Unit – 2

Highway Geometric Design

2.1 Highway Geometric Design

• The geometric layout of roads is the department of motorway engineering worried with the site of the bodily factors of the roadway consistent with requirements and constraints.
• The primary goals in geometric layout are to optimize performance and protection whilst minimizing fee and environmental damage. Geometric layout additionally impacts an rising 5th goal called "livability," that's described as designing roads to foster broader network goals, together with imparting get admission to to employment, schools, agencies and residences, accommodate various tour modes which includes walking, bicycling, transit, and automobiles, and minimizing gasoline use, emissions and environmental damage.
• Geometric roadway layout may be damaged into 3 fundamental parts: alignment, profile, and go-segment. Combined, they offer a 3-dimensional format for a roadway.
• The alignment is the path of the road, described as a sequence of horizontal tangents and curves. The profile is the vertical issue of the road, consisting of crest and sag curves, and the immediately grade traces connecting them.
• The go segment suggests the placement and variety of automobile and bicycle lanes and sidewalks, in conjunction with their go slope or banking. Cross sections additionally display drainage features, pavement shape and different gadgets outdoor the class of geometric layout.

Key Takeaways:

• The geometric layout of roads is the department of motorway engineering worried with the site of the bodily factors of the roadway consistent with requirements and constraints.

2.2             Cross Section elements

a. Friction

• Friction determines the operating speed and distance requirements in stopping & accelerating the vehicles.
• Skidding occurs in a vehicle when transverse movement of tyre > circumferential movement of tyre.
• Slipping occurs when circumferential movement of tyre > transverse movement of tyre.
• Factors affecting friction are:

1. Type & condition of pavement surface
2. Type & condition of tyre & tyre-pressure
3. Speed of vehicles
4. Temperature of tyre & pavement

• Coefficient of friction decreases with increase in temperature, load & tyre pressure.
• As per IRC,

1. Longitudinal friction coefficient = 0.35 to 0.40
2. Lateral friction coefficient = 0.15

• When a longitudinal friction coefficient of 0.40 is allowed for stopping a vehicle, the resultant retardation is 3.93m/s2, which is not too uncomfortable for passengers.

b. Pavement unevenness

• A Bump Integrator is used to measure vertical undulations of pavement surface recorded per unit horizontal length of the road.
• Unevenness index < 150 cm/km for good pavement surfaces of high speed highways
• Unevenness index = 250 cm/km is satisfactory for speed up to 100 km/hr
• Unevenness index = 350 cm/km is uncomfortable to passengers even at 50 km/hr

c.  Camber/ cross-slope

• Camber is the slope provided to the road surface in the transverse direction to drain off the rain water from the road surface.
• The requirement of camber depends upon:

1. Type of pavement surface
2. Amount of rainfall

• Shape of camber:

1. Parabolic & elliptic – preferred by fast moving vehicles, because they require frequent crossing over the crown line during over-taking operations
2. Straight line – to provide very flat cross slope in cement concrete pavements

• Note: Superior the road, flatter the camber (RCC > CC > Thick BT > Thin BT > WBM > Gravel > Earth).

d. Width of Pavement/ carriageway

Considering that IRC recommends the maximum width of a vehicle to be 2.44m, the minimum width of pavement criteria is given as follows:

• Single Lane = 3.75m
• Two Lane (without kerb) = 7.0m
• Two Lane (with kerb) = 7.5m
• Intermediate Lane = 5.5m
• Multi-lane pavement = 3.5m/lane
• Width of single lane for village roads = 3m

e. Traffic separators/ medians

• As per IRC, minimum desirable width of 5m for medians of rural highways is recommended.
• It may be reduced to 3m where land is restricted.
• Width of median can be further reduced to 1.2 – 1.5m for long bridges.
• IRC Recommendations for Median Width:

      At urban intersection = 1.2m

      For vehicles making right turn = 4m to 7.5m

      For vehicles crossing at grade = 9m to 12m

f.       Kerb height

      A Kerb is the edging of a pavement (or a sidewalk) near a road. It indicates the boundary between the pavement and shoulder, or sometimes islands or footpath. The heights of different types of kerbs are:

Low-mountable = 10 cm

Semi-barrier = 15 cm

Barrier = 20 cm

Key Takeaways:

• Skidding occurs in a vehicle when transverse movement of tyre > circumferential movement of tyre.

2.3 Carriageways

• A carriageway is one side of a road on which traffic traveling in opposite directions is separated by a barrier.
• A carriageway or roadway consists of the width of the road on which a vehicle is not restricted by any physical barriers or separation to move laterally. A carriageway generally consists of a number of traffic lanes together with any associated shoulder, but maybe a sole lane in width.

Key Takeaways:

• A carriageway is one side of a road on which traffic traveling in opposite directions is separated by a barrier.

2.4 Camber

• Camber or Cant is the cross slope provided to raise the middle of the road surface in the transverse direction to drain rainwater from the road surface.
• The objectives of providing the camber are

Surface protection (especially for gravel and bituminous roads)

Subgrade protection (by proper drainage)

Quick-drying of pavement which in turn increases safety

• Depending on the type of road surface and amount of rainfall, the following camber is suggested by IRC:

• Excessive camber or cross slope should be avoided because
• Excessive camber will cause a transverse tilt of the vehicle making it uncomfortable for passengers. Also, the distribution of load to different wheels will not be uniform, leading to uneven wear and tear of wheels and damage to pavements.
• With excessive camber, heavy rain will result in the formation of heavy cross ruts.
• The central seeking tendency of vehicles will increase to avoid transverse tilt. 

Different types of Camber:

• Parabolic Camber
• Straight Line Camber

Combination of Straight and Parabolic Camber    

Key Takeaways:

• The objectives of providing the camber are

Surface protection (especially for gravel and bituminous roads)

Subgrade protection (by proper drainage)

2.5 Stopping & overtaking sight distances

• The safe and efficient operation of vehicles on the road depends very much on the visibility of the road ahead of the driver. Thus the geometric design of the road should be done such that any obstruction on the road length could be visible to the driver from some distance ahead. This distance is said to be the sight distance.
• For straight road on level ground, there is no problem of side distance. But Straight road on level ground is a rare case. Horizontal curves, vertical curves (Summit), intersections are potential places where there could be a restriction in terms of Sight distance 
• The actual distance along the road surface over which a driver from a specified height above the carriageway has visibility of the stationary or moving object.

The sight distance used for design are

• Stopping Sight Distance (SSD) or absolute minimum sight distance
• Intermediate Sight Distance (ISD) = 2*SSD
• Overtaking Sight Distance (OSD) for safe overtaking
• Headlight Sight Distance = SSD at night

Types of sight distance

• Sight distance available from a point is the actual distance along the road surface, over which a driver from a specified height above the carriageway has visibility of stationary or moving objects.
• Three sight distance situations are considered for design:

Stopping sight distance (SSD) or the absolute minimum sight distance

Intermediate sight distance (ISD) is defined as twice SSD

Overtaking sight distance (OSD) for safe overtaking operation

Headlight sight distance is the distance visible to a driver during night driving under the illumination of headlights

Safe sight distance to enter into an intersection.

• The most important consideration in all these is that at all times the driver traveling at the design speed of the highway must have sufficient carriageway distance within his line of vision to allow him to stop his vehicle before colliding with a slowly moving or stationary object appearing suddenly in his own traffic lane.

The computation of sight distance depends on:

The reaction time of the driver

• The reaction time of a driver is the time taken from the instant the object is visible to the driver to the instant when the brakes are applied.
• The total reaction time may be split up into four components based on PIEV theory. In practice, all these times are usually combined into a total perception-reaction time suitable for design purposes as well as for easy measurement.
• Many of the studies show that drivers require about 1.5 to 2 secs under normal conditions. However, taking into consideration the variability of driver characteristics, a higher value is normally used in the design. For example, IRC suggests a reaction time of 2.5 secs.

Speed of the vehicle

• The speed of the vehicle very much affects the sight distance. The higher the speed, the more time will be required to stop the vehicle. Hence it is evident that, as the speed increases, sight distance also increases.

Efficiency of brakes

• The efficiency of the brakes depends upon the age of the vehicle, vehicle characteristics, etc. If the brake efficiency is 100%, the vehicle will stop the moment the brakes are applied. But practically, it is not possible to achieve 100% brake efficiency.
• Therefore the sight distance required will be more when the efficiency of brakes are less. Also for safe geometric design, we assume that the vehicles have only 50% brake efficiency.

Frictional resistance between the tyre and the road

• The frictional resistance between the tyre and road plays an important role to bring the vehicle to stop. When the frictional resistance is more, the vehicles stop immediately. Thus sight required will be less.
• No separate provision for brake efficiency is provided while computing the sight distance. This is taken into account along with the factor of longitudinal friction. IRC has specified the value of longitudinal friction between 0.35 to 0.4.

The gradient of the road.

• The gradient of the road also affects the sight distance. While climbing up a gradient, the vehicle can stop immediately.
• Therefore sight distance required is less. While descending a gradient, gravity also comes into action and more time will be required to stop the vehicle. Sight distance required will be more in this case.

Key Takeaways:

• The safe and efficient operation of vehicles on the road depends very much on the visibility of the road ahead of the driver. Thus the geometric design of the road should be done such that any obstruction on the road length could be visible to the driver from some distance ahead. This distance is said to be the sight distance.

2.6 Horizontal alignment

• Horizontal alignment is one of the maximum crucial functions of a toll road layout. Its right layout can bring about excessive overall performance concerning velocity, safety, efficiency, and comfort. In addition, it can bring about the saving of economic system and boom the toll road capacity.

• The layout of horizontal alignments calls for the information of layout velocity and horizontal curves. Horizontal alignment consists of a street contain a sequence of hetero strains referred to as tangents with the supply of curves to alternate direction. It additionally consists of the layout of super elevation, more widening, set again distance, transition curve layout, etc.

2.7 Curves

Horizontal curve:

• Horizontal alignment design involves the understanding of the design aspects such as design speed and the effect of the horizontal curve on the vehicles.
• The horizontal curve design elements include the design of super elevation, extra widening at horizontal curves, design of transition curve, and set back distance.
• The presence of a horizontal curve imparts centrifugal force which is a reactive force acting outward on a vehicle negotiating it. The centrifugal force depends on the speed and radius of the horizontal curve and is counteracted to a certain extent by transverse friction between the tyre and pavement surface.
• On a curved road, this force tends to cause the vehicle to overrun or to slide outward from the center of road curvature.

Forces acting on a Vehicle on Horizontal Curve

b: Distance between two wheels or total width of the wheelbase

h: height of the center of gravity of vehicle above the road surface

W: Weight of Vehicle

P: Centrifugal Force acting outwards

Vertical Curve:

• Vertical Curves are the second one of the 2 crucial transition factors in geometric layout for highways, the primary being Horizontal Curves.
• A vertical curve offers a transition among sloped roadways, permitting a automobile to barter the elevation price extrade at a sluggish price instead of a pointy cut.

2.8 Design of super elevation

To counter the effect of centrifugal force and to reduce the tendency of the vehicle to overturn or skid, the outer edge of pavement is raised w.r.t. Inner edge. The transverse inclination of the pavement surface is known as ‘Super elevation’.

• With Super elevation parallel to the road surface, a component of centrifugal force will be acting and a component of weight (because of super elevation) will be resisting the centrifugal force.
• Super elevation e = tanθ
• v: Speed in m/s
• e: Rate of super elevation or angle of super elevation

Equilibrium Super elevation or Balanced S.E.

• Normally, the centrifugal force is supposed to be balanced by super elevation along with the coefficient of lateral friction.
• If f=0, then the entire centrifugal force is balanced by super elevation alone. This super elevation is known as Equilibrium or Balanced Super elevation.
• With equilibrium super elevation, the pressure on the inner and outer tyre will be the same as that of a vehicle moving on a straight road (without super elevation).
• But since only Super elevation counteracts centrifugal force, this results in a very high value of super elevation. So, super elevation along with friction is preferred.
• Equilibrium super elevation is best suited for vehicles moving with design speed.
• If the vehicles are moving slowly over the curve with Equilibrium Super elevation, there is a chance of skidding or overturning inwards. So, equilibrium super elevation is not practically used.

Maximum and Minimum Super elevation [IRC]

• Camber and Super elevation both are cross slope. Camber is for drainage purposes; the cross slope is for countering centrifugal force.
• If the calculated super elevation is less than or equal to camber, then Super elevation equal to camber should be provided from drainage consideration. Else, drainage problems might be there affecting the pavement surface quality.

Key Takeaways:

• Horizontal alignment design involves the understanding of the design aspects such as design speed and the effect of the horizontal curve on the vehicles.

2.9 Widening

• When a vehicle takes a turn to negotiate a horizontal curve, the rear wheels do not follow the same path as that of the front wheels. Normally the rear wheels follow the inner path on the curve as compared with front wheels.
• The vehicle has occupies more width that it occupies on straight portion of the road. To compensate this, the carriageway width increased on the entire curved portion of the road, which is called extra widening of pavement on curve.
• T is located that pavement width, on the horizontal curves, are incredibly large than the width at the instantly roads. This greater width is the end result of greater widening of the pavement on the curves. The motives at the back of this are as follows. The wheelbase of the cars is inflexible and consequently at the same time as taking the turn, best the front wheels are capable of extrade direction. The direction travelled via way of means of the the front wheels can be one-of-a-kind and at positive distance outwards from the direction traced via way of means of the rear wheels.
• There is an inclination of the driving force to take the outer direction on the curves to have greater sight distance seen ahead. While overtaking operations on horizontal curves motive force will want greater spacing from the opposite cars to experience safer.

2.10 Transition curves

• The transition curve is provided to change the horizontal alignment from straight to circular curve gradually and has a radius that decreases from infinity at the straight end (tangent point) to the desired radius of the circular curve at the other end (curve point).
• The rate of change of radius of the transition curve depends on the shape and equation of the curve.

Necessity of Transition Curve

• Introduce gradually the centrifugal force between the tangent point and the beginning of the circular curve avoiding a sudden jerk on the vehicle.
• Enable the driver to turn the steering gradually with comfort and safety (easy to follow the path for drivers).
• Minimize encroachment on adjoining traffic lanes tend to promote uniformity in speed.
• Enable gradual introduction of designed super elevation
• Enable the gradual introduction of required extra widening.
• Improve the aesthetic appearance of the road.

Different Types of Transition Curves:

• Cubic Parabola
• Lemniscates
• Spiral (Clothoid)
• Up to a deflection angle of 9 degrees, there is no significant difference in these curves.
• Radius decreases with an increase in length for all these curves.

Lemniscates and Cubic Parabola:

• Rate of change of radius and hence rate of change of centrifugal acceleration is not constant for large deflection angle.

Spiral:

• Radius is inversely proportional to the length and the rate of change of centrifugal acceleration is uniform throughout the length of the curve.

Ideal Shape of Transition Curves:

• The rate of introduction of centrifugal force or rate of change of centrifugal acceleration should be consistent.
• The length should be inversely proportional to the radius.
• Spiral fulfills the condition of an ideal transition curve.
• The geometric property of the spiral is such that the calculations and setting out of the curve in the field are simple and easy (LR= constant).
• The spiral transition curve simulates the natural turning path of a vehicle.

Design Criteria:

Centrifugal Force / Comfort Criteria

Ls: Length of Transition Curve

v:  Speed in m/s

R: Radius of Horizontal Curve in m

C: Rate of change of Centrifugal Acceleration in m/s3

2.11 Vertical curves

• Vertical curves facilitate a gradual change between two different gradients.
• Vertical curves should be simple in application and result in a design that is safe and comfortable in operations, pleasing in appearance, and adequate for drainage.

Types of Vertical Curves:

• Summit Curves
• Valley Curves 

Summit Curve:

• Many curve bureaucracy may be used with best results, the not unusual place exercise has been to apply parabolic curves in summit curves. This is mostly due to the benefit with it may be laid out in addition to permitting a cushty transition from one gradient to another.
• Although a round curve gives identical sight distance at each factor at the curve, for extremely small deviation angles a round curve and parabolic curves are nearly congruent.
• Furthermore, using parabolic curves had been observed to offer exceptional driving consolation In figuring out the kind and duration of the vertical curve, the layout issues are consolation and protection of the driver, and the arrival of the seasoned le alignment. Among those, sight distance necessities for the protection is maximum vital on summit curves.
• The preventing sight distance or absolute minimal sight distance ought to be supplied on those curves and in which overtaking isn't always prohibited, overtaking sight distance or intermediate sight distance ought to be supplied as a long way as possible.
• When a quick transferring car travels alongside a summit curve, there's much less soreness to the passengers. This is due to the fact the centrifugal pressure could be appearing upwards even as the car negotiates a summit curve that's towards the gravity and subsequently part of the tyre strain is relieved. Also if the curve is supplied with ok sight distance, the duration might be enough to ease the surprise because of extrade in gradient.
• Circular summit curves are same because the radius stays identical at some point of and subsequently the sight distance. From this factor of view, transition curves aren't suitable because it has various radius and so the sight distance will even vary.
• The deviation perspective supplied on summit curves for highways are very large, and so the a easy parabola is nearly congruent to a round arc, among the identical tangent points. Parabolic curves is straightforward for computation and additionally it have been observed out that it affords precise driving consolation to the drivers.
• It is likewise smooth for discipline implementation. Due to these kind of reasons, a easy parabolic curve is favored as summit curve.

Valley Curves:

• When grades meet on the valley (sag) and the curve may have convexity downwards, the curve is truely referred because the valley (sag) curve.
• As withinside the case of horizontal curves, the specific sorts of curves in line with geometrical configuration are: Circular Quadratic parabola Cubic parabola and different types of transition curves V curve or sag curves are vertical curves with convexity downwards.
• They fashioned whilst gradients meet in any of the subsequent 4 ways: When a descending gradient meets any other descending gradient. When a descending gradient meets a flat gradient. And When a descending gradient meets an ascending gradient. When an ascending gradient meets any other ascending gradient.

Key Takeaways:

• The rate of change of radius of the transition curve depends on the shape and equation of the curve.

References:

1. Pavement Design: Yoder and Witzak Wiley

2. Traffic Engineering: L.R.Kadiyali Khanna Publishers