Given how many people treat speed limits as suggestions, at best, having your vehicle obey the limit would turn some people off of them.
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.
The major roads are already nigh impossible to walk across. Finding a way to raise the speed just makes it harder to be a pedestrian in yet more places.
I, too, love the idea of networked autonomous swarm agents behaving in an even more efficient setup. The problem is that if the only focus is on moving cars faster at the cost of people’s comfort, access, energy, and walkable anything we lose out on reasons to ever be outside of the car at all.
More cars and faster cars in our cities makes the city worse, even if they’re self driving.
I’ll echo that there are safety issues. Motorcyclists (I ride my much more fuel-efficient bike when I don’t have something too big where I need my car), bicycles (which I also ride for leisure and errands close by), and pedestrians (same here) aren’t going to work very well in that. You’re not going to mandate that every person carry some kind of transponder and that it must always work. Where I live, many students are walking and cycling. I also think tractors, dump trunks, and other special equipment will still have human drivers for at least part of the journey for the foreseeable future.
Also, smacking into an animal at 160kph is terribly dangerous and potentially damaging. A blowout at that speed also has much scarier implications for control. A lot of hazards would need to have issues solved here as well.
