Before a megaproject gets the nod to go ahead, a very unique set of circumstances must first be considered.
There is no blueprint or shortcut for building a world-changing megaproject. In fact, each one must be designed and built from the ground up, often amidst considerable amounts of red tape and criticism. Builders of megaprojects embrace the unknown, even when faced with incredible amounts of risk and massive cost overruns.
At the same time, successful megaprojects can accomplish things that have never been done before. They can be pinnacles of human achievement, and spectacles such as the International Space Station and the U.S. Interstate Highway System have already changed the world
Source: http://www.visualcapitalist.com/worlds-largest-megaprojects/