Planning is considered as a pre-requisite before attempting any development programme in the present era. This is particularly true for any engineering project, as planning is the basic requirement for any new project or an expansion programme. Thus there is a need for planned development of the road network and the links. Highway planning is of great importance when the funds available are limited whereas the total requirement is much higher. In developing countries like India it is important to utilize the available funds in the best possible manner by resorting to best planning principles. The objects of highway planning are briefly given below.
- To plan overall road network for efficient and safe traffic operation, but at minimum cost. Here the costs of construction, maintenance and resurfacing or strengthening of pavement layers and the vehicle operation cost are to be given due consideration.
- To arrive at the road system and the lengths of different categories of roads which could provide maximum utility and could be constructed within the available resources during the plan period under consideration.
- To divide the overall plan into phases and to decide priorities.
- To fix up date-wise priorities for development of each road link based on utility as the main criterion for phasing the road development programme.
- To plan for future requirements and improvements of roads in view of anticipated developments.
- To work out suitable financing system phases of highway planning.
Highway planning includes the following phases.
- Assessment of road length requirement for an area (it may be a district, state or the whole country)
- Preparation of master plan showing the phasing of plan in five year plans or annual plans. In order to plan the road system in the selected region, state or country, different studies and surveys are to be carried out to collect the data required. The data collected are to be processed and analysed to arrive at the best possible road network and to arrive at the optimum length of the road system.
Geometric Design
Geometric design for transportation facilities includes the design of geometric cross sections, horizontal alignment, vertical alignment, intersections and various design details. These basic elements are common to all linear facilities, such as roadways, railways and airport runways and taxiways. Although the details of design standards vary with the mode and the class of facility, most of the issues involved in geometric design are similar for all modes. In all cases, the goals of geometric design are to maximize the comfort, safety and economy of facilities, while minimizing their environmental impacts.
The geometric design of highways deals with the dimensions and layout of visible features of the highway. The features normally considered are the cross section elements, sight distance consideration, horizontal curvature, gradients and intersection. The design of these features is to a great extend influenced by driver behaviour and psychology, vehicle characteristics, traffic characteristics such as speed and volume. Proper geometric design will help in the reduction of accidents and their severity. Therefore, the objective of geometric design is to provide optimum efficiency in traffic operation and maximum safety at reasonable cost. The planning cannot be done stage wise like that of a pavement, but has to be done well in advance.
Factors Affecting Geometric Design
1) Design speed
Design speed is the single most important factor that affects the geometric design. It directly affects the sight distance, horizontal curve and the length of vertical curves. Since the speed of vehicles vary with driver, terrain etc., a design speed is adopted for all the geometric design.
2) Topography
It is easier to construct roads with required standards for a plain terrain. However, for a given design speed, the construction cost increases multi form with the gradient and the terrain.
3) Traffic Factors
It is of crucial importance in highway design, is the traffic data both current and future estimates. Traffic volume indicates the level of services (LOS) for which the highway is being planned and directly affects the geometric features such as width, alignment, grades etc., without a suitable traffic data it is very difficult to design any highway.
4) Design Hourly Volume and Capacity
The general unit for measuring traffic on highway is the Annual Average Daily Traffic volume, abbreviated as AADT. 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.
5) Environmental and Other Factors
The environmental factors like air pollution, noise pollution, landscaping, aesthetics and other global conditions should be given due considerations in the geometric design of roads.
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