Hello friends, in this article I will deal with the geometric design of highways The design of the highway is an important part of civil engineering. All the highway component is essential for the road design before the construction of the road. So, friends, you can learn about the geometric design of roads in this article.
Geometric Design of Highway and street
Geometric Design of Highway
The geometric design of the highway deals with the dimension and layout of the visible features of the highway such as alignment, sight distance, camber, superelevation, curve, etc.
The geometric of the highway should design to provide optimum efficiency in traffic operation with maximum safety at a reasonable cost.
This design of highway deals with the following elements :
Cross-section elements
Traffic lands, carriageway, shoulder, median strips, right of way, side slopes.
- Camber
- Superelevation
- Sight distance across the road.
The element of Horizontal alignment
- Horizontal curve radius
- Length
- Transmission curve
- Sight distance
The element of vertical alignment
- Gradient
- Length of the vertical curve.
Factor affecting Geometric Design
The geometric design is directly affected by some factors. Which factors are too selective when the designer should design the highway? Following are the main factors to affect the geometric design of highways:
- Design speed
- Topography
- Traffic volume and composition
- Traffic capacity
- Road user behavior
- Design vehicles
Camber
Camber is the convexity provided to the cross-section of the carriageway. It provides on the sight road by raising the center of the carriageway with the respect to edges, forming the crowner highest point on the center. The rate of camber slope design by 1 in n or n%.
The main objective provided of the chamber are:
- Drain out surface water.
- Prevent the infiltration of surface water in underline prevent layers.
- Separate the traffic into two directions.
- Improve the appearance of the road.
The highway has a steep cross-slope but it is not desirable because of the following relevance.
- The transverse tilt of the vehicle case uncomfortable side thrust and a drag on the steering.
- Discomfort caused the throw of vehicles when crossing the crown.
- The problem of the topping of highly loaded trucks.
- Formation of the cross ruts due to the rapid flow of water.
- The tendency of most of the vehicles to follow the centerline.
Types of camber
On the highway mainly threw types of camber should be designed. Which is the most important in the geometric design of the highway. Followings are the main types of camber which use in the highway.
Straight camber
A Camber is made of two equal straight-line slopes.
Parabolic camber
A camber with the shape of a simple quadratic parabola.
Adopted for fast-moving vehicles.
Composite camber
It may either compose of partly parabola and partly straight or two straight lines having a different slope.
Used for slow-moving vehicles.
Superelevation
The superelevation is the amount by which the outer edge of pavement raises with the respect to the inner edge of pavement along the horizontal curve to counteract the effect of centrifugal force and lateral skidding of vehicles. It is also known as banking.
It denotes by ‘a a’ and is expressed as the ratio of the height of the outer edge concerning horizontal width.
Method of introducing superelevation
Elimination of the crown of the cambered section. This may be done by two methods.
In the 1st method, the outer half of the cross slope rotates about the crown at the desired rate such that the surface fails on the same plane as the inner half and the elevation of the centerline is not altered.
This method has a drawback in that the surface drainage will not be proper at the outer half during a short stretch of the road with a cross slope less than a camber.
In the second method of elimination of crown known as the diagonal crown method, the crown progressively shifts outwards thus increasing the width of the inner half of the cross-section.
This method is not usually adopted as a portion of the outer half of the pavement has an increasing value of negative. Superelevation before the crown eliminate.
Rotation of pavement to attain full superelevation.
By rotating the pavement cross-section about the centerline depressing the inner edge and raising the outer edge each by half the total amount of supper elevation with the respected center.
By rotating the pavement cross-section about the inner edge of the pavement section raising both the center as well as the outer edge.
This is the method of introducing superelevation.
Sight Distance
Sight distance available from a point is the actual distance along the road surface which a driver from a specified height above carriageway has visibility of a stationary or moving object. In other words, sight distance is the length of the road visible ahead to the driver in any instance.
Stopping sight distance
The minimum sight distance available on a highway at any spot should be sufficient length to stop a vehicle traveling at design speed, safely without collision with any other obstruction. This distance is called the stopping sight distance. The sight distance available on the road to a driver at any instant depends on:
Feature of the road
Height of driver eye above the road surface
Height of the object above the road surface.
Factor affecting stopping sight distance
- The total reaction time of the driver
- Speed of the vehicle
- The efficiency of the brakes on vehicles
- Frictional resistance between the road and the types.
- The gradient of the road.
The total reaction time of the driver
The driver the time taken from the instant the object is visible to the driver than the instant effectively applied the brake. The stopping sight distance increase with the increase in the reaction time of the driver.
Total reaction time = perception time + brake reaction time
Perception of time
It is the time required for the driver to realize that the brakes must apply. The perception time varies from driver to driver and also depends upon several factors such as the speed of vehicle distance of object age of driver etc.
Brake reaction time
It is the time that elapses between the moment the foot is removed from the accelerator paddle and placed on the brakes and the time to actuate the brakes.
Speed of vehicles
The higher the speed higher will be the stopping sight distance.
Efficiency of brakes
The braking efficiency is said to be 100%. If the wheels fully locking preventing them from rotating on the application of the brakes. This will result in 100% skidding which is normally undesirable except in an utmost emergency. To avoid a skid, the braking force should not exceed the frictional force between the wheels and types.
Skid
A skid happens when the slide is without spinning or when the wheels halfway rotate. That when the path travel along the road surface is more than the circumferential movement of the wheel during their rotation.
Slip
A slip occurs when the wheel revolves more than the corresponding longitudinal moment along the roads.
Frictional resistance between road and tire.
The braking distance increase with a decrease in skid resistance.
In the upgrade, the stopping distance is less than in the downgrade.
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