Forget hybrids, gas taxes, or pay-as-you-drive insurance. The best way to reduce automobile emissions just might be "personal rapid transit" systems—or so says USC public-policy professor Catherine Burke. The PRTs would essentially be glorified monorails made up of small, personal "podcars." (Couldn't think of a trekier-sounding name if I tried.) The pros? Burke writes:

[Podcars] ride on small, overhead guideways -- like a monorail or people mover -- above existing roads, and are powered entirely by electricity. Picture the car as an elevated, driverless taxi. It's under computer control, so there would be no accidents, thereby saving lives and lowering insurance costs.

Podcars operate on demand, waiting at off-line stations; they can be summoned if one is not available when you arrive at the station. Each vehicle can hold four people, yet the system can be cost-effective even with a single rider for each trip.

The capital cost is low, about $25 million to $40 million a mile for the first systems, which include guideways, vehicles and stations, compared to $100 million to $300 million a mile for light-rail or subway systems. Because it operates over existing streets and sidewalks, there are few costs for rights of way or taking of private property. It is also inexpensive to operate and thus can be available 24/7 and still make an operating profit, depending on pricing policies.

Heathrow Airport is working to implement a PRT system that would navigate its sprawling terminals, and—for an airport of Heathrow's size—this kind of personal, destination-specific transportation system makes sense. It's less clear how it might work in cities, though. Hundreds of miles of above-ground rail would need to be laid, shadowing our cities with webs of twisting metal. And, according to the website LightRailNow!, in a PRT, commuters would travel to many different destinations from a single station and would likely be unwilling to share their cars with strangers, which would require stations to use up to 100 cars to accommodate rush hour traffic—creating podcar congestion and long waiting times.

As Burke points out, "There have been no significant service innovations in public transit in more than 100 years"—a fact that should spur us on to create greener and more-efficient public transit systems. But it's not clear that the podcar is the answer. (Plus, haven't good people been fooled by the promises of the monorail in the past?)

--James Martin