O'Toole says that on average, public transit runs at about 1/5th of capacity:
"...there are very good reasons why public transit occupancy rates will never rise much above their current levels of about one-fifth full. Suppose you take a bus or train to work during rush hour and it seems full. But it really only seems full as it approaches the center of town. It is likely to be nearly empty when it starts its journey in the suburbs, and be nearly full only when it gets close to the city center."
And, says O'Toole, the "actual energy costs of buses and other forms of transit tend to be not significantly better (and in the cases of buses much worse) than cars."