Actually, I should say, transportation period. We talk about the need to move people, but the problem isn't how to move those people, it's moving people period. People itself (well, ourselves) don't really require much space or energy - we sit there, in front of our cubicles or TVs staring at energy-efficient LED screens all day. All those global warming, energy consumption, land and resource issues can be traced down to how we have to move from being in front of our TVs to in our work cubicles. All the land to build the highways, all the land to build rail and bus networks, all the energy spent on moving cars and trains. None of that would be necessary if we co-located work and living space.
So, what I'm saying is, we should think of solutions where people can live within a 5-minute walk of their work (or work from home) instead of transportation-related infrastructure solutions. This would require an entirely new type of infrastructure, however (like a complete rebuild of nearly every metropolitan area).
Also, I lied. Killing off the world's population is the most overrated solution to the population problem.