That's a big question. Of course the internal combustion engine is notoriously inefficient, but it's all we've got right now. I think what we're seeing now in the market and with a lot of experience with different alternatives over the last 10 to 15 years, whether it's hybrid technology--propane, natural gas, hydrogen, or electric--is that we really need to be smarter about focusing first on the niches where these technologies fit the best.
Natural gas, of course, on the first go-around, was going to fuel everything with wheels in this country. I think there were a lot of hard learnings from that first go-around, both on the government and the private sector side. This time, starting with the medium and heavy vehicles, particularly a tractor trailer, that's not a particularly good application for electric because of the battery demands. But the passenger vehicle or urban delivery could be a very good fit with electric, and we're going to see that now as more of the automaker products come into the market.
Just to give you an idea, though, the heavy vehicles are only 4% of our vehicles but a third of our onroad carbon. We really think this portfolio approach is very important to move forward.