The various factors which control the highway alignment in general may be listed as
(a) Obligatory points
(b) Traffic
(c) Geometric design
(d) Economics
(e) Other considerations
In hill roads additional care has to be given for :
Stability
Drainage
Geometric standards of hill roads, and
Resisting length
(a) Obligatory Points There are control points governing the alignment of the
highways.These control points may be divided broadly into two categories
0 Points through which the alignment is to pass.
(i) Points through which the alignment should not pass.
Obligatory points through which the road alignment has to pass may cause the
alignment to often deviate from the shortest or easiest path. The various examples of this
category may be bridge site, intermediate town, a mountain pass or a quarry.
When it is necessary to cross hill range. mountains or high ridges the various
alternatives are to cut a tunnel across or to go round the hills or to deviate until a suitable hill pass is available. The suitability of these alternatives depend on many other factors, like the topography and site conditions and cost considerations. the straight alignment
is deviated along the hill side pass, thus avoiding a tunnel or heavy cutting.
(6) Traffic: The alignment should suit trafic requirements. Origin and Destination
study should be carried out in the area and the desire lines be drawn shoWing the trend of traftic flow. The new road to be aligned should keep in view the desired lines, traffic flow patterns and future trends.
(c) Geomeric Design: Geometric design factors such as gradient, radius of curve
and sight distance also would govern the final alignment of the highway. lf straight
alignment is aimed at, often it may be necessary to provide very steep gradients. As tar as possible while aligning a new road, the gradient should be flat and less than the ruling or design gradient. Thus it may be necessary to change the alignment in view of the design speed, maximum allowable superelevation and coefficient of lateral friction. It may be necessary to make adjustment in the horizontal alignment of roads keeping in view the
minimum radius of curve and the transition curves.
The absolute minimum sight distance, which should invariably be available in every
section of the road, is the safe stopping distance for the fast moving vehicles. Also there
should be enough distance visible ahead for safe overtaking operations of vehicles
speed on the road. Hence the alignment should be finalised in such a moving at design way that the obstructions to visibility do not cause restrictions to the sight distance requirements.
(d) Economy: The alignment finalised based on the above factors should also be
economical. In working out the economics, tne initial cost the cost, of maintenance and
vehicle operation should be taken into account. The initial cost of construction can be decreased if high embankments and deep cutings are avoided and the alignment is chosen in a manner to balance the cutting and filling.
(e) Other Considerations: Various other Tactors which may govern the alignment are drainage considerations, hydrological factorS, political considerations and monotony. The vertical alignment is often guided by drainage considerations. The subsurface water level, seepage flow and high flood level are the factors to be kept in view
