How do they decide the way mountain roads twist and turn?
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My brother used to do this professionally. There are two things to note before I get into some of the details: 1) The way it is done has changed a lot over time. 2) A lot of the basics are universal, but there are differences in different areas. This is based on the conditions in the Cascade Mountains of Washington State, USA.
Some of the big considerations on road alignment are:
– What grade (steepness) is acceptable, and how tight can corners be.
– Who owns the property — do you need to avoid other owners property.
– What physical objects do you need go around (lakes, rivers, cliffs)
– What’s the cheapest way to build.
Logging in Washington State got going strong in the age of the steam locomotive. Many of our roads started out as railroad grades. Railroad grades were put in before good topographic maps were available. People (nicknamed Scenery Inspectors) went out in the woods, walked the ground, and decided on the best route. Many of these railroad grades eventually turned into roads.
Later, after aerial photography became available, good topographic maps became available. Engineers could use these to plan roads back in the office. Minor adjustments might still be made in the field.
Most mountain roads need to make switchbacks to climb hills. Engineers will try hard to find little flatter spots to locate the hair-pin turns for these. Putting a 180 degree turn on the side of a steep hill is causes problems. The locations available for these turns often have a big influence on the rout of the road.
When it comes to where every little twist and turn is, this comes down to balancing the speed that the traffic will travel against the cost of the road. Moving dirt, blasting rock, and building bridges all cost money. The straighter you want the road, and the more constant you want the grade, the more construction you must do. Small, slow speed roads will turn into every little valley, and go out around every little ridge — they tend to just follow the local topography. This reduces the amount of dirt that needs to be moved.
On larger roads, the turns will be of larger radius and less frequent to improve safety and traffic flow. This of course is more expensive, and usually more disruptive to the natural environment.
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