You should account for parking. I've heard estimates around 200 billion USD per year for opportunity cost and maintenance of parking lots.
I think you also ought to count green space requirements against cars too. Cities seem to be forcing builders to insert green spaces and space buildings out even more than the parking requires. This is more opportunity cost for land, more miles driven, more roads, and less ability to walk.
Then you've got to count the roads. Sure, bikes need roads. We don't need as many (that should be counted in the parking) as wide and I highly doubt we put on the wear but you'd probably have to ash the Dutch what bike path maintenance is like compared to road maintenance: I don't know that the US has any bike paths that would qualify as busy year round.
If you're counting fuel against the cars make sure you do so for the bikes too. I'd subtract out the calories that contribute to obesity (well, maybe half of them -- let's be realistic).