There are a number of factors to be considered when deciding the position of a road, such as:
The position of towns and villages
The positions of rivers
The positions of passes in mountains
The positions of flood plains
The position of material required for the construc ion of the road.
The position of materials likely to give future rouble, such as highly expansive clays, collapsing ands, deleterious salts, wind blown sand belts.
Faced with all these factors it is a problem to decide exactly where the road ought to go. As far as Saudi Arabia is concerned, the main factors are:
The positions of towns and villages
The position of sand dune belts-these vary very much in width and height and if possible the narrowest and least high should be chosen so that the road can pass over the sand dunes at the specified grade without having to cut into them or disturb them unduly.
Mountain passes and escarpments.
Flood plains which may contain deleterious salts.
The position of construction materials, particularly rock which can be crushed either for concrete or for use as a base material with or without bitumen.
The availability of potable water for use in concrete. n considering all these factors aerial photographs are of immense importance, but they are not always available at the reconnaissance phase, so that the selection of alternative routes for detailed consideration