IMPORTANCE OF GEOMETRIC DESIGN
The geometric design of a highway deals with the dimensions and layout of visible features of the highway such as alignment, sight distances and intersections.
The geometrics of highway should be designed to provide optimum efficiency in traffic operations with maximum safety at reasonable cost. The designer may be exposed to either planning of new highway net work or improvement of existing highways to meet the requirements of the existing and the anticipated traffic.
It is possible to design and construct the pavement of a road in stages; but it is very expensive and rather difficult to improve the geometric elements of a road in stages at a later date. Therefore it is important to plan and design the geometric features of the road during the initial alignment itself taking into consideration the future growth of traffic flow and possibility of the road being upgraded to a higher category or to a higher design speed standard at a later stage.
Geometric design of highways deals with following elements :
(i) Cross section elements
(ii) Sight distance considerations
(iii) Horizontal alignment details
(iv) Vertical alignment details
(v) Intersection elements
Under cross section elements, the considerations for the width of pavement, formation and land, the surface characteristics and cross slope of pavement are included. The sight distance or clear distance visible ahead of a driver at horizontal and vertical curves and at intersections govern the safe movements of vehicles.
The change in the road directions are made possible by introducing horizontal curves. Super-elevation is provided by raising the outer edge of pavement to counteract the centrifugal force developed on a vehicle traversing a horizontal curve; extra pavement width is also provided on horizontal curves. In order to introduce the centrifugal force and the super-elevation gradually, transition curves are introduced between the straight and circular curves. The gradients and vertical curves are introduced in the vertical alignment of a highway. Design of road intersections with facilities for safe and efficient traffic movement needs adequate knowledge of traffic engineering.
Highway geometrics are greatly influenced by the topography, locality and traffic characteristics and requirements of design speed.
The geometric design of highways depends on several design factors. The important factors that control the geometric elements are :
(i) Design speed
(ii) Topography
(iii) Traffic factors
(iv) Design hourly volume and capacity
(v) Environmental and other factors.
(i) Design Speed
The design speed is the most important factor controlling the geometric design elements of highways. The design speed is decided taking into account the overall requirements of the highway. In India different speed standards have been assigned depending upon the importance or the class of the road such as National/State Highways, Major/Other District Roads and Village Roads. Further the design speed standards are modified depending upon the terrain or topography. Similarly urban roads have a different set of design speeds. Design of almost every geometric design element of a road is dependent on the design speed. For example the requirements of the pavement surface characteristics, the cross section element of the road such as width and clearance requirements, the sight distance requirements, the horizontal alignment elements such as radius of curve super-elevation, transition curve length and the vertical alignments such as gradient, summit and valley curve lengths-all these depend mainly on the design speed of the road.
(ii) Topography
The topography or the terrain conditions influence the geometric design of highway significantly. The terrains are classified based on the general slope of the country across the alignment, as plain rolling, mountainous and steep terrains.
The design standards specified for different classes of roads are different depending on the terrain classification. For example the design or ruling speed of NH and SH on plain terrain with general cross slope up to 10% is 100 km/h whereas the speed on rolling terrain with general cross slope of 10 to 25% is 80 km/h and that on mountainous terrain with cross slope 25 to 60% is 50 km/h. As the speed standards affect every geometric design element, topography also affects the geometric design of roads. Further in hilly terrain, it is necessary to allow for steeper gradients and sharper horizontal curves due to the construction problems.
(iii) Traffic Factors
The factors associated with the traffic that affect geometric design of roads are the vehicular characteristics and human characteristics of road users. It is difficult to decide the design vehicle or the standard traffic lane under the mixed traffic flow condition prevalent especially on urban roads of developing countries. This is a complex problem. The different vehicle classes such as passenger cars, buses, trucks, motor cycles, etc. have different speed and acceleration characteristics, apart from having different dimensions and weights. However, it is often necessary to consider some standard vehicle as the design vehicle. The important human factors which affect traffic behavior include the physical, mental and psychological characteristics of drivers and pedestrians.
(iv) Design Hourly Volume and Capacity
The traffic flow or volume keeps fluctuating with time, from a low value during off peak hours to the highest value during the peak hour. It will be uneconomical to design the roadway facilities for the peak traffic flow or the highest hourly traffic volume. Therefore a reasonable value of traffic volume is decided for the design and this is called the design hourly volume. The ratio of volume to capacity affects the level of service of the road.
(v) Environmental and Other Factors
The environmental factors such as aesthetics, landscaping, air pollution, noise pollution and other local conditions should be given due consideration in the design on road geometrics. Some of the arterial high speed highways and expressways are designed for higher speed standards and uninterrupted flow of vehicles by providing grade separated intersections and controlled access.