I thought bikes were assumed to take up the same space in a lane as a car. Granted, bikers tend to reduce the following distances, but I wouldn't think that would make all that much difference in a single-lane environment.
On a slightly-related subject, do you happen to have any stats showing the relative safety of bike commuting vs. car commuting? I haven't been able to find anything specific to that, and don't really have the patience to plow through the raw DOT/NHTSA data to reconstruct it into a meaningful (read: unbiased) picture of motorcycling.
Motorcycling is much more dangerous than driving. The thing is, almost anyone can drive a car without getting into too much trouble if they are careful. That is not enough for handling a motorcycle.
In a few years I put over 40,000 miles on my bike and I can tell you that it not only requires your constant attention, but even that is not enough for many people. It really is an aptitude thing. Many people simply should never own or ride a motorcycle, no matter how careful they are. They are quite literally an accident waiting to happen.
The size thing matters because a motorcycle can pass between cars if necessary. It can also easily use the shoulder for an escape route when it would be impractical or impossible with a car. Motorcycles also compress following distance when following other motorcycles. Without riding side by side, one can take the left rut and the other take the right. Although one is following the other technically, they are in separate "motorcycle lanes" within a single car lane.
Also, I am able to exploit gaps in traffic that even a tiny hybrid could not navigate without hitting something. I do it several times per commute.