Why do engineers blast through hills instead of building highways over them?


Can’t get back to all the comments, thanks all <3

EDIT: I used HILLS and not MOUNTAINS on purpose lol, c’mon guys I’m not *that* clueless. Tunnels absolutely do not figure into my question. Thank you everyone for answering this has been enlightening.

EDIT 2: I don’t think it has occurred to some browsers of this sub that the people asking these questions have probably considered their own question and come up with hypotheses already. Trust me, I had 2 hours longer to drive and think after I posted the question at a gas station haha. has confirmed some of my hypotheses about this and also added many new pieces of information to the puzzle which I am grateful for. Some of you taking time out of your day to say “stupid question” or something along those lines, please don’t consider becoming teachers, and go forth in this sub operating under the assumption that a lot of the questions asked here are not just asked out of curiosity, but also just seeking confirmation of hypotheses before going out into the world and spewing BS lol.

In: Engineering

Because then highways would be like rollercoasters and be super dangerous to drive on, and impassable by larger heavy trucks. Imagine you’re going 65MPH on the highway then suddenly there’s a hill going up at 30º

Roads have a slope limit, called the road’s grade. If the road is too steep, it will be unsafe. Mountains are natural features, and they are often more steep than this. So, to go over a mountain, you have to make a very long road to maintain the grade limit. You can do this on the mountain, with a series of switchbacks. Unfortunately, switchbacks are also dangerous and significantly increase travel time. This longer road is also much more expensive.

At some point, the economics of tunneling through the mountain are cheaper than the long road in terms of construction costs plus all the time/fuel used to traverse the road. Engineers usually build the cheapest solution, even when it’s a tunnel through a mountain.

This is both an engineering and an economics question. Good answers here on the engineering reasons. The economic answer is that tunnels probably cost more to build, but reducing auto accidents and saving driving time have more economic value over the life of the tunnel.

I have a fun colloquial fact (never found any actual data on this)
People have been building roads for millennia, check out the Egyptians and the Romans. I was told that before we knew these hard science engineering type math numbers for road grade. It was just a matter of trusting the animals the pulled wagons and carried loads.

So by allowing the animal it’s choice on where to walk while pointing in the right direction it would instinctually walk what we now accept is a safe road grade %.

In a situation where you ran into mountains you then understand why we have many many miles of winding twisting roads trying to get over that mountain while respecting that natural road grade limit.

If you want to save miles and miles of winding roads up and over and back down guess what you do? Build a tunnel or just blast a valley through the mountains. Of course this level of risk in engineering is something that modern humans mastered much better than our ancient counterparts but I bet there’s some cool story out there of a brilliant commander who tunneled through a mountain pass to surprise an enemy force!

They used to. It faster and better driving to blast through hills, though it costs more, obviously. Often there will be 2 routes, the old hilly route, and the new tunnel. No one takes the old route.

Weather and ice conditions will also play a role. Even if the hill isn’t too steep it can be an unnecessary slope in an area with terrible winter driving conditions. The hill can be moved, an idiot cannot be taught how to drive on ice. Ft. Worth taught us that recently

Come to California, where because of earthquakes we build over mountains instead of through them. Everything takes longer and the roads are shittier and more difficult to drive.

It comes down to the fact that if you don’t blast through hills, your roads have to either go up and down a lot, have a lot of curves, or both. The more curves you have and the more you have to go up and down, the slower you have to drive. A lot of curves and up’s and down’s also means that the road is longer than if it goes in a practically straight line, so even if you could drive at the same speed, it would still take longer.

Another Problem if the road goes up and down a lot is that it’s a lot harder on the cars and trucks driving over them. Think of when you’re on a bicycle. What wears you out more, the straight, flat bike bath, or the one that goes up and down and has a lot of curves? Well, the same applies to the cars and trucks.

Great easy-to-understand treatment about road design in general: https://youtu.be/9XIjqdk69O4

He addresses your specific question beginning at about 7:55.

The composition of the landscape is also a factor. It’s a lot easier to blast through loose earth and hills than it is solid granite or sedimentary layers like shale that won’t be structurally sound even after reinforcement.

Spent my childhood in Colorado and western South Dakota. You’d be AMAZED at the number of crazy roadways when a “simple” tunnel would suffice.

Its cheeper to blast hill tops for back fill. You need both cut and fill to prep for a road so they are able to use the environment around the road to make a better road.

Lots of good and right answers here. I’ll just add that if an engineer can achieve something equally with an explosion or using other means, he will choose the explosion.

Just sayin’


OK OK… I’ll be honest… explosions will be used when possible. Its just so cool!

My ex boyfriend in NZ used to bitch and moan all the time about how the roads there snake around mountains when places like Switzerland get all those neat tunnels going straight through, it’s just cheaper though

Too steep. My dad had to excavate the entire tip off of a hill because cars would stall trying to climb it. Had to rebuild the entire highway after he cut the hill down

Many roads are built to be “more level” to save fuel and meet modern fuel economy standards. Or so I once heard.

Railroads in my area are so much more level than the roads next to them. Hilarious to see actually.

I actually work for a company that build roads in mountainous region. Besides economics, there are few other reasons

1. Weather – there are mountains where it snows all year round. It becomes unmanageable in winters and these kind of roads are usually closed for 5-6 months during the winter.

2. Safety – some hilly roads are prone to mudslides and avalanches.

3. Time – roads in mountains take a lot of time to traverse, if the road is an important connection between two points, usually a tunnel can decrease the time by hours.

Go drive across Costa Rica sometime. You’ll know exactly why. You might want to bring some Dramamine.