Project 1 - Geometric Design Parameter- Prerequisites

The highway elements required for road geometric design are 

1. Cross-sectional elements of road 
2. Pavement surface characteristics
3. Road margin
4. Topography 
5. Environment 
6. Economy
7. Sight distance 
8. Horizontal design
9. Superelevation 

1. Cross-sectional elements of road :The cross-sectional elements will be one of the major input before we design the geometry pf a road 

• The cross-sectional elements are : a.Camber b.Width of carriageway c.Kerbs d.Pavement surface charateristics e.Road margin f.Width of formation

1. Camber: Camber is a transverse slope provided with respect to horizontal,

Camber is expressed in terms of percentage,roads are not flat but are designed and constructed with drainage gradient for water runnoff known as camber 

b.Width of carriageway: Width of a road design on many factors like traffic density, the present capacity, level of service and the future growth 

The width decision also depends upon the economy, the minimum width depends upon the width of vehical and the clearance 

c.Kerbs: It indicates the edge of the pavement/ carriageway

Kerbs is provided not only to identify the edge but also for may safety reason 

Kerbs is provided on the median and foothpath side so that the vehical does not eaisly jump to ther lanes and to the foothpath 

They are also provided to channelize the rain water to the drain 

2. pavement surface characteristics: 

Friction: friction between wheel and road surface is an important factor in deciding the safe operational speed and also the curve design. the force that develops between the wheel and road is the load acting multiplied by a factor called co-efficient of friction 

Light reflection: white roads causes glare during day time but provides good visibility at night.black roads have poor visibility and less glare.road surface should necessarily visible at night and reflection of light is the factor that answers

Drainage: Abolutely impermeable pavement surface should be used to prevent seepage of water into the pavement layers. further both the geometry and texture of pavement surface should help in drainage out the water from the surface in less time 

3.Road margins: road margins is the portion of the road beyond the carriagway on the roadway. single lane 3.5m, two lane 7m, intermediate lane 5.5m, Multilane 3.5mx number of lanes 

Shoulder: shoulder are provided along the edge of the pavement to accommudate the stopped vehical and also act as emergency lane. generaly there are two types of shoulder that is paved shoulder and earthen shoulder

Median: median is generally provided in a divided carriageways like 4 lane,6lane or multilane roads. it seperates the different direction movement of vehical . median width varies depend upon the urban area and rural area

4.Topography : Topography is  a 3 dimensional representation of the surface featuures of the land

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 pf road

5.Environment: The factor like the forest area and noise pollution will play a role in the geometry of the road 

In the noise restricted zone like school zones we should not have too many curves and have better visibility 

The geometry of the forest area road has  to be planned in such a way we avoide disturbance to floora and founa and also at social places like religious worship places

Human: As a human  is going to actual road users or facility user 

Human behaviour as a road user in the from of driver and also as pedestrians need to be considered 

The reaction time of the driver the behaviour and mental conditions of pedestrain crossing the road also influnce the geometry 

6.Economy: This always plays a major role in deciding the geometry 

The road designed are to be economical as far as possible 

It should be match with the funds planned including the other costs like maintenance

7.Sight distance:  the actual distance along the road which the driver from a specified height above a carriageway has a visibility of stationary or moving objects 

It is important to have a better sight distance to ensure the driver can stop the vehical safely 

The sight distance ensure the driver can react to a situation and stop vehical

Types of sight distance 

i.Stopping sight distance or absolute minimum sight distance (SSD)

ii. Intermediate sight distance(ISD) 

iii. Overtaking sight distance(OSD) 

iv. Head light distance

v.Safe sight distance for intersection

i. Stopping sight distance(SSD): The minimum sight distance available on a highway to stop the vehical available at a design speed without collision with any obstruction

The parameter of safe stopping distance is the most important feature in the traffic engineering 

Safe stopping distance is the distance from the point it first perceive to the time the deceleration is complete 

Adequate time is necessary for the drivers for them to road to the abstacle spontaneously 

The Analysis of SSD is sum of i. Lag distance ii. Brake distance

i.Lag distance: The lag distance that is moved by the vehical in a time period 't' at a velocity 'v'in m/s

Hence  lag distance is 'VT' where v is design speed of vehical  and t is reaction time of driver 

ii.Braking distance: Braking distance is the distance traveled by a vehical during the braking operation 

For a level road, braking id obtained by equating the work done is stopping the vehical and the kinetuc energy of the vehical Íf F is the maximum friction force developed and the braking distance is 'L' then work done against friction in stopping the vehical is FL=fWL where W is the total weight of the vehical , f is co-efficient of friction

ii. Intermediate sight disance(ISD) and iii. over taking sight distance(OSD): 

8.Horizontal design:  The horizontal curve is one of the most important elements in the design of highway 

• The proper design of horizontal curve is essential for a smooth and safe driving, especially when the traffic is moving at a high speed 
• Roads must pass through the existing and different environment while delivering the needs for safety and cost effectiveness
• Roads are not always flat and straight they passess vertical and horizontal curves in their alignmment to be compatible with the existing conditios
• The constrains typically include topograpical property ownership, land use, cost and environment
•  

9.Superelevation: Superelevation is the banking of a roadway along a horizontal curve so motorists can safely and comfortably maneuaver the curve at reasonable speeds