I think the math in this discussion doesn't start with realistic assumptions. When people go hiking it's often in groups. Put 2 hikers in a car that gets 30mpg and you have the equivalent of 60mpg per person. Or 3 in a SUV that gets 20mpg. That puts those about even with the typical motorcycle.
These people may all commute one-per-vehicle during the week, but I usually see groups getting out of each car at a trail head.
The MPG race for motorcycles and hikers looks like a wash to me.
Bicyclists, in my experience, are more likely to arrive one-per-car.
Do my observations or math match what others see?
"The belief that there is only one truth, and that oneself is in possession of it, is the root of all evil in the world."-- Max Born, Nobel Physicist