Project 1 - Geometric Design Parameter- Prerequisites

1.List all the highway elements required for Road Geometric Design as prerequisite & necessary checklist during design. eg. Design Speed, Design Vehicle etc..

The highway elements required for Road Geometric Design Are

1. CROSS-SECTIONAL ELEMENTS OF ROAD AND PAVEMENT SURFACE CHARACTERISTICS

1.CAMBER

Camber or cant is the cross slope provided to raise middle of the road surface in the transverse direction to drain off rain water from road surfaceToo steep slope is undesirable for it will erode the surface. Camber is measured in 1 in n or n% and the value depends on the type of pavement surface.

The objectives of providing camber are:

1.Surface protection especially for gravel and bituminous roads

2.Sub-grade protection by proper drainage

3.Quick drying of pavement which in turn increases safety.

2.Width of carriage way

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

Single lane - 3.75m

Two lane (no kerb) -7m

Two lane (Raised kerb)-7.5m

Intermidiate carriage -5.5m

Mulitilane - 3.5m

3.KERBS

Kerbs indicate the boundary between the carriage way and the shoulder or islands or footpaths. Different types of kerbs are :

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

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

3.Barrier type kerbs : They are designed to discourage vehicles from leaving the pavement. They are provided when there is considerable amount of pedestrian traffic. They are placed at a height of 20 cm above the pavement edge with a steep batter.

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

4.ROAD MARGINS

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

1.Shoulders

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

2.Parking lanes

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

3.Bus-bays

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

4.Service roads

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

5.Cycle track

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

6.Footpath

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

7.Guard rails

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

5.WIDTH OF FORMATION

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

6.RIGHT OF WAY(ROW)

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

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

7. FRICTION

For safe and comfortable driving four aspects of the pavement surface are important; the friction between the wheels and the pavement surface, smoothness of the road surface, the light reflection characteristics of the top of pavement surface, and drainage to water.

Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also affect the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles.

1.Skidding happens when the path traveled along the road surface is more than the circumferential movement of the wheels due to friction

2.Slip occurs when the wheel revolves more than the corresponding longitudinal movement along the road.

Various factors that affect friction are:

1.Type of the pavement (like bituminous, concrete, or gravel),

2.Condition of the pavement (dry or wet, hot or cold, etc),

3.Condition of the tyre (new or old), and

4.Speed and load of the vehicle.

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

8.Unevenness

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

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

8.Light reflection

1.White roads have good visibility at night, but caused glare during day time.

2.Black roads has no glare during day, but has poor visibility at night

3.Concrete roads has better visibility and less glare

It is necessary that the road surface should be visible at night and reflection of light is the factor that answers it.

9.Drainage

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

2.TOPOGRAPHY

The topography is widely used, the results of topographic surveys are used to prepare maps of the surface of the land, above and below the sea, to establish limits on the lands of private and public property.

It is a very important stage before starting a construction. It is necessary to perform topographical works before, during and after the construction of works, such as roads, railways, buildings, bridges, canals, dams, among others.

Second important factor that affects the geometric design is the topography. For a plain terrain, it is very easy to construct the highway as per the standards. As the terrain and the gradient increases the construction cost will increase for a specific design speed. So, to keep the construction cost and time under control, the geometric standards vary with different terrain or topography. This is hence classified as steeper gradients and sharper curves

Topography will impact other factors like design speed and cost

Based on the topography conditions the design standards will vary and basically they are classified into

Plain terrain: Area of the country having cross slope varying from 0-10%

Rolling terrain: Area of the country having cross slope varying from 10-25%

mountains terrain or hilly terrain: Area of the country having cross slope varying from 25-60%

Steep terrian: Aera of the country having greater than 60%

Design speed: Design vehical consist of standard weight dimension and operating characteristics

The dimension, weight and the characteristics of the vehical influence the gradiant, turning radius, width of carriageway the clearence requirement and other geometry feature

The standard design vehical shall be applied in designing all the type of roads

Three types of commercial vehical have been recognized as single unit, semi trailer, truck trailer combination

The selection depends upon terrain condition, economic justification and importance Of road

3.MISCELLANEOUS FACTORS REQUIRED TO CHECK DURING HIGHWAY DESIGN ARE

1.VEHICEL CHARACERISTICS

2.HUMAN BEHEAVIOUR

3.ENVIRONMENT AND ECONOMY

4.TRAFFIC

5.MATERIALS

6.LOCATION AND FUNCTION

7.AESTHETICS

Large variety of vehicles are now made which range from tiny to massive units. The weight of the axle, the dimensions of the car and the characteristics of the vehicle influence greatly the design aspects. The design aspects involve the pavement width, the clearances, the radii of curve and the parking geometrics. To facilitate this requirement, a design vehicle is set which own a standard weight, operating characteristics, and dimension. This helps to establish a design controls so that vehicle of designated type is accommodated. The physical, mental and psychological characteristics of the human affect greatly the geometric design of the highway. Always a reasonable value of traffic is considered for the geometric design. The design for a higher traffic value result in a design that is uneconomical. This value is collected from various and previous traffic data collected and recorded. While developing a geometrical design, it is very essential to give importance to the environmental concerns like noise and air pollution. The design developed considering all the above factors have to be economical in nature. It should come within the capital and the maintenance cost that is allotted. The highway geometric design has to be such a way that the overall aesthetics of the environment is not affected

4.SIGHT DISTANCE

The distance along the center line of the road at which a driver has visibility of an object, stationary or moving at a specified height above the carriage way is known as sight distance.

Sight distance is the length of road visible ahead to the driver at any instance. The standards for sight distance should satisfy the following conditions :

1.Driver travelling at the design speed has sufficient sight distance or road length visible to stop vehicle, in case of any obstruction on the road ahead, without collision.

2.Driver travelling ahead should be able to overtake slow moving vehicles, at reasonable intervals, without causing obstruction or hazard to traffic of opposite direction.

3.The driver entering a uncontrolled intersection, he should have sufficient visibility to enable him to control his vehicle in order to avoid collision with another vehicle.

The sight distance depends upon following factors

1.Speed of the vehicle.

2.The frictional resistance of the road surface.

3.Height of drivers eye.

4.Slope of the road surface.

5.Perception time and brake reaction time

DIFFERENT TYPES OF SIGHT DISTANCES ARE

1.Stopping sight distance(SSD) - The minimum sight distance available on a highway to stop the vehicle avilable at a design speed without collision without any obstraction is called SSD

SSD = VT + V^2/ 2GF

2. ISD - The intermediate Sight distance is generally the sight distance similar to SSD. Which is generally twice of the SSD.

ISD = 2SSD

3. Overtaking sight distance (OSD)- The distance that is safely avilable for the vechicle to overtake the vechicle in front, without hitting the vechicle coming in the opposite direction , this distance is called OSD

OSD = d1+d2+d3

OSD= v_b·t_r + v_bT+ 2s+ v_cT metres

T =sqrt(4s/a) S= 0.7v_{b + 6}

4. Headlight sight distance (HLSD) - The distance of the road avilable during the illumination of headlings of vechicles during the night time , this distance is called HLSD .As per IRC HLSD is equal to SSD.

HLSD =SSD

5.Safe sight distance - Safe sight distance is generally designed at the intersection of roads . At intersection where two or more roads meet visbility should be provided for the drivers approaching the intersection from either sides . This distance is called Safe sight distance

5.HORIZONTALCURVE DESIGN

The horizontal Design is a series of horizontal tangents (straight roadway sections), circular curves, and spiral transitions used for the roadway’s geometry. This design shows the proposed roadway location in relation to the existing terrain and adjacent land conditions. The main objective of geometric roadway design is to integrate these elements to produce a compatible speed with the road’s function and location. Safety, operational quality, and project costs can be significantly influenced by coordinating the horizontal and vertical alignments.Roadway horizontal curve design is based on the laws of physics and driver reaction to lateral acceleration. Any geometric alignment needs to address curve location; curve sharpness; tangent lengths; and how they relate to the vertical profile. All of these components should be balanced to operate at appropriate speeds under normal conditions.
Elements of Curve Design:
-Curve radius
-Superelevation
-Side friction
-Assumed vehicle speed
Horizontal curves depend on specific values for a minimum radius (based on speed limit), curve length, and sight obstructions (sight distance). An increased superelevation (bank) may be required to assure safety for high speed locations with small curve radii. Designers must confirm sufficient sight distance around corners or curves in order to avoid crashes.

Different type of Horizontal curve are
a) Simple curve
b) Compound curve
c) Reverse curve
d) Transition curve

Factors affecting the design of horizontal curves .

1.Terrain

2.Vechicle characteristics

3.Driver limitaions/emotions

4.Design speed

5.Sight distance

6. Drainage conditions

7. Economy/ cost

8. Environment.

Design speed is the single most important factor that affects the geometric design. It directly affects the sight distance, horizontal curve, and the length of vertical curves. Since the speed of vehicles vary with driver, terrain etc, a design speed is adopted for all the geometric design.

It is easier to construct roads with required standards for a plain terrain.However, for a given design speed, the construction cost increases multi form with the gradient and the terrain.

The environmental factors like air pollution, noise pollution, landscaping, aesthetics and other global conditions should be given due considerations in the geometric design of roads.

Large variety of vehicles are now made which range from tiny to massive units. The weight of the axle, the dimensions of the car and the characteristics of the vehicle influence greatly the design aspects. The design aspects involve the pavement width, the clearances, the radii of curve and the parking geometrics. To facilitate this requirement, a design vehicle is set which own a standard weight, operating characteristics, and dimension

The physical, mental and psychological characteristics of the human affect greatly the geometric design of the highway. Always a reasonable value of traffic is considered for the geometric design. The design for a higher traffic value result in a design that is uneconomical.

6.SUPERELEVATION.

Superelevation is a method used in roadway curves where the outer edge of the pavement is raised above the inner edge. An aspect of the vertical alignment or “profile” of a road viewed in cross-section, it’s an important safety element in the design criteria of any road with curves.

Advantages of providing Super elevation:-

1.Super elevation is provided to achieve the higher speed of vehicles.
It increases the stability of fast-moving vehicles when they pass through a horizontal curve, and it also decreases the stresses on the foundation.

2.In the absence of super elevation on the road along curves, potholes are likely to occur at the outer edge of the road.

3.The Indian road congress(IRC) has prescribed the max value of Super Elevation is 1 in 15.

SUPERELEVATION FORMULA IS

IF VELEOCITY IN METERS THEN e + f = V²/ gR

IF VELOCITY IN KMPH THEN e+f = V²/127R

Where,

e = rate of Super elevation in %

V = velocity of vehicle in m/s

f = lateral friction factor = 0.15

g = acceleration due to gravity = 9.81 m/s²

R = radius of circular curve in meters.

7.VERTICAL CURVE DESIGN

Vertical alignment of the road comprises of gradients and vertical curve. Vertical alignment is defined as the height and depth in a vertical axis with respect to the horizontal axis, this height and depth, it creates a gradient and vertical curve.

Roadway vertical alignments consist of crest and sag curves with straight grades connecting them. The vertical profile is typically displayed as a graph with elevation on the vertical axis, and horizontal alignment distance on the horizontal axis. Geometric design of the proposed profile is governed by safety, vehicle operations, drainage and construction issues.

Vertical alignment of highways consists of

For grades flatter than 4% grade compensation is not required due to negligible loss of tractive force.

Grade compensation is

Maximum value of gradient compensation =%, R= radius of horizontal curve

Types of gradients

1.Ruling gradient

2.Limiting gradient

3.Exceptional gradient

4.Minimum gradient

Vertical Curves

Generally two types of vertical curves are there to adopt for the vertical alignment. They are

• Summit curve
• Valley curve

Summit curve Summit curve is a vertical curve adopted mainly when the gradient is upwards. In case of summit curve simple parabola is considered as best curve shape. There are four different cases are involved in adoption of summit curve as follows.

Case-1 When upward gradient meets a flat gradient

Case-2 When upward gradient meets another upward gradient

Case-3 When upward gradient meets downward gradient

Case-4 When downward gradient meets another downward gradient

Valley curve It is a vertical curve provided when the gradient is downwards. Generally when the vehicle meets downward gradient it accelerates more and discomfort arises. So, in the design of valley curve in vertical alignment comfort is considered along with sight distance. Here also four cases are considered but case 2 and case 3 are same as summit curve and the other cases are as follows.