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?