If there was 100% adoption of self-driving vehicles with a inter-vehicle communication network, there is no reason why the left lane couldn’t go 100+ mph. There still would be lower speeds outside of the highway, but they could be substantially higher than today on most major roads.
Human drivers are why speed limits exist. People follow too close, people are impatient, people are aggressive, people are risky, people don’t know what the vehicles in front of them are going to do, people don’t use turn signals, people hit the brakes and cut across multiple lanes of traffic because they weren’t paying attention or missed their exit, etc.
Networked autonomous cars can communicate and collaborate, allowing for faster and safer travel. The left lane could have no speed limit because every car using it, leaving it, or entering it are all in agreement on what needs to be done and what to do and when to do it. Cars on major roads would slow down so another car can turn without causing the cars behind it to stop. Oncoming cars could slow to allow for an opening that a turning car can use instead of waiting for an opening in irregular traffic, or taking a risky turn that causes an accident.
Getting to that system will require laws against manual driving and mandating that all new vehicles have full autonomous driving. I hope I am dead before that happens because that future sounds awful to me.