A highway has many visible dimensions both in the horizontal plane and in the vertical plane. The art of design of the visible dimensions is known as Geometric Design. Proper geometric design can help in the reduction of accidents. Therefore, the objective of geometric design is the provide optimum efficiency in traffic operation and maximum safety at reasonable cost.
Highway geometric design covers elements such as design vehicle dimensions, user characteristics, terrain, highway classification, design speed, horizontal curves, vertical curves, gradient, sight distances, cross-sectional features, junctions, interchange etc.
Geometric design is influenced by a number of factors such as:
(a) Road user characteristics
(b) Vehicle characteristics
(c) Safety requirements
(d) Environmental considerations
(e) Economy in construction, maintenance and operation of vehicles
(g) Functional classification of roads
(h) Traffic volume and composition
(i) Design speed
During design the factors like Safety, environmental needs and economy are considered.
ROAD USER CHARACTERISTIC
1. A driver takes a certain amount of time to respond to a traffic situation. This can be called as reaction time.
2. The action of applying break on seeing a vehicle or obstruction on the road is not an instantaneous phenomenon. But it is a time-consuming phenomenon based on the psychological process involved.
3. We can call these processes as perception, intellection, emotion and volition (PIEV)
4. Perception Time is the time required for transmission of the sensations received through eyes, ears and body to the brain and the spinal chord by the nervous system. After perception intellection occurs, that is the formation of new thoughts and ideas. Recalling old memories of similar occasion.
5. Linked with these two stages is emotion, based on the situation, like fear or anger. This has vital influence on the final message or decision sent by the brain to the muscle. This actual act of talking a decision to produce action is done through volition time.
6. The total time required for PIEV, that is, from the instant the object comes in the line of sight of the driver to the instant he arrives at a decision, say, to slow down or to overtake under normal circumstance is called reaction time.
7. This could vary from 0.5 second for simple situations to 3 to 4 seconds for complex situation. The reaction time is affected by the condition of the driver-fatigue, disease, alcohol consumption etc., his habits, skill, judgment and environmental conditions like climate,
8. season, weather, time of duty, altitude and light.
9. Driver characteristics that influence safety are vision and hearing.
10. Pedestrian characteristics that influence the design of pedestrian facilities are speed and space occupied. A speed of 1.2 m/sec is commonly taken for design (AASHTO).
The topography of the land through which the road passes called as the terrain which controls the geometric design. The following terrain types are identified as controls for design in India:
Percentage cross-slope of country
0 to 10
10 to 25
25 to 60
Greater than 60
If cross slope is large, increase in radius of curvature of road will lead to increase in construction cost. Hence, design speed is reduced so that radius of curve reduces leading in reduction in cost of construction.
CROSS SECTIONAL ELEMENTS
1. The features of the cross-section of the pavement influences the durability for longer life of the pavement as well as the riding comfort and safety. Of these, pavement surface characteristic affect both of these.
2. Camber, kerbs, and geometry of various cross-sectional elements are important aspects to be considered in this regards.
PAVEMENT SURFACE CHARACTERISTICS
For safe and comfortable driving, following aspects of the pavement surface are very important:
(a) The friction between the wheels and the pavement surface.
(b) Smoothness of the road surface.
(c) The light reflection characteristics of the top pavement surface and
(d) Drainage of water.