*The geometric design of highways deals with the dimensions and layout of visible features of the highway. The emphasis of the geometric design is to address the requirement of the driver and the vehicles such as safety, comfort, efficiency, etc.
*Geometric design of highway deals with the following elements. Cross-section elements. (Width of pavement,
formation, and land, etc.,), Sight-distances, Horizontal alignment details, Verticals alignment details, Intersection elements, Design speed, Gradient, etc.
- DESIGN SPEED: This is the most important factor controlling the geometric design elements. It is decided on the basis of the type of road or class of roads such as NH, SH, MDR, etc. and topography of the region.
- Topography: Design speed or ruling speed of NH and SH on different terrains is as follows. (a) Plain terrain (cross slope up to 10%)-100kmph (b) Rolling terrain (cross slope of 10% to 25%) 80kmph (c) Mountainous terrain (cross slope of 25% to 60%)-50kmph
- Traffic factors: These include
(a) Vehicular characteristics.
(b) Road user characteristics.
It will be uneconomical to design the road for peak traffic flow. Therefore a reasonable value of traffic volume is selected as the design hourly volume which is determined from the various traffic data collected.
- Design Hourly Volume and capacity: It is uneconomical to design the road facilities for the peak traffics flows, hence a reasonable value of traffic volume is decided for the design which is called the design hourly volume.
Friction between the wheel and the pavement surface is an important factor in design.
The method of approach includes full-scale experiments on the British Road Research Laboratory’s track using braked and rolling wheels under carefully controlled conditions, laboratory investigations of the viscoelastic properties of rubber together with rubber friction tests, and theoretical considerations linking the different aspects.
A design vehicle which has standard weight, dimensions and operating characteristics are used to establish highway design controls to accommodate vehicles of a designated type. The dimensions, weight of the axle, and operating characteristics of the vehicle influence the design aspects such as the width of the pavement, radii of the curve, clearances, parking geometric, etc.
- I.R.C. recommends the longitudinal friction coefficient values of 0.35 to 0.40 and lateral coefficient of friction of 0.15.
- Skid occurs when the wheels slide without revolving or when the wheels partially revolve i.e. when path traveled along the road is more than the circumferential movement of the wheel.
- Slip occurs when the wheel revolves more than the corresponding movement along the roads.
Factors affecting friction :
Different aspects of tread pattern design and tread material are considered in relation to factors external to the tire, the major ones being the influence of water depth over a range representative of conditions on the road, the interaction of road surface texture, and the effect of speed.
The object of the investigations described in this paper is to put in perspective the relative roles of the “drainage” and “energy loss” properties of tires in providing good adhesion on wet roads. This work should lead to a better understanding of the mechanism of tire/road adhesion on wet surfaces and in particular to draw the distinction between, and relative importance of, lubrication and aquaplaning.
(i) Type of pavement surface and conditions of pavement for e.g. wet or dry smooth or rough etc.
(ii) Type and condition of the tyre.
(iii) The temperature of tyre and pavement.
1. The coefficient of friction decreases with increases in temperature, tyre pressure and load. Smooth and worn out tyre offer higher friction factors on dry pavements than new tyres with treads due to large areas of contact, but on wet pavements, new tyres have higher friction factors.