It's all about Passenger Occupancy

One of the arguments for public transportation systems is that they are both cleaner and cheaper than the automobile. It's pretty clear that they aren't cheaper, but what about cleaner. I've seen a few studies but a new one out of UC Berkley makes a significant point. What really matters is the passenger occupancy rate. From the Next 100 Blog:

The worst energy hogs shown in the paper's charts--and the worst greenhouse gas emitters--are urban diesel buses running off peak, with only a few passengers. But the most energy-efficient and generally cleanest vehicles are also urban diesel buses, running full at peak periods. (All comparisons are per passenger-mile traveled.)

HT: Knowledge Problem