The major danger on highways is difference in speed between cars ("variable traffic"), it's much safer to go with the flow of traffic. Absolute speed has little to do with it.
Additionally, speed limits are set for the benefit of police (increasing ticket costs) and for the benefit of politicians who can now say they are "making roads safer".
Well, there are a lot of other reasons for the speed limits as well.
Lack of sufficient climbing lanes on most of the original interstate system. Poorly designed junctions, including merges into the high-speed lanes Inadequate road thickness (autobahn is 55-75cm, US is 28cm), initial quality, drainage capability and maintenance. Inadequate driver training. Lack of enforcement for failing to bear right (though the US is doing this more now) and other minor moving violations that disrupt traffic flow.
The US interstate system would need considerable re-engineering, more funding to keep up with the required maintenance, and both tougher licensing requirements and stricter enforcement of minor moving violations that increase risk. It's a huge cost, that doesn't provide proportionate benefit in all parts of the country, and a questionable ROI. Trust me, I'd love higher speed limits, but I realize the giant mountain of money and effort that would be required to make it happen.
The debate over the exact causes of traffic accidents, fatalities, with regard to speed limits, intoxication, texting, etc. can be simplified by just agreeing that the largest general cause is driver inattention or error, without drilling down any deeper.
Driverless cars are fully attentive, and should have a lower error rate than normal / tired / drunk / distracted / angry / frustrated humans. This I welcome.
Additionally, speed limits are set for the benefit of police (increasing ticket costs) and for the benefit of politicians who can now say they are "making roads safer".
Case in point: the Autobahn http://autos.aol.com/article/driving-the-autobahn/
(but I am also still welcoming driverless cars! I'm okay getting places slower if it means I can read or study.)