Why not? The range between 1-10 MPH over the speed limit is statistically the safest. (Yes, I know, good luck finding a citation for a study I read 20 years ago...) It's hard to argue against the notion that you're safest when you keep up with prevailing traffic. That means that self-driving cars shouldn't be programmed to go slower.
Speed limits are usually too low. Often they have nothing to do with engineering or safety considerations, but are primarily revenue-oriented. If the advent of autonomous vehicles forces regulators to listen to the engineers for a change, that will be an unalloyed Good Thing.
You have to remember that many people on here probably don't own a car, or necessarily even hold a driver's license. They most likely either live at home or at school, or take a bus to work (NTTAWWT).
As far as I understand your comment, driving at the speed limit is unsafe because everybody is speeding. Sounds like a self-fulfilling thing. Maybe it would be even safer if the majority would obey the speed limit?
Did you read the link I posted? Theres lots of empirical data out there that says that drivers drive the same speed on a road, regardless of the posted limit. That speed is based on their judgment of the road - sight lines, shoulder width, sharpness of curves, etc.
Speed limits are usually too low. Often they have nothing to do with engineering or safety considerations, but are primarily revenue-oriented. If the advent of autonomous vehicles forces regulators to listen to the engineers for a change, that will be an unalloyed Good Thing.