Project 1 - Geometric Design Parameter- Prerequisites
the highway elements required for Road Geometric Design as prerequisite and necessary checklist during the design:-
1. Cross Sectional Elements of Roads:-
This is a major input element before degning and construction of any road
The cross sectional elements are :-a.Camber, b.Width of carriageway, c.Kerbs, d.Pavement surface characteristics, e.Road margin, f.Width of formation
Camber: Camber is a transverse slope which is provided w.r.t horizontal. they are expresed in terms of percentage. They are mainly provided for drainage.
Width of carriageway: Width of a road design on many factors like traffic density, the precent capacity, level of service and the future growth. the width depends upon economy
Kerb: Edge of the pavement/carriageway.They are provided on the median and footpath side so that the vehicle does not easily jump to the lanes aand to the footpath.Provided to channelize the rain water to the drain
2 Pavement surface charateristics: there are lot more reasons which defines the pavement
i. Friction: friction plays the major role for surface characteristics. The friction between the 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
ii. Light reflection: white roads causes glare during day time but provides good visibility oa night. black roade have poor visibility and less glare.
iii. Drainage: Absolute 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 Topography: Topography is the 3 dimensional representation of the surface features of the land. It will impact other factors like design speed and cost. Based on the topography, the design standards will vary and basically the are classified into plain terrain (Cross slope varying from 0-10%), rolling terrain (cross slope varying from 10-25%), mountain terrain or hilly terrrain (cross slope varying from 25-60%) and steep terrain (cross slope more than 60%)
4 Road Margins: It is the portion of road beyond the carriageway on the roadway.
single lane (3.5m), two lane (7m), intermediate lane (5.5m)
Shoulders: they are provided along the edge of the pavementto accomodate the stopped vehilce and also act as emergency lane. there are 2 types of shoulders paved and earthern shoulders
Median: median is generally provided in a divided carriageway like 4 lane, 6 lane or multilane roads. it seperates the different direction movement of vehicle. median width varies depending upon the urban and rural area
5. Enviroment: The factors like the forest area and noise pollution will play a major role in the geometry of the road
near hospitals, schools and residential zones (which are noise restricted zones ) should not have too many curves and have better visibility
The geometry of the road must be planned without affecting the flora and fauna.
The reaction time and mood of the driver will also affect the geometry of the road design must be planned according to this.
6. Economy: THis always plays a major role in deciding the geometry. The road design should be economical as far a possible. unnecessary wastages should be avoided by the project precisely.
7. Sight distance: The actual distance along the road which the drivers 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 the vehicle safely.
the sight distance will ensure the driver can react to a situation and stop the vehicle
SSD - Stopping sight distance and Absolute minimum distance
ISD - Intermediate sight distance (Twice the SSD)
OSD - Overtaking sight distance
Head light sight distance
Safe sight distance for intersection
SSD - Stopping sight distance and Absolute minimum distance
The minimum sight distance available o a highway to stop the vehicle 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 perceives to the time the deceleration is complete
Adequate time is necessary for the driers for them to react to the obstacle spontaneously
The required entity for the calculation of the SSD are 1. lag distance and 2. brake distance
SSD = VT + V^2/2g(f ± .01n)
ISD - Intermediate sight distance (Twice the SSD)
The requirement of OSD is not always be satisfied. In such cases overtaking is prohibited by using regulatory signs.
To provide an opportunity for overtaking operation on horizontal curves or in restricted areas, we provide intermediate sight distance, I.e, equals to twice of stopping sight distance.
ISD
OSD - Overtaking sight distance
This is the distance provided in a two way direction travel road. THe distance that is safely available for the vehicle to overtake the vehicle in front, without hitting the vehicle coming in the opposite direction
OSD =d1 + d2 + d3
Head light sight distance
The distance f the road available during the illumination of the head light of vehicle durig night time
As per IRC, the HLSD is taken as equal to SSD
HLSD has to be designed very carefully during the design of vertical sag curve
Safe sight distance for intersection
Safe sight distance is generally designed at the intersection of roads.
At intersections where two or more roads meet, visibility should be provided for the drivers approaching the intersection fro either sides.
They should be able to perceive any hazard and stop the vehicle if required
8. Horizontal Designs: One of the most important elements in the design of roadway
The proper design of horizontal curves 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 passes vertical and horizontal curve in their alighnment to be compatible with the existing condition
the constraint are topographical property ownership, land use, cost and environment
9. Horizontal curves
Types of Horizontal Curves:it is essential for a smooth and safe driving, especially when the traffic is moving at a high speed
1.Simple curve
Simple curve is a arc of a circle and most commonly used. The radius of the circle determines the sharpness and flatness of the curve. Larger the radius flatter the curve. they are generally used for higher radius
2.Compound curve
This curve normally consist of two simple curves curving in the same direction and joined together.
3.Reverse Curve
The reverse curve is the combination of two curves, but in opposite direction. for the safe uses the designers are nt suppose to use this curve unless it is absolutely necessary
4.Spiral Curve
Spiral curve is the curve with varying radius used in railroads and some modern highways. it provides a transition for tangent to a simple curve or between simple curves in a compound curves.