Energy goes into the construction of every highway and byway, whether it's made of asphalt, concrete, or even gravel; whether it's a narrow ribbon winding around mountains, an endless flat stretch across the prairie, or a congested freeway. Moreover, the way that roads are built and maintained has a significant impact on how much energy is burned by the vehicles that roll-or crawl-on the surface.

As governments weigh spending money to restore or expand existing roadways in the developed world and to construct entirely new ones in the developing world, they are asking questions about sustainability that were never considered in the early years of infrastructure build-out. The answers are not easy.

With public investment about $190 billion per year on highway,