Given how many people treat speed limits as suggestions, at best, having your vehicle obey the limit would turn some people off of them.
Agreed—and to be clear, I’m not advocating for self-driving lanes. But I think one of the potential motivations for the creation of such lanes is that human drivers would feel more comfortable if they weren’t sharing lanes with self-driving cars, just like they feel more comfortable not sharing lanes with buses. And by the same token, bus drivers and self-driving cars aren’t going to want to share lanes with each other, so there would be pressure to have different lanes for each type of traffic.