How do they decide the way mountain roads twist and turn?
In: 200
Im not a mountain roadologist, lol, but i would think that the surveyors would follow the contours of the land to determine the bezt route, and which hills they need to blast in half? Not sure, great question, hopefully a real roadologist will chime in with the 411
They plot a path that gets the road through the hills as safely and effectively as possible. They don’t want steep rises and drops, they don’t want too tight turns. So often they go along a valley floor, or along the lower slopes of hills, and try to find a path that minimizes sudden changes in directions. If they have to, they use tunnels or bridges to even things out. If necessary they cut through the tops of ridges. They work with topographical maps or 3D models and lay it all out ahead of time.
Often much older roads originally followed trails worked out over centuries by people on foot or horseback.
Many of the mountain roads you may find started out as trails or paths. The path was whatever made sense to the people (or animals, as many started out as deer trails, etc) as the easiest way to get through the terrain. The path was then widened for horseback, then for carriages. Sometimes the paths were re-routed for fallen trees or washed out sections of the trial. Eventually these paths were improved with gravel or planks. When automobiles came along the paths were further improved and rerouted to make life easier on drivers. Houses and landmarks were established and the roads were routed to or around them appropriately. Eventually a new section is built for a particularly difficult or treacherous section.
Some roads are a bit more deliberate than this, but many mountain roads you will find evolved over more than a century in one way or another.
Roads on plains are about getting from Point A to Point B without much change in elevation. Mountain roads are much more about elevation, about ascending or descending the mountain from Point A to Point B. The road between them is a combination of following the shape of the land (into gullies and out over ridges) while steadily going up or down that change in elevation. The steepness (grade) of the road is kept below that which makes it difficult for cars to drive. If there is not enough distance between Point A and Point B to do that, the road doubles back on itself (switchbacks) to increase the distance the amount of elevation is spread out over.
When designing a road there are a lot of factors. You have minimum and maximum slopes the road can be, you have to consider sight distance (the distance a driver can see without it being obstructed by the road or terrain) so you can’t have curves or dips in the road too sharp, you have to design curves so that the driver feels comfortable driving at the speed limit, and you have to consider cost. To reduce cost the most you don’t want any retaining walls, and you want to reduce the amount of moving dirt to the minimum possible. You also have to consider how far you move the dirt, so you want your cut areas (areas that require you to remove dirt) and your fill areas (areas you need to fill in dirt) to be near each other. This basically means the best path is to follow the contours of the mountain, which is why they all end up twisty and you have a steep slope on either side of you
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.
Usually the path of least resistance. Animals aren’t engineers, but they know how to navigate their native terrain. That develops into trails, which humans can turn into roads. Since some humans are engineers, we can make the trails somewhat more efficient with bridges and tunnels.
Driving on the roads (and hills) around Jerusalem, my joke is that “I’ve driven on the roads that Jesus walked” and it’s probably true because those routes make the most sense and likely have been used for millenia.
Road builder (off-highway) from British Columbia here. Our clients basically draw a line on the satellite images of where they want the road, then another company will walk it and ribbon off a trail to follow, then we go out, clear the right of way, and build the road. If it is steep terrain it gets designed by engineers but other than that we basically just figure it out.
I have an on-topic reason for posting a link to another sub. Open r/DesirePath and you’ll see images of natural walking paths across lawns, through foliage, and so on. It’s the literal origin of the term ‘the beaten path’.
Now imagine you’re an expeditioner (I forget if this is the correct term) looking for a way through a mountain 500 years ago. The first thing you may look for is a desire path from animals always going the same way through an area. If a specific route is easier, it naturally leads to more traffic.
One of these paths ends up taking you a pretty circuitous route through a mountain, but despite being circuitous, it pretty well beaten so a lot of animals use this path. So you and your party do too. With horses.
When you get back, you tell your friends that you found a route through the Rockies and here’s a map. Now you have a lot of people going this same route, beating it even more. Perhaps even wagons. Or elephants, for Hannibal in the Alps.
Anyway, your route get’s so well known that it’s been colloquially named after you, Mr. John I-70. And now, because there’s so much traffic, someone should really pave it, but due to budget cuts, it’s simpler to just leave the potholes in and what was I talking about again?
(Just read that last paragraph as “Basically, the more known a path is and the more traffic that uses it, the more ‘upgraded’ it eventually becomes until some government formally recognizes it and decides to start maintaining it as a road.”)
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