals. The cross-section drawings should extend at least up to the proposed right of way. The cross-section number, the reduced distances and the area of filling and/or cutting should be shown on cross section drawings.

vi) Land Acquisition Plans

Land acquisition plans and schedules are usually prepared from the survey drawings for land acquisition details. These plans show all general details such as buildings, wells, nature of gradients and other details required for assessing the values. The scale adopted may be 1 cm = 40m or less.

vii) Drawings of Cross Drainage and Other Retaining Structures

Detailed design for cross drainage and masonry structures are usually drawn to scale of 1 cm = 1 m. For details of any complicated portion of the structure enlarged scales up to 8 cm = 1 m or up to half full size may be employed. However, the size of drawing should not exceed the standard size. Cross sections of streams should be to a scale of not less than 1 cm = 10 m.

viii) Drawings of Road Intersections

Drawings of road intersections should be prepared showing all details of pavement, shoulders, islands etc. to scale.

ix) Land Plans Showing Quarries, etc.

Where quarries for construction materials are to be acquired for new projects, separate land plans should be prepared. The size of these maps and scales may be similar to those suggested under land acquisition.

Estimates

The project estimates should consist of general abstract of cost and detailed estimates for each major head. If the project work is proposed to be executed in stages, the estimate should be prepared for each stage separately.

Project Report

The first phase of project report soon after completing the preliminary surveys, feasibility and EIA studies is to prepare a 'Feasibility Report'. The Detailed Project Report (DPR) should be prepared after completing all the detailed studies including final location survey, preparation of longitudinal and cross sections, soil and material surveys, drainage studies, etc. The design details of the pavements and all Cross Drainage structures including major bridges should be carried out and the relevant drawings prepared as specified in the terms of reference for the project preparation.

Highway Projects

In a new highway project, the engineer has to plan, design and construct either a network of new roads or a road link. There are also projects requiring redesign and realignment of existing roads of upgrading the geometric design standards. Once a highway is constructed, development takes place along the adjoining land and subsequent changes in alignment or improvements in geometric standards become very difficult. A badly aligned highway is not only a source of potential traffic hazard, but also causes a considerable increase in transportation cost and strain on the drivers and the passengers. Therefore, proper investigation and planning are most important in a road project, keeping in view the present day needs as well as the future developments of the region.

New Highway Project

The new highway project work may be divided into the following stages.

1. Selection of route, finalization of highway alignment and geometric design details.
2. Collection of materials and testing of sub grade soil and other construction materials, mix design of pavement materials and design details of pavement layers.
3. Construction stages including quality control.

i) Route Selection

The selection of route is made keeping in view the requirements of alignment and geological, topographical and other features of the locality. However special care should be taken as regards the geometric design standards of the road for possible upgrading of speed standards in future, without being necessary to realign the road. After the alignment if finalized, the plans and working drawings are prepared.

ii) Materials and Design

The soil samples collected from the selected route during the soil surveys are tested in the laboratory in order to design the required pavement thickness and the design of embankment and cut slopes. The basic construction materials such as selected soil, aggregates etc. are collected from the nearest borrow pits and quarries and stacked along the road alignment after subjecting these materials to the specified laboratory tests. In order to design the mixes for the pavement component layers and to specify quality control test values during road construction, mix design tests are carried out in the laboratory.

iii) Construction

The construction of the road may be divided into two stages,

i) Earth work

ii) Pavement construction

The earth work consists of excavation and construction of the embankments. During the excavation for highway cuts, the earth slopes, their protection and construction of drainage network are taken care of. Highway embankments may be best constructed by rolled fill method by compacting the soil in layers under controlled moisture and density using suitable rollers. In the case of high embankments, the stability of the embankment foundation and slopes and the possible settlement of the embankment with time are to be investigated. The pavement construction is subsequently taken up starting with the preparation of sub grade and the construction of sub base, base and surface courses of the pavement.

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Highway Alignment

Once the necessity of the highway is assessed, the next process is deciding the alignment. The highway alignment can be either horizontal or vertical.

Alignment

The position or the layout of the central line of the highway on the ground is called the alignment. It is an arrangement in a straight line or in correct relative positions. Horizontal alignment includes straight and curved paths. Vertical alignment includes level and gradients. Alignment decision is important because a bad alignment will enhance the construction, maintenance and vehicle operating cost. Once an alignment is fixed and constructed, it is not easy to change it due to increase in cost of adjoining land and construction of costly structures by the roadside. A new road should be aligned very carefully as improper alignment would result in increase in one or more of the following

a) Construction cost

b) Maintenance cost

c) Vehicle operation cost

d) Accident rate

Requirements

The basic requirements of an ideal alignment between two terminal stations are that it should be:

a) Short

b) Easy

c) Safe and

d) Economical

a) Short

It is desirable to have a short (or shortest) alignment between two terminal stations. A straight alignment would be the shortest, though there may be several practical considerations which would cause deviations from the shortest path.

b) Easy

The alignment should be such that it is easy to construct and maintain the road with minimum problems. Also, the alignment should be easy for the operation of vehicles with easy gradients and curves.

c) Safe

The alignment should be safe enough for construction and maintenance from the view point of stability of natural hill slopes, embankment and cut slopes and foundation of embankments. Also, it should be safe for the traffic operation with safe geometric features.

d) Economical

The road alignment could be considered economical only if the total life cycle cost considering the initial cost, maintenance cost and vehicle operation cost is lowest.

Factors Controlling Alignment

It is not always possible to satisfy all these requirements mentioned above. Hence we have to make a judicial choice considering all the factors. For an alignment to be shortest, it should be straight between the two terminal stations. This is not always possible due to various practical difficulties such as intermediate obstructions and topography. A shortest route may have very steep gradients and hence not easy for vehicle operation. Similarly, there may be construction and maintenance problems along a route which may otherwise be short and easy. Roads are often deviated from the shortest route in order to cater for intermediate places of importance or obligatory points.

A road which is economical with lowest initial construction cost, need not necessarily be the most economical in maintenance or in vehicle operation cost. It may also happen that the shortest and easiest route for vehicle operation may work out to be the costliest of the different alternatives from construction view point. Thus, it may be seen that an alignment can seldom fulfil all the requirements simultaneously; hence a judicial choice is made considering all the factors. The various factors that control the alignment are as follows.

1) Obligatory Points

These are the control points governing the highway alignment. These points are classified into two categories. Points through which it should pass and points through which it should not pass. Obligatory points through which the road alignment has to pass are generally due to the topographic and other site conditions including natural obstructions. Some of the examples of this category include location of a mountain pass, suitable location of bridge to cross a river, presence of quarry or an intermediate town to be connected. These obligatory points necessitate deviation of the road alignment from the straight alignment with shortest or easiest path. Some of the examples are:

i) Bridge Site

The bridge can be located only where the river has straight and permanent path and also where the abutment and pier can be strongly founded. The road approach to the bridge should not be curved and skew crossing should be avoided as possible. Thus to locate a bridge the highway alignment may be changed.

ii) Mountain

While the alignment passes through a mountain, the various alternatives are to either construct a tunnel or to go round the hills. The suitability of the alternative depends on factors like topography, site conditions and construction and operation cost.

iii) Intermediate Town

The alignment may be slightly deviated to connect an intermediate town or village nearby.

These were some of the obligatory points through which the alignment should pass.

There are obligatory points through which the road should not pass and these locations may make it necessary to deviate from the proposed shortest alignment. The obligatory points which should be avoided while aligning a road include religious places, very costly structures, unsuitable land etc. Religious places like temple, mosque, church, grave or tomb have been protected by the law from being acquired for any purpose. Acquiring costly structures would mean heavy compensation resulting in increased cost. Marshy, peaty and water logged areas are generally unsuitable for road construction and should be avoided as far as possible. If a marshy land with peaty soil falls on the path of a straight alignment, it may be necessary to deviate the road alignment from the straight path and go around the unsuitable land or pond. The other alternative method is to resort to very expensive construction techniques. The points through which the alignment should not pass are given below.

i) Religious Places

These have been protected by the law from being acquired for any purpose. Therefore, these points should be avoided while aligning.

ii) Very Costly Structures

Acquiring such structures means heavy compensation which would result in an increase in initial cost. So the alignment may be deviated not to pass through that point.

iii) Lakes or Ponds etc.

The presence of a lake or pond on the alignment path would also necessitate deviation of the alignment.

2) Traffic

The road alignment should be decided based on the requirements of road traffic. Origin and Destination study should be carried out in the area and the desire lines be drawn showing the trend of traffic flow. The new road to be aligned should keep in view the desire lines, anticipated traffic flow, classified traffic volume, their growth and future trends.

3) Geometric Design

Geometric design factors such as gradient, radius of curve and sight distances also would govern the final alignment of the highway. If straight alignment is aimed at, often it may be necessary to provide very steep gradients. As far as possible while aligning a new road, the gradient should be flat and less than the ruling or design gradient. Thus, it may be necessary to change the alignment considering the design speed, maximum allowable super elevation and coefficient of lateral friction. It may be necessary to make adjustment in the horizontal alignment of roads keeping in view the minimum radius of curve and the transition curves. The absolute minimum sight distance, which should invariably be made available in every section of the road, is the safe stopping distance for the fast moving vehicles. Also, there should be enough distance visible ahead for safe overtaking manoeuvres of vehicles moving at design speed on the road. Hence the alignment should be finalized in such a way that the obstructions to visibility do not cause restrictions to the sight distance requirements.

4) Economics

The alignment finalized based on the above factors should also be economical. While working out the economics, the factors to be considered are,

i) initial construction cost of the road,

ii) regular and periodic maintenance cost of the road and

iii) vehicle operation cost in future years.

While trying to decrease the initial construction cost, either the future road maintenance cost or vehicle operation cost or both of these may increase considerably. Therefore, while carrying out economic analysis, it is essential to work out overall economics based on life cycle cost of the road project and not consider the initial cost of the road project only.

5) Other Considerations

Various other factors which may govern either the horizontal or vertical alignment of the road are drainage considerations, hydrological factors, political considerations and monotony. The vertical alignment is often guided by drainage considerations. The sub-surface water level, seepage flow and high flood level are the factors to be kept in view, while deciding the highway alignment.

Types of Alignment

1) Horizontal Alignment

Horizontal alignment in road design consists of straight sections of road, known as tangents, connected by circular horizontal curves. It is the design of the road in the horizontal plane. It consists of a series of tangents (straight lines), circular curves and transition curves. It should provide safe travel at a uniform design speed.

2) Vertical Alignment

Vertical alignment is the longitudinal section and it consists of straight grades joined by vertical curves. Vertical alignment specifies the elevations of points along the roadway. Once the road is aligned and constructed, it is not easy to change the alignment due to increase in cost of adjoining land and construction of costly structures by the road side, as the land value increases manifolds once the road is opened to traffic.

Principles of Highway Alignment

The alignment of a highway is a three-dimensional problem measured in x, y, and z coordinates. This is illustrated, from a driver’s perspective, in Fig.1. However, in highway design practice, three-dimensional design computations are cumbersome and the more important thing is the actual implementation and construction. As a consequence, the three-dimensional highway alignment problem is reduced to two two-dimensional alignment problems, as illustrated in Fig. 2. One of the alignment problems in this figure corresponds roughly to x and z coordinates and is referred to as horizontal alignment. The other corresponds to highway length (measured along some constant elevation) and y coordinates (elevation) and is referred to as vertical alignment. The horizontal alignment of a highway is referred to as the plan view, which is roughly equivalent to the perspective of an aerial photo of the highway. The vertical alignment is represented in a profile view, which gives the elevation of all points measured along the length of the highway.

Fig.1 Highway Alignment in Three Dimensions

Fig. 2 Highway Alignment in Two-Dimensional Views

Special Considerations while Aligning Roads on Hilly Areas

During alignment of hill roads, special care should be taken on the following points which pertain to the hill roads.

i) Stability of Hill Side Slopes

While aligning hill roads, special care should be taken to align the road along the side of the hill which is stable. A common problem in hill roads is that of landslides. The cutting and filling of earth to construct roads on hill-side causes steepening of existing slopes and this affect its stability of the hill slopes.

ii) Drainage of Surface and Subsurface Water Flowing from the Hill Side

Numerous hill-side drains should be provided for adequate drainage facility across the road. But the cross-drainage structure being costly, attempts should be made to align the road in such a way that the number of very expensive cross drainage structures is kept minimum.

iii) Special Geometric Standards for Hill Roads

Different sets of geometric design standards are followed on hill roads with reference to gradient, curves and speed, and they consequently influence the sight distance, radius of curve and other related features. The route should enable the ruling gradient to be attained in most of the length, minimizing steep gradients, hair pin bands and needless rise and fall.

iv) Resisting Length

The resisting length of a road may be calculated from the total work to be done to move the loads along the route taking the horizontal length, the actual difference in levels between the two stations and the sum of ineffective rise and fall in excess of floating gradient. In brief, the resisting length of the alignment should have kept as low as possible. Thus, the ineffective rise and excessive fall should be kept minimum.

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