then the railroad,

then the automobile.

Now, the greatest minds in tech are working on the next transportation revolution.

But not everyone agrees what that will be.

Eye-catching proposals like the Hyperloop and self-driving cars are dominating the headlines.

But some technologists are focused on solving a much more mundane problem: congestion.

One solution could be travelling through the air rather than on the ground.

Uber promises to have flying taxis operating by 2020 in LA, Dallas and Dubai.

The company says these flying taxis which look like a cross between a drone and helicopter will ultimately be the same price as cars.

It's working with Bell Helicopter and Boeing's Aurora to build them.

Google co-founder Larry Page has backed Kitty Hawk.

A flying car company that's completed more than a thousand successful flights.

But manufacturers face major challenges.

Electric aircraft require huge bursts of power for vertical takeoff, and that means large heavy batteries.

Also these aircraft have not been safety certified in the US, and aerospace rules would make it difficult to operate a large fleet in urban centers.

That's led others to look in the opposite direction, underground.

Elon Musk recently founded The Boring Company.

And his vision is for an urban network of underground tunnels where cars travel on an autonomous skate at up to 150 miles an hour.

Traveling on that would look something like this test run, carried out earlier this year.

Traditional tunnel building can be bureaucratic and expensive.

And digging can move slower than a snail. By reducing the tunnel diameter by half.

The company says it can dig faster and reduce costs by three to four times.

The problem is there's no indication yet it can bore its narrow tunnels as quickly and cheaply as hoped.

When it comes to revolutionizing transportation,

there are no easy solutions.

So the simplest projects may be the ones that take off first.