It all depends on the mountain. How wide the road needs to be but it normaly involves big machines explosives, and lots of people. You need to decide if you can go around or need to go through with a tunnel. You need to decide on the gradient you can accept, how steep you can afford the track to be. What traffic is going to use the road. How high is the road to be. Can you afford to spend enough money. do you have the machinery, do you have the skills required. You will need qualified surveyers.
Do you need a fully paved road? Would a gravel surface work out? Will heavy lorries use this road? There are many more things to take into account before you even consider starting out on a project like a mountain road.
__________________
There's them that knows and them that just thinks they know, whitch are you? Stir the pot and see what rises up. I have catalytic properties I get a reaction going.