The maximum speed at which vehicles can continuously travel safety under favorable conditions is known as design speed.It may also be thought of as the maximum approximate speed that will be adopted by most drivers. Choice of design speed has to be made carefully, so as to match the terrain condition and also to be acceptable to most road users.It is the basic parameter which determines all other geometric design feature.
CROSS SECTIONAL ELEMENTS :
The features of the cross-section of the pavement influences the life of the pavement as well as the riding comfort and safety. Of these, pavement surface characteristic affect both of these. 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, four aspects of the pavement surface are 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
Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also affect the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles.
Skidding happens when the path travelled along the road surface is more than the circumferential movement of the wheels due to friction. Slip occurs when the wheel revolves more than the corresponding longitudinal movement along the road.
VARIOUS FACTOR THAT EFFECT FRICTION :
Type of the pavement (like bituminous, concrete, or gravel), Condition of the pavement (dry or wet, hot or cold, etc),Condition of the tyre (new or old), and Speed and load of the vehicle.
The frictional force that develops between the wheel and the pavement is the load acting multiplied by a factor called the coefficient of friction and denoted as f. The choice of the value of f is a very complicated issue since it depends on many variables. IRC suggests the coefficient of longitudinal friction as 0.35-0.4 depending on the speed and coefficient of lateral friction as 0.15.These values have been taken by keeping optimum factor of safety in mind. The former is useful in sight distance calculation and the letter in horizontal curve design.
Unevenness affect the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tyres. Unevenness Index is a measure of unevenness which is the cumulative measure of vertical undulation of the pavement surface recorded per unit horizontal length of the road.
An unevenness index value less than 150 cm/km is considered as good, a value less than 250 cm/km is satisfactory up to speed of 100 kmph and values greater than 320 cm/km is considered as uncomfortable even for 55 kmph. Pavement surface condition is measured by Bump Indicator in terms of unevenness index.
LIGHT REFLECTION :
White roads have good visibility at right, but cause glare during day time.Black road has no glare during day, but has poor visibility at night when the surface is wet. Concrete roads has better visibility and less glare.
The pavement surface should be absolutely impermeable to prevent seepage of water into the pavement layers. Further, both the geometry and texture of pavement surface should help in draining out the water from the surface in less time.
Camber or Cant is the cross slope provided to raise middle of the road surface in the transverse direction to drain off rain water from road surface.
The objective of providing camber area:
• Surface protection especially for gravel and bituminous road
• Sub-grade protection by proper drainage
• Quick drying of pavement which in turn increases safety Too steep slope is undesirable because
(a) It will erode the surface.
(b) Due to too steep slope, transverse tilt of vehicles causes uncomfortable side thrust and a drag on the steering of automobiles. Also the thrust on the wheels along the pavement edges is more causing unequal wear of the tyres as well as road surface.
(c) Discomfort causing throw of vehicle when crossing the crown during overtaking operations.
(d) Problems of toppling over of a highly laden bullock carts and trucks.
(e) Tendency of most of the vehicles to travel along the centre line.