I read that stat to say at any given time 76% of vehicles have only one person in them, but I doubt that 76% of vehicles never have more than one person in them ever. My commuter car probably has only me 95% of the miles, only 1 passenger the other 4%, but 3 or 4 people 1% of the time, for that reason I expect to keep my car with 200k miles on it when I hopefully get my Elio.
I don't know the current sales #'s, but I think US vehicle sales are about 16-17 million this year and dipped to around 10 million at the lowest point in the recent recession. I'm sure someone can find the breakdown, but I'd guess at least 1 million subcompacts, 2 million compacts, and 2 million midsize sedans are sold each year. Light trucks, SUV's, and CUV's I think make up more than half of the sales. Elio just has to siphon off some of those small to midsize buyers to be successful.