Project 1 - Geometric Design Parameter- Prerequisites

Ans: The following Highway elements required for Road Geometric Design as prerequisite & necessary checklist during design.

1) Design speed:

• Design speed is the most important factor while geometric design.
• Design speed is known as the vehical can travel in a smooth alignment with high speed.
• Design speed is variying with different terrain conditions and different site conditions like plain and hilly and rural and builtup.

2) Topography:

• Topography plays an important role in the design of geometry.
• Topography is a 3 dimensional representation of existing surface of the land
• Topography will change based on terrain condition like plain and hilly
• It will impact the project cost and design speed of the road

3) Traffic:

• While design of geometry traffic is also important factor for consideration
• A reasonable traffic flow is considering for road design based on the estimations carried out after carriying various traffic surveys.
• the geometry of the road depends upon the design volume and capacity of the traffic

4) Design vehicle:

• Design vehicle consist of standard weight and characteristics and dimensions while considering in a geometric design
• The dimension and weight and characteristics of the vehicle influence the width of the carriageway and number of lanes and turning radius of the curve
• The standard design vehicle shall be applicable for design of roads

5) Environment:

• While designing the road environment plays an major role in the geometry
• Forest and wild life will play important role while geometry design
• While designing the highway the design shall consider flora and fauna
• while Designing the highways shall be less disturbance to the forest and wild life

6) Human:

• While designing the highways human behaviour as road user in the form of pedestrian need to be considered
• Reaction of time and mental condition of the pedestrain is very important for the design of geometry
• While designing the highways seperate pedestraina fecilities need to be given. that will not disturb the main traffic on the highways

7) Economy:

• Economy plays an important role while designing the road
• Economy should match with the fund planned
• Economy will change based on the terrain and site consition

8) Aesthetics:

• Aesthetic plays a major role in defining the geometry of the road and importance of that area, historical building and nature.
• While design the highway near aesthatic plases the designer should keep in mind to give the extra fecilities for road users

Cross sectional elements of the roads:

1) Camber:

• Camber is the transverse slope provided with respect to the horizontal. it is expressed in percentage
• Camber is mainly used for drainout the water from the surface of the road

2) Width of carriageway:

• Width of the carriageway depends upon the traffic density and present capacity and future growth
• Width of carriagway will affect the economy of the project

3) Kerb:

• Kerb indicates the boundary of the pavement and island or median, it is also used for safey reasons
• Kerb is provided a the median and between main carriageway and service road in built up sections due to this vehicle do not jump inti other side of the road.
• Kerb is also used for channalised the rainwater to the drain

4) Friction:

• The force developed between the wheel and road is the load multipled by a factor is called cofficient of friction
• Longitudinal Friction considered to be 0.35 to 0.4
• Lateral friction considered to be 0.15
• To avoid skidding sufficient friction to be provided

5) Drainage: Drainage is provided for drainout the rainwater from the pavement surface very fastly, improper drainage affect the pavement composition and it will affects undulation in the pavement

Sight distance:

1) Stopping sight distance:

• The minimum distance available on a highway to stop the vehicle available at a design speed without collision with any obstruction
• For the analysis of SSD we need a) Lag distance b) Brake distance
• Lag distance: time required for driver to analyse the situation

Lag distance=VT

• Braking distance:It is the distance travelled by a vehicle after applying the brake
• Braking distance=v^2/2gf

Stopping sight distance=VT+V^2/2gf

Stopping sight distance=Vt+v^2/2g(f+or-0.01n) (in gradient location)

2) Intermediate sight distance:

• Intermediate sight distance is used for design of highways in all the time in single direction.
• Intermediate sight distance=2XSSD

3) Overtaking sight distance:

• Overtaking sight distance is known as the distance i.e safely available for the vehicle to overtake the vehicle in front, without hitting the vehicle coming in opposite direction
• Overtaking sight distance=d1+d2+d3
• i.e d1+d2+d3=Vbxt+VbxT+2XS+VXT

4) Head light distance:

• The distance required for distance of the road available during the illumination of the head light of vehicle during night time
• HLSD is taken as equal to SSD
• It is more important in vertical design

5) Safe sight distance:

• Safe sight distance is generally designed at the intersection of roads
• If two or more roads meet, visibilty should be provided for drivers from either side

6) Overtaking zone:It is known as overtaking opprtunity for vehicles moving at a design speed should be given at frequent interval

overtaking zone =3x(d1+d2+d3)

Horizontal design of the road:

1) Alignment:

• Alignment is the linear picture of the road which represnt plan and profile
• Alignment will need to be change for the following factors
• a) Topography b) existing building structures c) Availability of land

2) Factors affecting horizontal design:

• Terrain:Terrain is the 3 dimension view of existing surface based on the terrain condition the design of the road will change. the economy of the road change with respect to the terrain
• Vehicle characteristics:Vehicle characteristics is the one of the most important factor design of roads. the vehicle characteristics will change the road width, curve design etc.
• Driver limitation: The road user li.e driver or pedestrian should know the traffic rules and regulations for safety while using the road
• Design speed: design speed varies with respect to the site conditions.
• Sight distance:Sight distance is most important factor for design of roads. it will vary with respect to terrain condition
• Drainage condition: Drainage is one of the most important factor for design of roads. proper drainage will drain the rainwater from the surface of the road.
• Economy:Economy of the road will meets the planning. economy will change w.r.t Terrain
• Environment: while design of highway designer should keep in mind to avoid flora and fuana

3) Horizontal curve:

• Simple curve: Simple curve is an arc of circle it is the most commonly used in horizontal curve design, larger the radius flater the curve
• Compound curve: compound curve consist of two simple curves curing in same direction and joined together.
• Reverse curve: Reverse curve consist of two simple curves joined together and curving in opposite direction, these curves not used unless it is absolutly reuired.
• Spiral curve: Spiral curve is a curve with variying radius used in rail and some modern highways

4) Superelevation:

• To avoid skidding in horizontal curves the surface of the road is given an inward tilt it is knowm as superlevation
• The rate of superelevation generally increases with speed and sharper curvature.
• Superelevation e=V^2/225R
• Advantage of super elevation is it allows the running of vehicle at a higher speed on curved path without any dangoerous

5) Method of providing super elevation:

• Elimination of the crown of the cambered section
• Rotation of pavement to attain full superelevation

6)Minimum curve radius: Rmin=V^2/127(emax+fmax)

7) Extra widening:

• Extra widening refers to the additional width of the carriageway that is requied on a curved section of road user and above that required on a straight alignment
• it is mandotory to provide extra widening curves below 400m radius
• Extra widening=Machanical widening+Psychological widening
• Machanical widening =L^2/2R
• Psychological widening=V/9.5 Squareroot of R

8) Transition curve:

• A Curve is provided between straight portion and circular curve
• It is provided for smooth change is a direction from straight line to circular curve
• Necessity of transition curve is is when a vehicle travel from staright to a circular curve it is suddenly subjected to an additional outward centrifugal force
• There are 3 types of transition curves i.e
• a) Lemniscale
• b) Cubic parabola
• c) Spiral curve
• Minimum length of transition curve should be greater than the values devided from considerations
• a) Rate of change of centrifugal acceleration = Ls= 0.0215v^3/CR
• b) Rate of change of superelevation = Ls= 2.7V^2/R

9) Set back distance:

• Due to obstruction like walls , cut slopes and buildings arise lack of visibility
• Required sight distance should be available across the inside of the horizontal curve
• Set back distance=R-(R-n)cos θ

Vertical design:

• Vertical curves provided a gradual change from one tangent gardient tp another
• It consist of gradients and vertical curves
• Gradient: Rate of rise or fall along the length of the road w.r.t horizontal is known as gradient
• it is expressed in a ratio of 1 in x
• There are different types of gradient i.e
• a) Ruling gradient: Maximum gardient that is used in design is called ruiling gradient
• b)Limiting gradient:Limiting gradient may be used where the topography of the places necessity. it is used in forest sections
• c) Exception gradeint: It is provided in extreame difficult conditionssteeper than limiting. for plain terrain it is 7% and 8% for hilly

1)Deviation angle: It is the algebric difference between two gradients i.e N=n1-n2

Garde compansation = (30+R)/R

2) Summit curves: When a vehicle approaches a summit curv, the view of the road is cut off beyond the summit

• Simple parabolic curve is generally used as summit curves
• Length of summit curve depends upon Deviation angle and Required sight distance which may be OSD, ISD or SSD

Length of summit curve exceeding the required sight distance: L=NS^2/9.6

Length of curve less than the required sight distance: L=2S-9.6/N

In the case of SSD

Length of summit curve exceeding the required sight distance: L=NS^2/4.4

Length of curve less than the required sight distance: L=2S-4.4/N

3) Valley curves: Valley curves are provided at the place of descending gradients

• During night time valley curves design must ensue that the roadway is properly illuminated
• Critieria for determining the valley curve is

Head light sight distance:

Length of valley curve exceeding the required sight distance: L=NS^2/1.5+0.035S

Length of curve less than the required sight distance: L=2S-(1.5+0.035S)/N