I'll tell you what, try riding a motorcycle sometime. You'll gain a lot more appreciation of what it means for you and the people all around you to be moving at 55mph (let alone 70+) in 2 ton metal boxes, in relatively close proximity. I think the problem is not that speed limits are too slow, it's that modern cars are too comfortable, lulling you into a false sense of security.
The problem is that 'safe driving speed' is completely dependent on road conditions(weather, traffic, pavement quality, time of day, day of the week(lots of drunk drivers on weekend nights), traffic makeup, etc.). Metal boxes with seat belts and airbags are far safer than motorcycles, but it's not appropriate to go 80 when motorcycles are nearby.
