Assume that you're travelling 100 miles. The first 4 miles are in town, with lots of crossings, lights, etc. The middle 92 miles are on highways with a few turn offs. The final 4 miles are back in town with lights and crossings again. let's assume UK speed limits for everything (30mph in town, 70mph on highways). We'll ignore acceleration.

In an autonomous car you can do the speed limit for the entire journey. That's 8 miles at 30mph and 92 miles at 70mph. The journey takes you 95 minutes.

In the human-driven car you're slowed to an average of 20mph in town, but on the highway you travel at 85mph because that's what everyone else does. The journey takes you 89 minutes.

Obviously the 95 minutes in the autonomous car can be a productive 95 minutes whereas driving yourself you can't do much else for those 89 minutes, which is a huge difference, but until all the cars are autonomous (or there are superfast autonomous lanes at least) then it's probably going to be quicker to drive yourself because to a human a speed limit is a guideline rather than a rule.