On average, buses are far worse than cars for energy efficiency because of the low average load factor.
On what data is this assertion based? I spent a few minutes seeing if such data exist. I could not find data to support your claim that buses are far worse.
I found the following. A bus fuel efficiency is about 5 mpg [1]. That is with fifty-five passengers, which is the maximum capacity and therefore our lower bound. In my county, the average load-factor over all of 2012 was 479 million passenger miles divided by 44 million vehicle miles, or 10 passengers per mile.
Our average fuel consumption over number of passengers then is 50 mpg, which is not far worse than cars for energy efficiency. In 2006, the average mpg of a private vehicle on the road was about 20 mpg. Even with two people in such a vehicle, the average-loaded bus is better.
I did not dig very deeply; I was more trying to find your data and stumbled into data that seems to paint a different picture. It's quite possible that my data paints the wrong picture and you were using much more sound data, but because you did not provide it, I must ask for a citation now.
Which data had you used?
[1] http://www.nrel.gov/docs/fy00o...
[2] http://metro.kingcounty.gov/am...
[3] http://www.project.org/info.ph...