Transportation Lecture 3 to Onward(1)

Transportation Engineering

Akhtar Abbas Assistant Professor

The University of Lahorewww.uol.edu.pk/ce

Department of Civil Engineering, (25-02-2013)

Geometric Design

• The geometric design of roadways deals with the positioning of the physical elements of the roadway according to standards and constraints.

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Geometric Design

• Objective of geometric design • The basic objective in geometric

design is to provide 1.A smooth-flowing2.Crash-free facility

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Geometric Design

• Geometric roadway design can be broken into three main parts:-

1.Alignment2.Profile3.cross-section

. Combined, these provide a three-dimensional layout for a roadway

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Geometric Design

• AlignmentThe alignment is the route of the road, defined as a series of horizontal tangents and curves

• ProfileThe profile is the vertical aspect of the road, including crest and sag curves, and the straight grades connecting them.

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Geometric Design

• Cross Section The cross section shows the position and number of vehicle and bicycle lanes and sidewalks, along with their cross slope or banking. Cross sections also show drainage features, pavement structure and other items outside the category of geometric design.

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Typical-cross-section-of-highway-4-lane

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Geometric Design

• The Geometric Design of highways deals with the following elements

1.Cross-section elements:- pavements, width, formation width, surface characteristics, camber

2.Sight distance3.Intersection elements

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Geometric Design

4. Horizontal and vertical alignment details• Highway geometrics are

generally influenced by the topography, locality and traffic characteristics etc of the region.

• Factors like speed, design traffic, traffic capacity, benefit cost ratio etc govern the geometric design of highway.

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Requisite of Good Road

• A good road surface should posses the following characteristics:-

1.It should remained dry throughout the year.

2.It should have good carriage way.3.It should have smooth gradients,

smooth and large curves.

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Requisite of Good Road

4. Its initial cost and maintenance cost should be minimum.

5. It should have a good impervious wearing surface.

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HIGHWAY ENGINEERING

Elements of typical cross section of road

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• Number of travel lanes to be provided

• Width and location of shoulders

• Medians

• Slopes

• Embankments and Ditches

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Cross section of Road

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Components of a flexible pavement


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• Carriage way width or pavement widthThe carriage way width depends upon the traffic lane and the number of lanes. The carriage way intended for one line of traffic movement is called traffic lane.The lane width is determined on the basis of vehicle width and the minimum side clearance provided for safety purposes

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If the side clearance is more, operating speed of may also be increased which enhances the capacity of traffic lane.Thus for a 2.44 meter wide vehicle having sufficient clearance of 68 cm on both sides, a carriage way of 3.8 meter width is sufficient for a single lane.

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SEPARATOR

• The main object of these separators is to prevent the head on collision between two vehicles moving in opposite directions in the adjacent lanes. It reduces the head light glare.These separators may be in the form of marking, physical dividers or area separator

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SEPARATOR

• Pavement marking is simplest and cheap.• Mechanical separators should be designed

in such a way that even if the wheels of vehicle encroach on it, but no damage should be done to the vehicle.

• Theoretically it is desirable to have a wide area separators of 8 to 14 m width, but

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SEPARATOR

actually it should be decided in conformity with the availability of land and its cost.

• To reduce the head light glare, a minimum width of 6 m is required

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SEPARATOR

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SEPARATOR

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SEPARATOR?

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KERBS

• Kerbs indicate the boundary between the pavement and shoulder.

• Karbs may be divided into three groups1.Low kerbs

The height of this class of kerbs is 7 to 8 cm. The vehicle can climb on these kerbs easily. They are also useful for longitudinal drainage system

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KERBS

2. Urban Parking Kerbs• This class of kerbs are 15 to 20 cm high

with 1:1 better to prevent scraping of tires.

• This type of kerbs prevent encroachment of the foot path or parking space by slow moving vehicles.

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KERBS

3.High speed barrier• This class of kerbs is used in hilly

roads or on bridges. They are 23 to 45 cm high and do not allow the vehicle to leave the pavement.

• These have steep batter.

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KERBS

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KERBS

• .

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KERBS

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KERBS

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ROAD MARGINS

• Various elements included in road margins are

1.Shoulders 2.Parking Lanes3.Drive way4.Frontage Road5.Cycle track

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ROAD MARGINS

6. Foot Path7. Guard rails

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SHOULDERS

• Shoulder are provided along the edge of the pavement on both sides to provide lateral support.

• They also serve as an emergency lane for damaged vehicle or otherwise vehicle to leave the main lane.

• The minimum width of shoulders should not be less than 4.6 m

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Parking Lane

• In Urban areas parking lanes are also provided

• Parking lanes should have sufficient 3 m width,

• Generally parallel parking should be allowed , as it is safer, to drive.

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ROAD MARGINS

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ROAD MARGINS

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ROAD MARGINS

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ROAD MARGINS

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ROAD MARGINS

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ROAD MARGINS

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ROAD MARGINS

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Drive Ways

• Drive way connect the main highway with the service or fuel station.

• They should be located away from intersections

• For safe crossing, the width of drive ways should be kept minimum and radius maximum.

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Drive Ways

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Frontage RoadService Road

• These roads are provided to give access to the properties along an important highway

• These roads may run parallel to the highway and be isolated by a separator, with approaches at selected points.

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Cycle Track

• In urban areas cycle tracks are provided if the cycle traffic is very high on the road

• Generally the minimum width of cycle track should not be less than 3 meter.

• In case a double lane cycle track is provided, its width may be kept as 3 meter

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Foot Path

• In urban areas where vehicular as well as pedestrian traffic is very high, footpath is provided

• Foot paths generally provided on both sides of pavement. The width of foot path should not be less than 1.5 m.

• Its surface should be smooth so no inconvenience is caused to the pedestrians

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Guard Rails

• These are provided if the highway is constructed on embankments higher than 3 meters to prevent the vehicles from running off the pavements

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Guard Rails

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Width of Formation

• The width of formation includes the width of pavement shoulders and separators, if any.

• In the case of embankment, formation width is equal to the top width of embankment and in case of cutting it is taken as the bottom width of cutting minus the side drains

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Right of Way

• It is the area of land acquired for the road along its alignment

• The width of this land is known as land width and depends upon the importance of road and its possible future development.

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Camber or Cross Slope

• Camber is the slope provided in the transverse direction of the road to drain off the rain water from road surface

• Usually camber is provided in the straight roads by raising the centre of the carriage way with respect to the edges forming highest point at centre

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Camber

• At horizontal curves, surface drainage is done by raising the outer edge of the pavement with respect to inner edge.

• The rate of camber depends upon the following two factors:

1.The amount of rainfall2.Type of pavement surface

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Camber

• Types of cambers depend upon the type of pavement surface

1.Barrel camber2.Sloped camber3.Composite camber

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Barrel Camber

• It consists of continuous curve either parabolic or elliptical.

• In this type of camber the shape of the surface is flat at the middle and steeper towards the edges, This type of camber is preferred for fast moving vehicles as they have to cross the centre or crown line frequently during overtaking operation.

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Sloped Camber

• This type of camber consists of two straight slopes joining at centre

• This type of camber is provided in relative impervious pavement surface such as cement concrete pavement

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Composite Camber

• This type of camber consists of two straight slopes with parabolic portion at centre.

• This type of camber is preferred for slow moving vehicles such as bullock drawn iron tyred carts

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Medians

• Medians are provided on divided multi-lane highways to provide a separation of opposing traffic lanes.

• Medians can also provide space for1.Snow storage2.Collecting surface drainage3.Refuge for pedestrians at crosswalks

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Medians

4. Installation of traffic control devices5. Adding future lanes• Median widths are always measured

between the inside edges of opposing travel lanes

• Medians operate best when they are highly visible during the day or night and are at a width that provides for the predominant usage 62

Medians

• There are three basic types of medians:

1.Flush medians2.Curbed (raised) medians, and3.Depressed medians.

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• Flush medians consist of a relatively flat paved area separating the traffic lanes with only painted stripes on the pavement

• This type is generally used only for lower-speed urban arterials

• Painted medians need frequent repainting to maintain their visibility at night and under inclement weather conditions

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Flush Medians

• Curbed raised medians are most commonly used on lower-speed urban arterials

• They have the same basic advantages and characteristics of flush medians except the separation is more clearly defined than for painted lines do not need frequent repainting and are more easily seen at night and during inclement weather 65

Curbed Median

Depressed Median

• Depressed medians are most commonly used for high-speed expressways, freeways and rural arterials

• Depressed medians are uncurbed grass areas with flat slopes drained by open ditches

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Depressed Median

• Normally, the widths of depressed medians are considerably greater than for either flush medians or raised medians

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Median

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Median

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Median

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Median

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Pavement Surface Characteristics

• The important characteristics of pavement surface are:

I. FrictionII.EvennessIII.Light reflecting characteristicsIV.Drainage

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FRICTION

• It is very important characteristics of road surface

• Friction between the pavement surface and the tyre is the factor which effects the operation speed, stopping and acceleration distances of the vehicles. Thus frictional force is an important factor in the acceleration and retardation ability of vehicle

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FRICTION

• Skid Skid occurs when the wheels

slide without revolving or when the path travelled along the road surface is more than the circumferential movement of wheels due to their rotation. On applying the brakes, the wheels are locked fully or partially,

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FRICTION

If in this condition vehicle moves forward that is known as longitudinal skid.When vehicle moves on horizontal curves, if the centrifugal force of the vehicle is more than the centripetal force, then lateral skid takes place. This is dangerous as the vehicle goes out of control leading to accident.

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FRICTION

• SlipIf the wheel revolves more than its corresponding longitudinal movement along the road, then it is called slip.

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FRICTION

• Factors Affecting Frictions1.Type of pavement surface2.Degree of dryness of pavement surface3.Condition of tyre4.Speed of vehicle5.Load and tyre Pressure6.Temperature

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FRICTION

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• Smooth and even surfaces allow high operating speed of the vehicle than rough and uneven surface

• Pavement unevenness thus effects operational cost, comfort and safety

• Fuel consumption, wear and tear of tyres also increases with increase in unevenness of surface

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Pavement Unevenness

• Visibility in night driving depends very much on the light reflecting characteristics of the pavement surface

• The glare caused by the reflection of head lights is considerably on wet surfaces than on dry surfaces. At night , white or light coloured surfaces offer good visibility but during bright sunlight they produce glare and strain on eyes

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Light Reflecting Characteristics

Sight Distance OR Visibility

• It is the actual distance along the road surface at which a driver from a specified height, above the road surface can see obstructions, stationary or moving objects clearly.

• Restrictions on sight distance may be caused at horizontal curves, on vertical curves and at intersections.

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Sight Distance

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• Sight Distance can be classified into three categories depending upon the situation:

1.Stopping sight distance2.Safe overtaking sight distance3.Sight distance at intersection

Stopping sight distance

• It is the minimum distance required within which a vehicle moving at design speed can be stopped with out colliding with a stationary object on the road surface

• The sight distance available on road to a driver at any instance depends on features of the road ahead, height of the drivers eye above the road surface and height of object above the road surface

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• The factors affecting SSD:1.The speed of the vehicle2.Efficiency of brakes3.Total reaction time of driver4.Longitudinal slope of the road5.Friction (Vehicle tyre and road

surface)

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• It is recommended that to avoid skidding the braking force should not be more than the frictional force b/w the wheel tyres and road surface

• Reaction time It is the time in seconds which a driver can take from the instant, the object is visible to him to the instant the brakes are effectively applied.

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• This time vary from driver to driver and so many factors like speed, distance of object and environmental conditions

• Reaction time can be divided into perception period and break reaction time Perception time is the time taken by an average driver to realize a danger ahead before actually trying to apply the brakes

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• The short time lag between perception of danger and the effective application of brakes is called the brake reaction time

• Stopping distance is depend highly on speed of the vehicle.

• SSD = 0.28Vt + 0.01V2

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Overtaking Sight DistanceOSD

• All the vehicles do not move at design speed and hence necessity of overtaking slow moving vehicles by faster ones arises

• For overtaking manoeuvre, road should be visible sufficiently ahead of the vehicle to be overtaken, it has to leave its own track and use the track for traffic from opposite direction for some distance.

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• The distance visible to the driver of a vehicle intending to overtake another slow moving vehicle, without causing any inconvenience or possibility of accident to the traffic in the opposite direction is called overtaking sight distance or safe passing sight distance.

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• Factors which effect the minimum OSD1.Skill and efficiency of driver intending

to overtake2.Space b/w both vehicles intending to

overtake and to be overtaken 3.Speeds of three vehicles namely

vehicle overtaking, overtaken and vehicle coming from opposite direction.

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4. Acceleration rate of overtaking vehicle

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Sight Distance at Intersection

• There should be clear view across the corners for a sufficient length, to avoid collision of vehicles at road intersection, particularly at uncontrolled ones.

• Longer line of sight across the corners can be obtained by making corners rounded or sloping or by giving a suitable set back to the abut building at the corner.

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• Sight distance at road intersections can be decided according to the following conditions

1.Suitable sight distance should be provided

2.Traffic on main road should allow free movement and therefore, it would be the responsibility of the driver on minor to stop or change the speed.

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3. Enabling the stationary vehicle to cross the main road.This condition occurs when vehicle enter to the intersection from minor road is controlled by stop sign.

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Sight Distance

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Sight Distance

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Sight Distance

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Sight Distance

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Sight Distance

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Sight Distance

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Design Speed

• Design speed is the speed which can be allowed on a road without risking safety conditions

• Design Speed depends on1.Type of road2.Importance of road3.Surface characteristics of the road

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Design Speed

4. Type of traffic5. Intensity of traffic6. Road geometry 7. Topography of Area

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Design Speed

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SUPER ELEVATION

• When a vehicle passes from a straight to a curved path or in other words when a vehicle negotiates a horizontal curve following two forces act on the vehicle:

1.Centrifugal Force2.Weight of the vehicle

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SUPER ELEVATION

1. Centrifugal Force• The centrifugal force is a function of

the speed of the moving vehicle. P=Wv2/(gR)

• It always acts at centre of gravity of vehicle. Its direction always tends to out-side, i.e., it always tends to push the vehicle out of track

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SUPER ELEVATION

• To counter act this tendency, outer edge of road is raised above the inner edge

• This rise of outer edge is called super elevation or cant or banking.

• Thus super elevation e is the ratio of the height of outer edge with respect to horizontal width.

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SUPER ELEVATION

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SUPER ELEVATION

• Thus e= tanθ, In practice the value of θ is kept 40 or slope of 1 in 15 with horizontal.

• Total height of outer edges w.r.t. inner edge E = e X width of road

= e.B.

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SUPER ELEVATION

• The centrifugal force P= Wv2/(gR)W= weight of vehiclev = velocity of vehicle R= Radius of circular curveP = Centrifugal forceg = acceleration due to gravity

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SUPER ELEVATION

Effect of centrifugal force1. Tendency to overturn the vehicle.2.Tendency to skid the vehicle laterally. Stability Condition Against

Overturning Let us consider a vehicle moving on

horizontal curve. Forces acting on the vehicle are

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SUPER ELEVATION

I. Centrifugal force P acting outward at C.G

II. Weight W acting downward at C.G

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SUPER ELEVATION

• Stability Conditions Against OverturningLet h be the height of C.G. of the vehicle

above the road level. The overturning moment due to centrifugal force

= P x hThe resisting moment = W x b/2 where b is the centre to centre distance of wheels of vehicle

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SUPER ELEVATION

• For equilibrium P x h = W x b /2P/W= B/(2h)

When the centrifugal ratio, i.e., P/W is equal to b/(2h) there is danger of overturning

Thus to avoid overturning, centrifugal ratio should always be less than b/(2h)

As P/W=v2/(gR) So v2/(gR) < b/(2h)113

SUPER ELEVATION

• Thus to avoid overturning , h should be as small as possible.

• Only due to this reason the modern cars have low centre of gravity

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SUPER ELEVATION

• Stability Conditions Against SkiddingThe lateral thrust P= Wv2/(g.R) is resisted

by the frictional between tire and pavement surfaces.

If the lateral resisting friction is less than the central force p, then skid will occur

For Equilibrium P= Maximum lateral friction developed

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SUPER ELEVATION

P= Fa +Fb = f(Ra + Rb) = fW

Thus when the centrifugal ratio, attains the value equal to the lateral coefficient of friction, there is danger of lateral skidding

If f<b/(2h) skidding would occurIf f> b/(2h) overturning at outer edge

would occur 116

SUPER ELEVATION

• Minimum Super elevationIf the calculated value of super elevation is

equal to or less than the camber value, minimum super elevation to be provided, should be equal to camber slope, to effect the effective drainage of the road surface.

In this case crown is eliminated and a uniform transverse slope equal to camber is maintained from outer edge to inner.

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SUPER ELEVATION

In case of horizontal curves having large radius, value of centrifugal force shall be nominal, and hence, section of road is retained as normal section and no super elevation is provided.

In such condition outer half of road surface shall have negative super elevation but sum of this –ve e and centrifugal force will be considerably less than friction 118

SUPER ELEVATION

• Maximum Super elevationIn the mixed traffic conditions, different

vehicles move with wide range of speed and hence design of super elevation under such conditions become a complex problem.

Super elevation if provided according to maximum design speed , may provide easy conditions for fast moving traffic,

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SUPER ELEVATION

but causes lot of inconvenient to slow moving traffic and vice versa.

To strike a balance between two extreme conditions, and also by taking certain other factors into consideration, it is recommended that the super elevation should be provided, to fully contract the centrifugal force according to 75% of design speed

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SUPER ELEVATION

The limitation of maximum Super elevation should be 1 in 15

If calculated value of super elevation as per 75% of design speed is less than limiting value of 1/15 or 0.067 the full value of super elevation is provided.

But if this value exceeds limiting value of 1/15 or 0.067, provide max. super elevation equal to 1/15

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Gradient

• The rate of rise or fall of road surface along its length with respect of horizontal distance is termed as gradient.

• It may also be defined as longitudinal slope in the road pavement.

• A rising or ascending gradient is denoted by plus (+) sign while descending gradient by negative (-) sign.

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Gradient

• Gradient is expressed as a ratio of 1 in x (one unit vertical to x units horizontal) and also sometimes as percentage. 1 in 20 gradient shows 1 m rise or fall of road level in 20 m horizontal distance.

• The angle which measures the change of direction at the intersection of the two grade lines is called DEVIATION ANGLE

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Gradient

• Deviation angle is denoted by N which is equal to the algebraic difference between two grades.

• Gradient in the road should not be very steep. Steep grades are not only difficult to climb but also increase the operational cost of vehicle. Engineers should try to provide as easy a gradient as possible.

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Horizontal Curve Radius

• Horizontal CurveWhen highway changes its direction in

horizontal plan. A horizontal curve has to be used to bring about this change.

For certain speed of vehicle the centrifugal force depend on the radius of the horizontal curve.

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Horizontal Curve

In order to keep the centrifugal ratio low, the radius of curve has to be correspondingly increased. It has already been provided in super elevation that centrifugal force is counteract by the combined effects of super elevation and lateral friction i.e.e + µ = v2/(gR)

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Horizontal Curve

Where e= the super elevation, µ= coefficient of lateral friction v = speed of vehicle in m/sec V = speed of vehicle in Km/hour R = Radius of curve in metres

The ratio of maximum allowable super elevation has been fixed 1 in 15 about 7%

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Horizontal Curve

Design coefficient of friction µ is taken as 0.15

Hence e + µ = 0.067 + 0.15 = v2/(gR) =V2/127 R

If the design speed of highway is decided , then minimum radius to be adopted can be determined from this relationship

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Horizontal Curve

Thus R=v2/(e+µ) = V2/(127R)• When minimum design speed V’

Km/h is adopted in the formula, the absolute minimum radius of horizontal curve Rmin is obtained. If V is ruling design speed, the ruling minimum radius of horizontal curve is obtained by above equation

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Extra width of road at Horizontal curve

• On horizontal curves, increased carriageway width is provided than the normal width on straight reaches.

• The increased width is called the extra widening of the pavement .

• Its value depends upon the sharpness of the curve. For sharper curves extra width to be provided shall be more.

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• Reasons of providing extra width of pavement at horizontal curves

I. Drivers have a tendency to keep away from the edge of the carriageway, while driving on curve.

II.The clearance between the vehicles, crossing or passing each other over horizontal curve, kept more that on straight road due to psychological effect

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III. While negotiating a horizontal curve the front steering wheels are turned and thus more space of road is occupied by the vehicle

IV. While travelling on horizontal curve, rear wheel do not trace the same path as the front steering wheels do. This is called off tracking, so more width of road is required for vehicle.

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• V. At more than designed speed if super elevation and lateral friction can not be counter act the centrifugal force fully. Out ward skidding of rear wheels may occur thus more width of road is covered. This condition occur at very high speed.

• VI. At start of curves, drivers has a tendency to follow outer edge of pavement to have

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to have better visibility and large radius curved path

VII. Trailer units require even large extra width at curves Out of all the points listed above, first two points come under psychological effectBy IRC recommendations psychological widening as

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Ws = V/(9.5sqrt®)Where V is design speed in km/hrMechanical wideningWm= l2/(2R) where l is longest vehicle

base in meters for more than one lanes = l2/(2B)

We= Ws + Wm

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Horizontal Transition curve

• Transition curve is such a curve whose radius changes from infinity to some finite value or between two finite values.

• This curve is also called easement curve • This curve is introduced between straight

road and starting point of simple circular curve.

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• As its radius gradually decreases to definite value equal to the radius of simple circular curve , centrifugal force is also developed gradually and passengers in a vehicle do not feel any inconvenience.

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• Objects to providing transition curve areI. To obtain gradual and easy

transformation from straight to circular curve and from circular curve to the straight roads

II.To obtain a gradual increase of curvature from zero at the tangent point , to that of circular curve at their junction point

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III. To obtain a gradual increase of extra widening from zero at tangent point to a specific value at junction point with the circular curve. ADVANTAGES

Transition curve is introduced to have a smooth change of direction of a vehicle from straight to the curved path and thus, avoid jerks add comfort to passengers

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• To avoid shock and inconvenience to the passengers.

Transition curve should satisfy the following conditions

I. It should meet straight and simple circular curve tangentially

II.Rate of increase of curvature and super elevation should be same

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III. It should have the radius of curvature equal to that of circular curve at their junction point and infinite at junction point with the straight road.

Types of Transition curve1. Lemniscates2. Spiral3. Cubic parabolic

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• Types of Transition Curves• Clothoid or spiral – the one that most

commonly used• Lemniscates – used for large

deflection angles on high speed roads

• Cubic Parabola Unsuitable for large deflection angles

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Vertical Curve

• In vertical alignment of the highway when two different or contrary gradients meet, they are connected by a curve to smoothen out the vertical profile. These curves are known as vertical curves.

• The vertical curves are provided to ensure gradual change in the grade and to avoid abrupt change of grade at apex or

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Vertical Curve

• To ensure smooth and gradual change in the grade at apex to provide case for the fast moving vehicle.

• Objectives of Vertical curveI. To obtain adequate visibility and safe

driving.II.The secure comfort to the passengers.

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Vertical Curve

• Types of Vertical CurvesI. Summit curvesII.Valley Curves Summit Curve When the convexity of the curve is

upward, it is called summit curve. This occur when ascending gradient intersects descending gradient or

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Vertical Curve

When ascending gradient meet another ascending gradient or ascending gradient meets a horizontal gradient.

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Vertical Curve

• Length of Summit Curve• Generally for summit curve

parabolic curves are adopted whose equation is given by

• Y=x2/a where a = 2L/N• L= Length of curve• N= Deviation angle in radians

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Vertical Curve

• The length of summit curve is governed by stopping sight distance and over taking distances separately. But curves with greater lengths are aesthetically better. In deciding the length on the basis of stopping sight distance two cases may arises.

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Vertical Curve

I. When the length is greater than stopping sight distance.

II. When the length is less than stopping sight distance

When L>stopping site distanceL=NS2/((sqrt(2H) + sqrt(2h))2

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Vertical Curve

L=length of parabolic curve in meterS= Stopping site distanceN= Deviation angle in radianH= Height of drivers eye level above

surface of pavement in metersh = height of object above pavement

surface

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Vertical Curve

II. When L< Stopping site distanceL=2S-(sqrt(2H)+sqrt(2h))2 +

(sqrt(2h)2)/NAccording to the over taking site distanceI. L>overtaking site distance then

L=NS2/(8H)II. L< overtaking sight distance, L=2S-

8H/N

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Vertical Curve

Valley Curves: When the convexity of the curve is downward, the curve is called as a valley curve. Valley curve is formed (i) when a descending gradient intersects an ascending gradient (ii) When a descending gradient meets an other descending gradient (iii) When descending gradient joints a horizontal path.

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Vertical Curve

• In valley curves the maximum deflection takes place in case (i). In valley curves there is no sight distance problem in day light, but at night the sight distance gets reduced.

• In the design of valley curves the comfort to passengers is the most important factor.

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Vertical Curve

• The lowest point is decided on the drainage consideration.

• The best shape of valley curve is a transition curve for gradually introducing and increasing the centrifugal force acting downwards.

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Vertical Curve

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• Superelevation.htm

Documents

Transportation Lecture 3 to Onward(1)