Horizontal Curve

As discussed earlier, the objective of a horizontal curve in a plan is to provide a change in the direction to the central line of a road. When a vehicle negotiates a horizontal curve, the centrifugal force acts horizontally outwards through the centre of gravity of the vehicle. For a certain speed of a vehicle, the centrifugal force is dependent on the radius of the horizontal curve. To keep the centrifugal ratio, $\frac {P}{W}$, within a low limit, the radius of the should be kept correspondingly high.

The centrifugal force which is counteracted by the superelevation and lateral friction is given by,

$e+f=\frac{v^2}{gR}$

If the design speed is fixed for a highway, then the minimum radius to be provided can be found from the above relation.
The ruling minimum radius of a curve for ruling speed $\text{v m/s}$ is given by,

${R}_{ruling}=\frac{v^2}{g(e+f)}$

If minimum speed $v'$ of vehicle is adopted instead of design speed $v$, then minimum radius of horizontal curve ${R}_{min}$ is given by,

${R}_{min}=\frac{(v')^2}{g(e+f)}$

Minimum Radii of horizontal curves for different terrain conditions in $m$

Classification of Roads	Plain Terrain		Rolling terrain		Mountainous Terrain
					Area not affected by snow		Snowbound areas
	Ruling min	Absolute min	Ruling min	Absolute min	Ruling min	Absolute min	Ruling min	Absolute min
NH & SH	360	230	230	155	80	50	90	60
MDR	230	155	155	90	50	30	60	33
ODR	155	90	90	60	30	20	33	23
VR	90	60	60	45	20	14	23	15

Classification of Roads	Steep Terrain
	Area not affected by snow		Snowbound areas
Ruling min	Absolute min	Ruling min	Absolute min	Ruling min
NH & SH	50	30	60	33
MDR	30	14	33	15
ODR	20	14	23	15
VR	20	14	23	15

Widening of Pavement on Horizontal Curves

When a vehicle travelling at high speed negotiates a horizontal curve, there is a tendency of a vehicle to move outward. To have a safe negotiation of vehicles sometimes the width of the pavement at the curve is widened because of the following reasons:

1. Whenever a vehicle goes around a horizontal curve, it goes off track due to rigid wheelbase of the vehicle.
2. If the vehicle is moving at a very high speed, superelevation and lateral friction developed won't be able to fully counteract the outward thrust due to the centrifugal force which may lead to transverse skidding and rear wheel may take the outward path instead following the front wheel path.
3. In the case of trailer units, the path traced by the units is also likely to be on either side of the central path of towing vehicle.
4. While negotiating a horizontal curve, a driver has a tendency to use the outer side of the curve to have a greater visibility at the curve.
5. While two vehicle cross or overtake at horizontal curve there is a psychological tendency to maintain a greater clearance between the vehicles.

The extra widening of pavement on horizontal curves is divided into two parts:

1. Mechanical Widening: The widening required to account for the off-tracking due to the rigidity of wheel base is called mechanical widening.
   ${W}_{m}=\frac{nl^2}{2R}$
        $n=\text{number of lanes}$
        $l=\text{Length of the wheel base}$ (normally taken as 6 m for commercial vehicles, if not known)
        $R= \text{Radius of the horizontal curve, m}$


2. Psychological widening: Extra widening is also provided for psychological reasons such as, to provide for greater manoeuvrability of steering at higher speeds, to allow for the extra space requirements for the overhangs of vehicles and to provide greater clearance for crossing and overtaking vehicles on the curves.
   
   ${W}_{ps}=\frac{v}{9.5\sqrt{R}}$
   

If radius is more than 300 m than there is no need to provide extra widening.

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Superelevation

The objective of a horizontal curve in a plan is to provide a change in the direction to the central line of a road. When a vehicle negotiates a horizontal curve, the centrifugal force acts horizontally outwards through the centre of gravity of the vehicle. This centrifugal force is counteracted by the transverse frictional resistance developed between the tyres and the pavement surface which enables the vehicle to change the direction along the curve and to maintain the stability of the vehicle.

Centrifugal Force, P $P=\frac{W\times{v^2}}{gR}$

     $W=$ Weight of the vehicle, KG
     $v=$ Speed of vehicle, $m/sec$
     $R=$ Radius, $m$
     $g=$ Acceleration due to Gravity $=9.81 m/{sec^2}$

The ratio of the centrifugal force to the weight of the vehicle, $\frac {P}{W}$ is known as the centrifugal ratio or the impact factor which is equal to $\frac{v^2}{gR}$.

The centrifugal force acting on a vehicle negotiating a horizontal curve has 2 effects:

1. Overturning Effect:
   
   
   
   The equilibrium condition for overturning will occur when $Ph=\frac{Wb}{2}$ or when $\frac{P}{W}=\frac{b}{2h}$. This means that there is a danger of overturning when the centrifugal ratio $\frac{P}{W}$ or $\frac{v^2}{gR}$ attains a value of $\frac{b}{2h}$.


2. Transverse Skidding Effect:
   
   The centrifugal force developed has also the tendency to push the vehicle outwards in the transverse direction. If the centrifugal force $P$ developed exceeds the maximum possible transverse skid-resistance due to friction, the vehicle will start skidding in the transverse direction.
   In other words, when centrifugal ratio $\frac{P}{W}$ attains a value equal to the coefficient of lateral friction there is a danger of lateral skidding.

Condition for no overturning and no skidding,

$\frac{P}{W}=\frac{v^2}{gR} < \frac{b}{2h}$

$\frac{P}{W}=\frac{v^2}{gR} < f $

To counteract the effect of centrifugal force and to reduce the tendency of overturning and skidding of the vehicle, the outer edge of the pavement is raised with respect to the inner edge, thus providing a transverse slope throughout the length of the horizontal curve. This transverse inclination to the pavement surface is known as Superelevation or Cant or Banking.

The superelevation $'e'$ is expressed as the ratio of the height of outer edge with respect to the horizontal width.

$e=\frac{E}{B}=tan\theta$

The forces acting on a vehicle while traversing a circular curve of radius $\text{R m}$ at a speed of $\text{v } m/sec$ are:

1. The centrifugal force, $P=\frac{W\times{v^2}}{gR}$ acting horizontally outwards through the centre of gravity, CG.
2. the Weight, $W$ of the vehicle acting vertically downwards through the CG.
3. The frictional force developed between the tyres and the pavement counteracts transversely along the pavement surface towards the centre of the curve.

After analysing the forces shown in the figure, we will get result as,

$e+f=\frac{v^2}{gR}$

The value of $f$ is recommanded by IRC is $0.15$

Maximum Superelevation

IRC has recommended the maximum value of superelevation to take care of the mixed flow condition and the loading condition of veḥicles as a truck/bullock cart can be loaded with less dense material such as cotton due to which the CG of the vehicle will be high and it will not be safe for it to move on a road with high rate of superelevation.

Recommended Maximum Values of Superelevation

For plain and rolling	7.0%
Hills bound by snow	7.0%
Hills not bound by snow	10.0%
Urban roads with frequent intersections	4.0%

Minimum Superelevation

Minimum superelevation is required to drain off the surface water. If the calculated superelevation is comes out to be less than the minimum camber of the road surface then the minimum superelevation will be equal to the minimum camber to be provided.
IRC has given the values of radii beyond which normal cambered section may be maintained and no superelevation is required for curves,

Design Speed	Radius (m) of horizontal curve for camber of
KMPH	4%	3%	2.5%	2%	1.7%
20	50	60	70	90	100
25	70	90	110	140	150
30	100	130	160	200	240
35	140	180	220	270	320
40	180	240	280	350	420
50	280	370	450	550	650
60	470	620	750	950	1100
80	700	950	1100	1400	1700
100	1000	1500	1800	2200	2600

Superelevation Design

The design of superelevation for a mixed traffic condition is complex problem because different vehicles run on the road with different speeds. If we rely fully on to counteract the centrifugal force and neglect the lateral friction and provide maximum superelevation, it would be benefical for fast moving vehicles but might cause problem for slow moving vehicles on the other hand if we rely fully on to counteract the lateral friction and provide minimum superelvation, it would be unsafe for fast moving vehicles.
As a compromise and from practical consideration it is suggested that the superelevation should be provided to fully counteract the centrifugal force due to 75 percent of the design speed (by neglecting the lateral friction developed).

Steps for superelevation design:

1. Calculate Superelevation for 75 percent of design speed $(\text{v } m/{sec})$ by neglecting lateral friction
   
   

   $e=\frac{{0.75 \times {v}}^2}{gR}$



2. If the value of $e$ calculated above comes out to be less than 0.07 or 7 percent then the value is so obtained is provided.
   If it is greater than 0.07 then the maximum superelevation equal to 0.07 will be provided and proceed to step 3 or 4.


3. Calculate the value of lateral friction for the maximum value of $e=0.07$
   
   

   $f=\frac{{v}^2}{gR}-0.07$

   
   If the value of $f$ calculated above is less than 0.15, the superelevation of 0.07 is safe otherwise, calculate the restricted speed as shown in step 4.


4. Restricted/Allowable speed at curve,
   
   

   $e+f=0.07+0.15=0.22=\frac{{{v}_{a}}^2}{gR}$
   ${v}_{a}=\sqrt {0.22gR}$

If the allowable speed is greater than the design speed, the design is adequate and provide a superelevation of 0.07. If speed is less than the design speed, the speed is limited to the allowable speed, ${v}_{a}$.
Appropriate warning sign and speed limit regulation sign are installed to restrict and regulate the speed at such curves when the safe speed ${v}_{a}$ is less than the design speed $v$.

Attianment of Superelevation

The road cross section at the straight portion is cambered with the crown at the centre of the pavement and sloping down towards the edges. But the cross-section in the circular curve portion of the road is superelevated with uniform tilt sloping down from the outer edge of the pavement up to the inner edge.
The crowned camber section at the straight before the start of the transition curve should be changed to a single cross slope equal to the desired superelevation at the beginning of the circular curve.

The attainment of superelevation may be split up into two parts:

1. Elimination of Crown of the cambered section:
   
   This can be done by two methods:
   
   1. Rotation of outer edge about the crown: In this method, the outer half of the cross slope is rotated about the crown at a desired rate such that the surface falls on the same plane as the inner half and the elevation of the centre line is not altered.
   2. Diagonal Crown Method: In this method, the crown is progressively shifted outwards, thus increasing the width if the inner half of cross-section progressively. This method is not usually adopted as a portion of the outer half of the pavement has increasing values of negative superelevation on to a portion of the outer half, before the crown is eliminated.
   
   
   


2. Rotation of pavement to attain full superelevation:
   
   When the crown of the camber is eliminated, the superelevation available at the section is equal to the camber. But the superelevation to be provided at the beginning of the circular curve may be greater than the camber. Hence the pavement section is further rotated till the desired superelevation is obtained and this can be attained by two methods.
   
   
   1. By rotating the pavement cross section about the centre line. In this method, the vertical profile of the pavement remains unchanged and it also balanced the earthwork. But this method creates drainage problem if the subgrade is not in an embankment.
   2. By rotating the pavement cross section about the inner edge of the pavement section raising both the centre as well as the outer edge of the pavement. This method does not create drainage problem however additional earth is required as the entire shoulder is rotated along with the cross slope.

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Sight Distance

Sight distance is the actual distance available from a point along the road surface which a driver from a specified height above the road surface has visibility of a stationary or moving object. In other words, sight distance is the length of road visible to the driver at any instant.

Three sight distance situations are considered in the design:

1. Stopping Sight Distance (SSD)
2. Safe Overtaking Distance
3. Safe Sight Distance for entering into uncontrolled intersections

Sight Distances considered by IRC in Highway design:

1. Intermediate Sight Distance which is equal to twice the stopping sight distance.
2. Head Light Sight Distance is the distance visible to a driver during night driving under the illumination of the vehicle head lights. It is critical at up-gradients and at the ascending stretch of the valley curves.

Stopping Sight Distance

The minimum sight distance available on a highway at any spot should be of sufficient length to stop a vehicle travelling at design speed, safely without collision with any other obstruction.
IRC has suggested the height of eye level of a driver as 1.2 m and height of the object as 0.15 m above the road surface.

Stopping Sight Distance depends on the following factors:

• Total reaction time of Driver
• Speed of vehicle
• Efficiency of brakes
• Frictional resistance between tyres and the road
• Gradient, if any.

$\text{SSD = Lag Distance + Braking Distance}$

Lag Distance is the distance covered by the vehicle during reaction time and IRC recommended 2.5 Seconds as total reaction time.

$\text{Lag distance} = v\times t$

Braking Distance is the distance covered by the vehicle after the application of the brakes.

$\text{Braking Distance}=\frac{v^2}{2gf}$

$SSD=v\times t+\frac{{{v}^{2}}}{2gf}$

        $v=$ Speed of vehicle in $m/sec$
        $f=$ Coefficient of friction (0.4 to 0.35 depending on speed, from 30 to 80 KM/H)
        $g=$ Acceleration due to gravity $=9.81 m/sec^2$

Stopping Sight Distance at slopes,

$SSD=v\times t+\frac{{{v}^{2}}}{2g\left( f\pm \frac{n}{100} \right)}$

        $n=$ slope of the pavement
        $+$ is for Ascending Gradient and $-$ is for Descending Gradient.

Over Taking Sight Distance (OSD)

Different vehicles travel at different speeds and this condition is called mixed traffic condition. Some of the vehicles move at a speed which is below design speed while some vehicles move at a speed higher than design speed, in such circumstances, it is necessary for the fast moving vehicles to overtake the slow-moving vehicles.

The minimum distance visible to the driver of a vehicle intending to overtake a slow vehicle ahead with safety against the traffic of opposite direction is known as the minimum overtaking sight distance or the safe passing sight distance available.

Overtaking Sight Distance depends on following factors:

1. Speed of
   
   • Overtaking Vehicle
   • Overtaken Vehicle
   • Vehicle coming from opposite direction, if any
2. Distance between the overtaking and overtaken vehicles
3. Skill and reaction time of the driver
4. the rate of acceleration of overtaking the vehicle
5. Gradient of the road, if any

$OSD={d}_{1}+{d}_{2}+{d}_{3}$

     ${d}_{1}=$ Distance travelled by the overtaking distance during the reaction time $t$ from position $1$ to $2$
             $t$ can be taken as $2sec$
    ${d}_{2}=$ Distance travelled by the overtaking vehicle from position $2$ to $5$during the actual overtaking operation in              time $T$.
    ${d}_{3}=$ Distance travelled by oncoming vehicle from position $6$ to $5$ during overtaking operation in time $T$.

${d}_{1}={v}_{s}\times{t}={v}_{s}\times{2}$

${d}_{2}=b+2s$

$b={v}_{s}\times{T}$

$T=\sqrt{\frac{4s}{a}}$,       $s=0.7\times {{v}_{s}}+6$

${d}_{3}=v\times{T}$

     ${v}_{s}=$ Speed of slow (overtaken)vehicle in $m/sec$
     $v=$ Speed of overtaking vehicle in $m/sec$
     $a=$ acceleration in $m/{sec^2}$

$OSD=({v}_{s}\times{t})+({v}_{s}\times{T}+2s)+(v\times{T})$

The minimum length of overtaking zone should be 3 times the safe OSD and desirable length of the zone is 5 times the safe OSD.

Intermediate Sight Distance (ISD)

Sufficient overtaking sight distance should be available on most of the road stretches however on horizontal curves the OSD requirement may not be always fulfilled. In such cases overtaking should be prohibited by regulatory signs.
At vertical summits where OSD cannot be provided, Intermediate sight distance (ISD) equal to 2 times the SSD may be provided.

Sight Distance at Intersections

It is important that on all approaches of intersecting roads there is a clear view across the corners from a sufficient distance so as to avoid collision of vehicles. It is more important for uncontrolled intersections.

The design of sight distance at intersection may be based on the 3 possible conditions:

1. Enabling the approaching vehicle to change speed: The sight distance should be sufficient to enable either one or both the approaching vehicles to change speed to avoid the collision. The vehicle approaching from the minor road should slow down.
   The total reaction time required for the driver to decide to change speed may be taken as 2 seconds and at least one more second will be needed for making the change in speed.


2. Enabling approaching vehicle to stop: The sight distance should be sufficient to enable either one or both the approaching vehicles to stop to avoid the collision.
   It is the responsibility of the drivers on the minor road who would cross or enter the main road, to stop or change speed to avoid the collision. The traffic of the minor road is generally controlled by an appropriate traffic sign.


3. Enabling stopped vehicle to cross the main road: This case is applicable when the traffic on the minor road is controlled by a stop sign and so these vehicles have to stop before they can proceed to cross the main road. In this case, sufficient sight distance should be available to start the stopped vehicle, accelerate and cross the main road before another vehicle travelling at its design speed on the main road reaches the intersection.

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Geometric Design of Highway

There are several design factors on which the geometric design of a highway depends, the most important are as follows:

1.  Design Speed
2. Topography
3. Traffic Factors
4. Design hourly volume and capacity
5. Environmental and other factors

Highway Cross Section Elements

The important surface characteristics of the pavement are the friction, unevenness, light reflecting characteristics and drainage of surface water.

1. Friction
   
   The friction between the tyres and the pavement is one of the main factors in the determination of operating speed and stopping sight distance of vehicles. Whenever a vehicle negotiates a horizontal curve lateral friction comes in to play and counteract the centrifugal force. The coefficient of friction or skid resistance offered by the pavement surface under various driving and surface condition is important with reference to the safety.
   A skid occurs when the slide without revolving or when the wheels partially revolve i.e., when the path travelled along the road surface is more than the circumferential movements of the wheel due to their rotation.
   A Slip occurs when a wheel revolves more than the corresponding longitudinal movement along the road.
   
   

   Factor affecting Friction:

   • Type of pavement surface (Cement concrete, Bituminous, WBM etc)
   • Its relative roughness
   • Condition of the pavement (Wet, Dry, Smooth, Rough etc)
   • Type and condition of tyre (New with good treads or smoothened or worn out)
   • Speed of vehicle
   • Brake efficiency
   • Load and tyre pressure
   • Temperature of tyre and pavement
   • Type of skid, if any
   
   Indian Road Congress (IRS) has recommended a value of 0.35 to 0.40 for the longitudinal coefficient of friction depending on the speed and a value of 0.15 in case of lateral friction.


2. Pavement Unevenness
   
   Even pavements are desirable to have higher operating speed, less tractive resistance, less fuel consumption and less fatigue and accidents.
   The pavement surface condition is measured by Bump Integrator in terms of unevenness index, which is the cumulative measure of vertical undulations of the pavement recorded per unit horizontal length of the road.
   A value less than 150 cm/KM is desirable for high-speed highways and a value of 250 cm/KM is satisfactory up to a speed of 100 KM/H and a value more than 350 cm/KM is considered very uncomfortable even at speed of 50 KM/H.


3. Light Reflecting Characteristics
   
   Night Visibility very much depends upon the light reflecting characteristics of the pavement surface. The glare caused by the reflection of the head light on Wet pavements is considerably more than on the dry pavement.
   Light coloured or white pavement surface gives good visibility at night particularly during rains but they produce glare and eye strain during bright sunlight.
   Blacktop pavement surface provides very poor visibility at night, especially when the surface is wet.


4. Cross Slope or Chamber
   
   Cross Slope is the slope provided to the road surface in the transverse direction to drain off the rainwater from the road surface.
   
   Camber is provided because of the following reasons:
   • To prevent the entry of surface water into the subgrade soil.
   • To prevent the entry of water into the bituminous pavement layers.
   • To drain off the rainwater as soon as possible and to allow the pavement to get dry soon after the rain.
   
   Camber of a pavement depends on:
   
   • Type of pavement surface
   • Amount of Rainfall
   
   
   
   
   

Width of pavement or Carriageway

The width of the pavement depends on the number of traffic lanes and width of the traffic lane. The maximum width of a vehicle as per IRC specification is 2.44 m.

Width of Carriageway

Class of Road	Width of Carriageway (m)
Single Lane	3.75
Two lanes, without raise kerbs	7.0
Two lanes, with raised kerbs	7.5
Intermediate carriageways (except on important roads)	5.5
Multi-Lane carriageway	3.5 per lane

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Roads in India

The roads in India have been in existence for at least 3000 years. In the 4th century the prime minister of Emperor Chandra Gupta Maurya, B.C. Kautilya has mentioned rules about the depth of road for different kinds of traffic in his book "Arthasastra". Later in the 5th century Asoka, the Great had improved the roads.

During the Mughal periods, the roads of India were greatly improved. Some of the highways either built or maintained by Mughals received great appreciation from the foreign visitors who visited India during that period.

Jayakar Committee

After the first world war, the use of motor vehicles increased and the demanded a better road network which can carry both bullock cart traffic and motor vehicles. Hence in 1927, the govenment appointed a Road development Committe headed by M.R. Jayakar.

Recommendations made by the committe:

1. Central Government should take the complete charge of roads as it is a matter of national interest.
2. It suggested to found "Central Road Fund" to create road development fund with an extra tax levied on petrol from road users.
3. It also suggested to found a semi official technical body to pool know how from various parts of the country and to act as an advisory body on various aspects of roads.
4. A research institute should be instituted to carry out research and development work and to be available for consultations.

Most of the recommendations were accepted by the government. The Central Road Fund was formed by the year 1929 and the semi official technical body called Indian Road Congress was formed in 1934 and Central Road Research Institute was formed in 1950.

Motor Vehicle Act

The Motor Vehicle Act was brought into force by the government in 1939 to regulate the road traffic in the form of traffic laws, ordinance and regulations.

The Motor Vehicle Act has been revised in the year 1988.

20 Year Road Development Plan

The Nagpur Plan (1943-63)

 A conference of the Chief Engineers of all the states and provinces was convened in 1943 by the Government of India at Nagpur.

Features of Nagpur Plan

1. National Highways which would connect several states and provinces and would be of national importance for strategic, administrative and other purpose.
2. State highways which would be connect imporant places within a state.
3. District Roads which would take traffic from main roads to the interior of each district.
   District Roads were further divided into Major District Roads and Other District Roads.
4. Village Roads which would connect a village or a group of villages to the road system.

Targets of Nagpur Plan:

1. 2 Lakh kms of road across the country within 20 years.
2. Construction of star and grid pattern.
3. Road density of 16 kms per 100 sq km.

 The Bombay Plan (1961-81)

 The second 20 year road development plan was initiated by the IRC and finalised in 1959 for the period 1961-81. This road development plan is known as Bombay Road Plan.

Targets of Bombay Plan: 

1. Total road length to be construct was about 10 Lakh kms.
2. Road density of 32 kms per 100 sq km.
3. 1600 KM of expressway
4. Traffic engineering cells in each state. 

The Lucknow Plan (1981-2001)

The 3rd 20 year development road plan was prepared by Road Wing of the Ministry of Shipping and Transport. 

Targets of Lucknow Plan:

1.  Road length to be construct by the year 2001 was 27 Lakh.
2. Road density of 82 km per 100 sq km.
3. National Highway of a square grid of 100 KM X 100 KM.
4. All weather road to connect villages or group of villages with a population of 500 and above. For villages with population less than 500, an all weather road shall be available at a distance of less than 3 km in plain areas and 5 km in hilly terrain.
5. Expressway to be constructed on major traffic highways.
6. The Major District Roads to connect all towns and villages with a population of 1500 and above.
7. The other District Roads to connect  villages with a population of 1000 to 1500.

 

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Early Roads

Roads have a very long history, Brick-paved streets were used in India as early as 3000 BC. Roads in the towns were straight and long, intersecting one another at right angles.

Only during the period of the Roman empire, roads were constructed in large scale. Many of the Roman roads were of elaborate construction. Some of these still exist after over 2000 years. The Appian Way was built in 312 BC extending over 580 KM which demonstrates the road building technique used by Romans.

Roman roads were built straight and they neglected the natural gradient. It was built after the soft soil was removed and hard strata were reached and that's why the depth of the pavement was as high as 0.75 m to 1.2 m although the load was very low. The wearing course was made of dressed large stone-blocks set in lime mortar.

Tresaguet Construction

In the18th century Pierre Tresaguet developed an improved method of construction of roads in France. 

The main feature of his method was that the thickness of construction need be only in the order of 30 cm. He considered the fact that drainage of water along the pavement surface and subsurface moisture content control the life of the pavement.

                                             Tresaguet Construction  (Source: Kullabs)

Telford Construction

Thomas Telford was the founder of the Institution of Civil Engineers in London. To keep the soil subgrade firm he used heavy foundation stones. He provided level subgrade of width 9m. He provided a definite cross slope for top surface by varying the thickness of the foundation stones.

                                                    Telford Construction (Source: Kullabs)

Macadam Construction

John Macadam was a Scottish road builder. He was the first one to notice that the use of large stone foundation is baseless since the stress below the wheels decreases as the depth increases. He also gave importance to the subgrade drainage and compaction. 

                                               Macadam Construction (Source: Kullabs)

Macadam's method of construction was recognised as a scientific method of construction and hence adopted by various countries with slight modifications.

One of the most popular methods which is even now prevalent in many countries is the Water Bound Macadam (WBM) construction. 

In this method broken stones of the base course and surface course, if any, are bound by the stone dust in presence of moisture. WBM roads are in use in India both as a finished pavement surface for minor roads and as a good base course for superior pavement carrying heavy traffic.

There are also bituminous bound macadam and penetration macadam which are adopted by our country.

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