When a fast moving vehicle passing from a straight path suddenly negotiates a horizontal curve is termed as Superelevation. The centrifugal force acts on the vehicle from inside towards the outside of the curve which tries to push the vehicle off the track. This centrifugal force always acts at right angle to the direction of motion. It disturbs the stability of the vehicle. The frictional resistance between the wheels and road surface will act in the opposite direction.

Read also How to construct cement concrete pavement

Superelevation

When the transverse inclination providing to the pavement surface on curves by raising the outer edge with respect to inner edge to counter act the centrifugal force. Also, To reduce the tendency of vehicle to overturn is termed as Superelevation. It is express as the difference of heights of two edges of the carriage way to the width of carriage way. It is denoting by ‘e’.

Read also more What is Sight Distance ?

Formula

e = V²/127 R

Where,

V = Speed of vehicle in km/hr

R = Radius of curve in metres.

Thus a superelevation of 1 in 15 means that for a 15 m wide road. The other edge of the carriage way is raising above the inner edge of the curve by 1 metre. It should be remember that greater the superelevation, more is the inconvenience to the slow traffic. Hence the superelevation providing should be minimum as possible but it can not be less than the camber giving to the remaining part of road for effective drainage. The super elevation generally varies from 1 in 14 to 1 in 16 but in special circumstances, the maximum superelevation can be provide up to 1 in 10.

METHODS OF PROVIDING SUPERELEVATION

Introducing super elevation on horizontal curves is an important feature of highway construction. The horizontal curve with a desire super elevation in road terminology is termed as a ‘Super elevated curve’. A super elevated curve consists of a circular curve, with transition curves on both the ends. A transition curve thus joins the straight portion of the road with one end of the circular curve.

At the end of the straight portion of the road curve, which is also the beginning of the transition curve, there is a provision of usual camber to the road surface. “The principle of introducing super elevation lies in the gradual changing of this camber surface at the start of the transition curve to a one way slope. i.e. the requiring super elevation at the end of the transition curve which is also the beginning of the circular curve”. This full and requiring super elevation is maintain on the entire circular curve up to its end which is also the beginning of the next or forward transition curve.

To change the cambered section of the road into a super elevated one, two steps are taking.

• Firstly, the camber on the outer edge is eliminate gradually till the road pavement has one straight tilt or slope from the inner to the outer edge (Elimination of crown method).
• Secondly, this straight line slope is gradually increase till the desire super elevation is attain. (Rotating pavement method).

It may be note that on the next transition curve, this design super elevation must be gradually remove. The cross-slope should be gradually change to the cambered road surface in the similar manner in which it was introduced but in the reverse order. The two important methods of providing super elevation to the curves are given below:

• Outer slope rotation method. (Elimination of crown method)
• Diagonal crown method.

Outer slope rotation method. (Elimination of crown method)

In the first method, the outer slope of the road is slowly rotate around the crown by progressively decreasing its inclination or gradually raising the outer edge. This inclination of the outer slope tangential to the crown surface firstly reaches horizontal. Then it is slowly increase till it has attained the same inward slope. The main disadvantage of this method is the difficulty of draining water in some portion of the road length where the slope on the outer edge is less (gradually decreased) than the camber provide on the road. This method is commonly use on Indian highways. The above disadvantage will not count much if the road has a good longitudinal slope.

Diagonal crown method.

In the diagonal crown method, the cambered section of the road is eliminate by progressively shifting the crown towards the outer edge along the extension of the inner slope (as shown in figure).

This shifting of the crown is continue, till it reaches the outer edge. This method is usually not recommend as some portion of the road will have an excessive slope more than the required super elevation (negative super elevation).

Once the profile of the road has been brought to a single straight line slope by any of the above methods. The designed super elevation is attained by any of the following methods;

• By progressively revolving the straight line slope about the central line. Thereby decreasing the inner edge and elevating the outer edge simultaneously both by half the total amount of super elevation (e). This method is preferred as grade in centre line remains unchanged. (No bumps).
• By progressively revolving the slope about the inner edge thereby raising the centre line and outer edge simultaneously. It is prefer in flat topographic areas where rainfall is heavy. (Easy drainage).

ADVANTAGES OF SUPERELEVATION

The super elevation provided to the horizontal curves affects the following:

• The stability of fast moving vehicle is increase and their tendency to skid or overturn decreases.
• The distribution of pressure on both the wheels will be equal and thus wear and tear of vehicles decreases.
• Intensity of stresses on the foundation decreases.
• In super elevated curves, the vehicles need not necessarily be slow down.
• No gullies or ditches are require on outer edges of super elevate curve.

The various construction steps for laying of cement concrete pavement slab are describe below:-

• Preparation of sub grade
• Preparation of base course
• Placing of form work
• Watering of surface
• Mixing and placing of concrete
• Compaction and finishing
• Belting, Brooming and Edging
• Curing
• Opening to traffic

Read more How to construct a hill roads

Preparation of sub grade

The sub grade is the natural soil which is properly compact by rollers and is brought to require camber and gradient. The cross and longitudinal profile should be check by suitable templates.

Preparation of base course

Over the prepare subgrade, base course or sub base course is sometimes providing. In certain cases, where the bearing capacity of subgrade soil is high, base or sub base layer may be omitting. The base for a concrete road may be WBM surface, compact granular material layer or stabilise soil base. The base or sub base layer not only provides a smooth level surface and a supporting layer, but it also reduces the thickness of concrete slab.

Placing of form work

After preparing the base, form work for concrete slab pavement is lay. The forms can be of steel or timber. The depth of form work should be equal to the thickness of slab. Form work should be rigidly fix in position and must be well in advance from the point where concrete mix is to be place. It should be oil properly from inside and must be check to line and grade.

Watering of surface

After laying form work, the surface of base or subgrade must be wet with water before placing of concrete mix. Water should not be allowed to stand on the surface during wetting operation. The main function of watering is to saturate the surface completely so that it should not absorb any water from the concrete mix.

Mixing and placing of concrete

The ingredients of concrete such as cement, sand and coarse aggregates are mix in a dry state in the ratio 1 2: 4. The mixing should be doing preferably in a concrete mixer. The water should be adding in a measure quantity to obtain the design water cement ratio.

The concrete mix is place in the lay form works by manual labour and it should be deposit in layers of thickness not more than 50 mm to 80 mm. The concrete mix should be lay on the entire width of the form work and always proceed lengthwise. The technique of construction may be alternate bay or continuous bay method. The top most layer should be lay 6 mm to 12 mm higher than the specify profile for further tamping work. The require transverse and longitudinal joints should be provide and it should be ensure that the top layer is lay to the desired camber and gradient.

Compaction and finishing

After placing of concrete mix, it should be compact by vibrating hand screeds or hand tampers. For large scale construction work, power drive vibrators or machines may be use. The hand tampers are use across the bay and tamping of the surface is doing along the length of the bay.

For finishing the concrete surface, wooden hand floats are usually used. The main function on floating is to develop a uniform and even surface pavement devoid of any waves or corrugations. The floating is always doing in longitudinal direction. Float is held in position parallel to the centre line of pavement and move gradually from one side to the other. Straight edge with 2 handles is use to check the finish pavement surface for its grade and level in longitudinal direction.

Belting, Brooming and Edging:

Belting is do in transverse direction to the carriage way by a 150 to 300 nm wide strip of canvas or rubber fit with handles at both ends. The belting is doing to finish the surface of concrete just before the concrete sets or hardens.

After belting, brooming of pavement surface is carrying out immediately with a fibre broom brush. Brooming is carrying out, to make the pavement surface rough and non-slippery having skid resistance. After brooming, edging tool is use for rounding the transverse and longitudinal edges of the pavement slab. The process of edging is carrying out before the concrete mix develops initial Setting.

Curing

Initial curing of finish pavement is carry out after 12 hours by covering the surface with gunny bags which are keep wet for at least one day. For final curing, gunny bags are removing and surface is cover with a sandy layer which is keep wet for at least 14 days.

Curing of finish pavement may also be carry by:

• Application of liquid impervious membrane.
• Ponding method in which whole surface is dividing into small bays by forming earthen banks which are fill with water. After curing is complete, a surface is clean properly and washes.

Opening to traffic

Road pavement is open to traffic after 28 days of curing slab or even earlier when the concrete has attained require strength. During this period, brick edging is constructing to protect the slab. Earth may be spread on the berms up to the top of brick edging to disallow the traffic.

The common tests to asses the the properties and requirements of paving grade bitumen are the viscosity tests, penetration test, ductility test and the softening point test. Also specific gravity test and flash and fire point test are need for use in paving applications. Additional tests like the matter soluble in carbon disulfide, loss on heating and penetration test on residue may also be caring out.

Earlier the classification of bitumen was base on the penetration ad ductility test results. It was later observing that bitumen from different sources possessing same penetration value at a specifying temperature may exhibit entirely different viscosity characteristics Hence different temperature susceptibility characteristics at the applications and service temperatures. Therefore it is important to determine the viscosity property of the binder in terms of ‘absolute viscosity’ and ‘kinematic viscosity’ test results.

Read also How to Construct WBM roads

Types of Tests on Bitumen

1. Penetration test
2. Viscosity tests
3. Ductility test
4. Softening point test
5. Specific gravity test
6. Flash and Fire point tests
7. Loss on heating test
8. Solubility test

Read also How to construct Hill roads

1. Penetration test on bitumen

The consistency of bitumen materials varies depending upon several factors such as constituents, temperature, etc. At temperature ranges between 25 and 50 degree celsius. Most of the paving bitumen grades remain in semi solid or in plastic state. Determination of absolute viscosity of bituminous materials is not so simple. Therefore the consistency of these materials is determining by indirect methods. Penetration test is one such indirect test to determine the consistency of paving grade bitumen, which is a very simple test.

Procedure

1. First of all, Sample of bitumen is softened to a pouring consistency, stirring thoroughly.
2. Then, Pour into containers to a depth at least 15 mm in excess of the expecting penetration.
3. The sample containers are then placing in a temperature control water bath at a temperature of 25 degree celsius for one hours.
4. The sample with container is taking out.
5. Place under Penetrometer.
6. The needle is adjusting to make contact with the surface of the sample.
7. The dial is set to zero or the initial reading is taking out.
8. The needle is release for 5 seconds.
9. Hence, The final reading is taking on dial gauge.

At least three penetration tests are make on this sample by testing at distances at least 10 mm apart. After each test, the needle is disengaged and wiped with benzene and dried. The depth of penetration is reporting in one-tenth mm units. The mean value of three measurements is reporting as a penetration value. It may be note that the penetration value is likely influence by any inaccuracy as regarding pouring temperature, size of needle, weight placing on the needle and the test temperature.

2. Viscosity Test on bitumen

Viscosity of a liquid is the property that retards its flow due to internal friction. It is a measure of resistance to flow of the liquid. The flow of liquid under an applying force will depend on its viscosity. Higher the viscosity, slower will be its movement or rate of flow.

Viscosity of liquid bituminous binders like bitumen emulsion and tar are determining by indirect method using orifice type viscometers. A specifying quantity of the binder is allowing to flow through specifying orifice size of the test-cup at a given temperature. The time taken in seconds is recording as the viscosity value. As per specifications of BIS, the viscosity values of bitumen emulsions are determine using ‘Saybolt Furol’ orifice viscometer at test temperature of 25 degree celsius and 50 degree celsius. The viscosity values of tar are determine using orifice viscometer termed as ‘Tar Viscometer’ using either 10 mm or 4 mm size orifice.

3. Ductility test on bitumen

In the flexible pavement constructions where bitumen binders are using, it is important that the binders from ductile thin films around the aggregates. The ductile film of binder improves the physical interlocking of the aggregate bitumen mixes. Under traffic loads, the bituminous pavement layer is subjecting to repeat deformation and recoveries. The binder material which does not possess sufficient ductility would crack and permit the surface water to enter into the pavement resulting in rapid deterioration and failure. Ductility test is carrying out on bitumen to test the adhesive property of bitumen and its ability to stretch. The bitumen may satisfy the penetration value, but may fail to satisfy the ductility requirements.

• The ductility value is expressing as the distance in centimetre (cm).
• In which the bitumen specimen of standard size can be stretch before the tread breaks.
• The standard briquette specimen has a minimum cross section 10 mm x 10 mm.
• The test is conducting at 27 degree celsius with a rate of pull of 50 mm per minute, until the stretch specimen breaks.
• The ductility machine functions as a constant temperature water bath with a pulling device at a Pre-calibrating rate.
• Two clips are thus pull apart horizontally at a uniform speed of 50 mm per minute.

Procedure

• The bitumen sample is heat and pour in the mould assembly place on a plate.
• The samples along with the moulds are cooled in air.
• Then in water bath maintaining at 27 degree celsius.
• The excess bitumen material is trim.
• The surface is levelling using a hot knife.
• The mould assembly containing sample is replaced in water bath of the ductility testing machine for 85 to 95 minute.
• The sides of the mould are remove.
• The clips hook on to the machine and the pointer is adjusting to zero.
• The distance up to the point of breaking of thread is report as ductility value, in cm.

The ductility value gets seriously affecting by factors such as pouring temperature, dimensions of briquette, level of briquette in the water bath. Also, a presence of air pockets in the specimen briquettes, test temperature and rate of pulling.

The ductility values of bitumen generally vary from 5 to over 100 for different bitumen grades. A minimum ductility value of 50 to 75 cm is generally specifying for bitumen using in pavement construction.

4. Softening Point test on bitumen

In this, temperature at which the substance attains a particular degree of softening under specifying condition of test. It is usually determining by Ring and Ball test.

Generally higher softening point indicates lower temperature susceptibility and is preferring in warm climates.

Procedure

• A brass ring containing test sample of bitumen is suspending in a breaker with liquid bath.
• Water is use as the bath if the softening point is less than 80 degree celsius.
• Glycerine is use for temperatures exceeding 80 degree celsius.
• A steel ball is place upon the bitumen sample.
• The liquid medium is then heat at a rate of 5 degree celsius per minute.
• The temperature at which the softened bitumen touches the metal plate place at a specify distance below the ring.
• It is record as the softening point of the bitumen.
• Harder grades of bitumen possess higher softening point than soft grade bitumen.

The softening point of various bitumen grades use in paving jobs vary between 35 degree to 70 degree celsius.

5. Specific gravity test

The specific gravity of a bitumen binder is a fundamental property. It is frequently use as an aid to classify the binders for use in paving jobs. In most applications, the bitumen is weighing, but finally when use with aggregate system, the bitumen content is convert on volume basis using density values. The density of bitumen is influence by its chemical composition. Increase amounts of aromatic type compounds or mineral impurities cause an increase in specific gravity.

The specific gravity of bituminous binder is termed as the ratio of the mass of a given volume of the binder to the mass of an equal volume of water, the temperature of both being at 27 degree celsius. The specific gravity is determining either by using a pyknometer. Then, preparing a specimen of cube shape in semi solid or solid state and by weighing in air and water. The specific gravity is obtaining by dividing the weight of the bitumen by weight of equal volume of water.

Generally the specific gravity of pure bitumen is in the range of 0.97 to 1.02. The specific gravity of cutback bitumen may be lower depending on the type and proportion of diluent use. Tars have specific gravity ranging from 1.10 to 1.25.

6. Flash and Fire point tests on bitumen

When a bituminous binder is heat continuously, above a certain temperature it starts emitting volatile vapours and these volatile vapours can momentarily catch fire causing a flash, though the binder itself does not catch fire and burn at this temperature. The temperature at which such behaviour occurs is found to differ for different types and grades of bituminous binders. This condition is very hazardous and therefore it is essential to determine the temperature at which the flash of fire can occur in each type and grade of bituminous binder.

Flash Point

Flash point test gives an indication of the critical temperature at and above which suitable precautions should be taken while heating the binder. In order to eliminate fire hazards during heating, mixing or application, the paving engineers should restrict the mixing and application temperatures well below this temperature.

The ‘flash point’ of a bituminous binder is defined as the lowest temperature at which application of a test flame causes the vapours of the binder to catch an instant fire in the form of flash under specified test conditions. Two types of test apparatus may be used for conducting flash point test on bitumen, namely the Pensky-Martens Closed Cup Tester and Open Cup Tester.

Fire Point

If the bituminous binder is further heat to a temperature higher than the flash point, the binder material itself catches fire and continues to burn, the lowest temperature causing this condition is termed as the ‘fire point’.

The fire point is always higher than the flash point of a material. The fire point is termed as the lowest temperature at which application of a test flame causes the binder material to ignite. And Burn at least 5 seconds under specifying test conditions. Pensky-Martens Open Cup Tester is made use of to determine the fire point of the bituminous binders.

7. Loss on heating test on bitumen

When bitumen is heat, it loses the volatiles and get hardened. To study the effect of heating, an accelerating heating procedure is adopting.

• About 50 grams of the sample is weighing.
• Then, heat to a temperature of 163 degree celsius for 5 hours in a special oven designed for this test.
• After the heating period, this specimen is cool and weighing again.
• Hence, The loss in weight is expressing as a percentage by weight of original sample.

Bitumen use in pavement mixes should not indicate more than one percent loss in weight. The penetration values of bitumen is 150-200 up to two percent loss in weight is allow. The residue after heating when subjecting to penetration test shows a reduction in penetration value. The reduction in penetration value should be less than 40% of the original penetration value of the bitumen.

8. Solubility test on bitumen

Pure bitumen is completely soluble in solvents like carbon disulphide and carbon tetrachloride. Hence any impurity in bitumen in the form of inert minerals, carbon, salts etc. It could be quantitatively analysing by dissolving the samples of bitumen in any of the two solvents.

• A sample of about 2.0 g of bitumen is dissolve in about 100 ml of solvent.
• The solution is filter and the insoluble material retaining is washing, drying and weighing.
• The insoluble material is expressing as a percentage by weight of original sample.
• Therefore, Insoluble material should be preferably less than 1%.
• In solubility test with carbon tetrachloride, if black carbonaceous residue is over 0.5%. The bitumen is considering to be ‘cracked’.
• Hence, The minimum proportion of bitumen soluble in carbon disulphide is specifying as 99%.

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All the vehicles do not keep moving along the roadway during the entire 24 hours of the day; they have to stop or park at the desired locations for different duration’s. In cities the problem of parking vehicles is becoming more and more acute day by day. When vehicles are parked on the road side even for a short while. Therefore, there is restriction to other vehicles passing by, resulting in congestion and accidents. In shopping centres, public places and localities with offices there is a shortage of parking facilities. Proper planning and design of parking facilities is essential in cities and large towns. Details of parking studies including estimation of parking demand. In this Article some particulars pertaining to design of parking facility are providing.

Read More about What is Sight distance ?

Parking facilities may be broadly divided into two types:

• On-street or kerb parking
• Off-street parking

On-Street or Kerb Parking

Types of kerb parking

In this type of parking, vehicles are park along the kerb which may be designed for parking. Kerb parking is quite convenient for those who find suitable space to park their vehicles near the place they wish. But for others who do not find a parking space it is a problem. Often they may have to park their vehicle at a far off place and walk down to the destination. Unless the kerb parking facility is adequately designing in advance while planning a new town; it might lead to a lot of inconvenience to moving vehicles. This result in congestion at the road stretches due to reduction in road capacity and increase in accidents.

Kerb parking facility may be either unrestricted or restricted type. The restricted kerb parking may either be controlled by police or by parking metres. Therefore, a specified parking fee is collected from those who park their vehicles for a certain duration of parking time.

Different patterns of kerb parking are ‘parallel parking‘ and ‘angle parking‘. The length of parking space required along kerb and roadway width occupied by the parked vehicles for different parking angles. The number of vehicles that can be park per unit length of the kerb.

Parallel parking

It needs less roadway width, but number of vehicles that can be parked per unit length of road is least. Parallel parking is generally prefer when the width of kerb parking space and the width of the street are limit. But the parking and unparking operations are more difficult needing a few forward; and reverse movements before parking properly or before taking out. Parallel parking may be either with equal spacing facing the same direction or two cars place closely leaving a gap as open space (for manoeuvring) between two-car units.

Angle parking

Angle parking may be at angles 30, 60 or 90 degrees. The width of the roadway required for parking; also the width required for parking and unparking manoeuvres increase with increase in parking angle up to maximum 90° angle. Angle parking accommodates more vehicles per unit length of kerb; maximum number of vehicles that can be parked per unit length of the kerb is with right angle parking. Angle parking is more convenient to the drivers for the parking and unparking manoeuvres than the parallel parking; however these cause much more obstruction to the through traffic resulting in more accidents than the parallel parking. Out of various angles used for angle parking; degree angle is considering the best taking into account all the above factors.

Off-Street Parking

Need for off-street parking facility

At locations where the parking demand is high and kerb parking cannot be permitting in view of traffic congestion, off-street parking facilities are providing at the nearest locations depending on the availability of space for this purpose. When a parking facility is providing at a separate place away from the roadside or kerb, it is termed as ‘off-street parking’. The main advantage of this method is that there is no undue increase in congestion, delay and hindrance to moving traffic due to on street parking on the already congested roads. But the main drawback is, some of the vehicle users will have to walk a greater distance after parking the vehicle at the off-street parking facility. It is also not possible to provide the off-street parking facility at very close intervals especially in business centres of a city.

Types of off-street parking

Two basic types of off-street parking facilities are, ‘surface parking lots’ and ‘multi-floor parking garages’. Both in the parking lots and the garages, the basic traffic operations consist of five steps namely, 

• entrance 
• acceptance 
• Storage
• Delivery
• exit. 

Hence some definite space is requiring in front of the parking lot or garage for vehicles during entrance acceptance and also for exit operations. This space providing is termed as ‘reservoir area’, the size of which depends on the average rate of arrival of vehicles to be park during peak hour. The average time required to dispose off one car and the number of attendants employed for storage-operations. However, for self parking there is no need for the reservoir space, as the additional space requirement at the entrance and exit will be minimum.

Surface parking lots

Surface parking lots may be convenient where sufficient space is available at comparatively low cost. When the parking of vehicles is doing by owners or drivers of the cars, the method is termed “self parking system’. If the vehicle is left by the driver at the entrance space and again collecting from there, the parking and delivering operations being carried out by attendants, it is termed ‘attendant parking system’. Most important advantage of attendant parking is less space requiring to store and manoeuvre the same number of cars.

Multi-storeyed parking garages

Multi-storeyed parking garages are restored to when the floor space available for the parking garage is less and is very costly. It is possible to construct multi-storeyed garages to park a large number of cars at a time. The garages can be above the ground or below the ground or an integral of both. It is necessary to provide the inter-floor travel facility for the vehicles, which may be either by ‘elevators’ or by ‘ramps’.

In mechanised garages the elevator may be designed to move both in vertical and in horizontal directions to carry and place the vehicle in the appropriate parking stall and to deliver it back. The mechanised garages are operated by trained parking attendants only. Therefore the owners/drivers of vehicles will have to hand over the vehicle and the keys to the parking attendant at the reserved space at the entrance; similarly, the vehicle is taking delivery also from the space near the exit. One of the main advantages of mechanised parking garages is parking space requirement is minimum and there is maximum utilisation of available space. The disadvantage of this method is that if there is a mechanical breakdown or a power failure, the functioning of the elevator system would come to a stand-still.

When ramps are providing for driving the vehicles to and from the parking stalls located at different floor levels, the total space requirement will be increased considerably. For self parking, the overall ramp space requiring will be still higher.

Sight distance is the length of road visible ahead to the driver at any instance. Sight distance available at any location of the carriageway is the actual distance a driver with his eye level at a specify height above the pavement surface has visibility of any stationary or moving object of specify height which is on the carriageway ahead. The sight distance between the driver and the object is measure along the road surface.

Restrictions to Sight distance

• At horizontal curves, when the line of sight is obstruct by object at the inner side of the curve. Here the sight distance is measure along the centre line of the horizontal curve when the vehicle driver is able to see another vehicle or object on the carriage way.

• At a vertical curve the line of sight is obstruct by the road surface of the summit curve ( i.e., a vertical curve of the road with convexity upward ).

• In this case also the sight distance is measure along the centre line of the vertical curve when the vehicle driver is able to see another vehicle or object on the road.
• At an uncontrolled intersection when a driver from one of the approach is able to sight to vehicle from another approach road proceeding towards the intersection. Here the sight distance for each vehicle driver is the distance from the position when the two an see each other up to the intersection point of the two roads.

Types of sight distance

Sight distance require by drivers applies to both geometric designs of highways and for traffic control. Three types of sight distance situations are consider in the design:

• Stopping sight distance (SSD) or absolute minimum sight distance.
• Safe overtaking sight distance (OSD) or passing sight distance.
• Safe sight distance for entering into controlled intersections.

Important points remember during the design of road geometric :

Safe Stopping

Driver travelling at the design speed has sufficient sight distance. Also length of road visible ahead to stop the vehicle without collision. Whenever, in case of any obstruction on the road ahead. As safe stopping is most essential requirement to avoid collision. So, this requirement has to be invariably fulfil all along the road.

Safe overtaking

Driver travelling at the design speed should be able to safely overtake the slower vehicles without causing obstruction or hazard to traffic of opposite direction, at reasonable intervals.

Safety at an uncontrolled intersection

Driver entering an uncontrolled intersection has sufficient visibility to enable him to take control of his vehicle. To avoid collision with another vehicle.

Apart from the three situations mention above, the following sight distances are consider in highway design :-

Intermediate sight distance

It is twice the stopping sight distance. When overtaking sight distance cannot be provide, intermediate sight distance (ISD) is provide to give limit overtake opportunities to fast vehicles.

Head light sight distance

This is the distance visible to a driver during night driving under the illumination of the vehicle head lights. This sight distance is critical at up gradients and at the ascending stretch of the valley curves.

Stopping Sight Distance

The minimum distance visible to a driver ahead or the ‘sight distance’ available on a highway at any spot should be of sufficient length to safely stop a vehicle travelling at desire speed, without collision with any other obstruction. Therefore this stopping sight distance (SSD) is also refer as absolute minimum sight distance. This is also some times termed as ‘non passing sight distance’.

The sight distance available to a driver travelling on a road at any instance depends on the following factors :

• Features of the road ahead
• Height of the driver’s eye above the road surface
• Height of the object above the road surface.

Features of the road ahead

The features of the road ahead which affect the sight distance are the horizontal alignment and vertical profile of the road, the traffic condition and the position of obstructions.

Height of the driver’s eye above the road surface

At vertical summit curves the height of the driver’s eye and the object above road level are more important factors affecting the visibility.  

Height of the object above the road surface

The height of an object to be considered for stopping a vehicle depends on what might be a source of danger to the moving vehicle. For the purpose of measuring the stopping sight distance or visibility ahead, IRC has suggested the height of eye level of driver as 1.2 m and the height of the object as 0.15 m above the road surface.

Hence the stopping distance available at a summit curve is the distance measured along the road surface at which an object of height 0.15 m can be seen by a driver whose eye is at a height of 1.2 m above the road surface.

Factors on which stopping distance depends

The distance within which a motor vehicle can be stopped depends upon the factors listed below:

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

Total reaction time of driver

Reaction time of the driver is the time taking from the instant the object is visible to the driver to the instant the brakes are effectively apply. The actual time gap or the reaction time of the driver depends on several factors. During this period of time the vehicle travels a certain distance at the original speed, which may be assuming to be the design speed of the road. Thus the stopping distance increases with increase in reaction time of the driver. The total reaction time, t may be split up into two parts:

• perception time 
• brake reaction time.

Perception time 

The ‘perception time’ is the time required for a driver to realise that brakes must be applied. It is the time from the instant the object comes on the line of sight of the driver to the instant he realises that the vehicle needs to be stopped. The perception time varies from driver to driver and also depends on several other factors such as the distance of object and other environmental conditions.

Brake reaction time

The ‘brake reaction time’ also depends on several factors including the skill of the driver. This type of the problems and various other environmental factors.

Generally total reaction time of the driver is taking together, as this is found to vary considerably depending on several factors. The total reaction time may be explain with the help of ‘PIEV’ theory.

‘PIEV’ theory

According to ‘PIEV’ theory, the total reaction time of the driver is split into four parts, viz., time taking by the driver for:

• Perception,
• Intellection
• Emotion
• Volition

Perception time

‘Perception time’ is the time requires for the sensations receiving by the eyes or ears of the driver to be transmitting to the brain through the nervous system and spinal cord. In other words, it is the time requires to perceive an object or situation.

Intellection time

Intellection time is the time requires for the driver to understand the situation. It is also the time requires for comparing the different thoughts. Regrouping and registering new sensations.

Emotion time

Emotion time is the time elapse during emotional sensations. As well as other mental disturbance such as fear, anger or any other emotional feelings like superstition etc. with reference to the situation. The emotion time varies for different drivers, but even for a particular driver the emotion time is likely to vary considerably depending upon the situation or the actual problem involve.

Volition time

‘Volition time’ is the time taking by the driver for the final action, such as brake application.

It is possible that the driver may apply brakes. To take any other avoiding action like turning, by the ‘reflex action’, without the normal thinking process,’ which is probably the minimum time for taking a preventive action like brake application.

Speed vehicle

The stopping distance depends very much on the speed of the vehicle. First, during the total reaction time of the driver the distance moved by the vehicle will depend on the speed. Second, the braking distance or the distance moved by the vehicle after applying the brakes, before coming to a stop depends also on the initial speed of the vehicle. Hence it is evident that higher the speed, higher will be the stopping distance.

Efficiency of brakes

The braking efficiency is said to be 100% if the wheels are fully locked preventing them from rotating on application of the brakes. This will result in 100 percent skidding which is normally undesirable, except in utmost emergency. Also skidding is considering to be dangerous. As it is not possible for the driver to easily control a vehicle after it starts skidding. Hence to avoid skid, the braking forces should not exceed the frictional force between the wheels and tyres.