I thought of this idea where society would move away from cars and roads and move to a different mode of transportation...zip-lines. 

The world has been desperate in finding alternate forms of energy for transportation for the past three decades. During this time, our scientists have found alternatives that are simply ineffective or too expensive. My question is why use something that was so expensive to make, so dangerous for the environment and so dangerous to travel in, when something so clean, safe, and free is out there all the time for us to use...gravity.