To what, 55 mph? It's the most fuel-efficient speed for the majority of vehicles, and is far less likely to kill people in an accident, than a collision at 70*1.15 = 80mph.
why stop there? pretty much any form of transportation is more dangerous and/or generates more emissions that humans on their own two legs. collectively, we have decided to trade some of that away to actually get places in a reasonable amount of time. but if you don't feel comfortable driving in 65+ mph traffic, there are plenty of surface roads that run parallel to interstates.
