Earlier this week, I questioned the sanity of pouring money into building roads when demand for oil will soon outstrip supply, not to mention that road transportation is the region's single largest source of greenhouse gas emissions.

In response to such concerns, some folks contend that electric cars will allow us to continue with car-dependent business as usual, and therefore that road building is just dandy.

The first big hole in that argument is that we're not going to have gobs of extra electricity lying around to power all those new electric cars.   I did some analysis on that a while back, and estimated that if Seattle's entire existing fleet was electric, it would add about 14 percent to the city's total electricity demand.

That doesn't sound so bad until you consider the fact that reductions in fossil fuel use—due to peak oil, greenhouse gas regulation, or both—are going to increase demand for carbon-free electricity for other things, such as powering buildings that now rely on natural gas or oil. And while it's true that cars can charge during off-peak periods to take advantage of unused electrical generation capacity, I can imagine a future scenario in which energy is such a precious commodity that we will be storing that excess capacity to supply buidlings and other non-transportation needs during peak demand.

There's also a second important, but less recognized, flaw in the electric car scenario: It's not just what comes out of the tailpipe that matters. There are also significant greenhouse gas emissions and energy consumption associated with manufacturing and maintaining cars, as well with building and maintaining roadway infrastructure.

It just so happens that some eggheads at UC Berkeley recently crunched the numbers on that, summed up in the blue pie below:

This pie chart shows the breakdown of greenhouse gas emissions from an average sedan, including the indirect contributions from car manufacture, maintenance, and infrastructure. This complete accounting adds fully one-third to the carbon footprint that would be assumed if only fuel use was counted. Thus, for example, Seattle's overall greenhouse gas inventory would rise significantly if these additional factors were included.

Improving fuel efficiency reduces the big light blue slice, but has little effect on the other slices. Efficiency and even going electric only get you so far when there's an extensive, energy-intensive network that's still required to support a car-centric transportation system. And given that energy prices are only going to keep rising, the cost of maintaining that network will become increasingly onerous.

The upside to all this—yes, there is an upside—is that creating urban environments that are less car-dependent will deliver a host of social and environmental benefits beyond those related to energy and greenhouse gas emissions.  Meanwhile, more and more people are becoming dissatisfied with car-dependent lifestyles, and demand for "walkable urbanism" is growing.

So then, can somebody please remind me why it is we're still planning to build so many roads?