Commuting using a 50+MPG vehicle is an option for the vast majority of humanity and would make a huge difference. So, it's not like the average person has zero options. People drive ~3.22 trillion miles in the US using 142.85 billion gallons = ~22MPG so a 50% reduction is completely viable.

Further the first 50% reduction is the lowest hanging fruit. We don't need to get to 100% to make a huge difference over time.