I don't see how mandating that someone who may be unable to safely pilot their vehicle at higher speeds, drive above their capability, is going to be safer than having that person safely able to keep their vehicle under control, albeit a bit slower.
OTOH, someone who can't maintain safe control of their vehicle while avoiding other slower moving traffic, may be more of an issue than a slow driver.
Forcing everyone to drive at higher speeds just isn't the wise move. I've seen too many people that have trouble keeping control at city street speeds.
Personally I drive at whatever speed feels comfortable. In the truck pulling the toyhauler, usually around 65 mph, and I usually slow down a tad in heavy traffic, just to keep some space around me. That truck and trailer do not swerve, or change directions all that quickly and safely. On the bike, I'll be the one passing nearly everyone. :)