As ExxonMobil (NYSE:XOM) made clear with both its algae investment and its comments about peak gasoline, and as the Department of Energy further underlines in its new Energy Outlook, the age of the gas-guzzling American motorist has reached its twilight years.

That's not to say we're not going to require a massive amount of oil to power our economy over the next few decades. Continental Resources (NYSE:CLR) should certainly keep up its efforts to increase domestic crude production. We just aren't likely to meaningfully grow our total gasoline consumption from here.

If you want to invest in the transportation trends of the future, you can play it a few ways.

You can focus on biofuels, which are going to account for most of the growth in liquid fuel supply going forward. That would suggest an investigation of bulletproof biofuel stock Green Plains Renewable Energy, or perhaps Valero Energy (NYSE:VLO), which increased both its ethanol and biodiesel exposure this week.

Alternately, you may, like T. Boone Pickens, see a bright future for natural gas as a bridge fuel to the future. Some interpret Exxon's purchase of XTO Energy (NYSE:XTO) as a vote on natural gas as a transportation fuel, but I would prefer to hear that straight from the horse's mouth.

The House of Representatives last week voted to extend tax credits for natural gas-based vehicle fuels, but the NAT GAS Act has yet to come to the floor. I think that natural gas has a decent shot at becoming a legislative favorite, but it's not a lock. Clean Energy Fuels (NASDAQ:CLNE) and Westport Innovations are direct bets on natural gas fueling infrastructure, but I haven't given either firm more than a cursory glance.

Aside from liquid fuels, there's the growing wave of hybrid and plug-in hybrid electric vehicles. Warren Buffett's cast his vote here with a winning investment in BYD, which this week had to rebuff rumors that its Chinese sales figures are inflated. BYD is looking to bring its electric cars to the U.S. late next year. Meanwhile, battery makers A123 (NASDAQ:AONE) and Johnson Controls (NYSE:JCI) have just formed joint ventures to develop clean energy vehicles with Chinese automakers.

I've already made one ill-advised investment in this space, with a biodiesel player blowing up in my face. I may be overly cautious as a result, but I'm not jumping into any additional alternative transportation names just yet. I'd love to hear your best ideas, though. Please share one in the comments section below.