Sadly, a lot of speed limits aren't set according to government standard 85th percentile, and local governments are easily swayed to lower speed limits just with a few letters from citizens, no study needed.
That's when you get things like 6 lane divided roads with a 50km/h speed limit. Just excruciatingly slow.
It's important to note that there isn't an obvious relationship between traffic volume and traffic speed. If you model traffic separation as a constant two seconds (front-to-front) your traffic flow is 1800 vehicles per hour at all speeds.
Of course, your average person cares more about how close the back of the car in front of them is, so flow rates at very low speeds are lower than at high speeds. At higher speeds you get other problems, though - traffic gets "patchy" as some people get a little nervous, as people drive at different speeds and so on.
You probably get the most people per hour through a lane of traffic at around 60km/h. Everyone's travelling at about the same separation and about the same speed, and they're all tailgating the guy in front of them trying to make him go a little faster. The speed leaves enough breathing room for everyone to drive consistently, which is supposed to be the big benefit of these self-driving cars.
Of course, these lower limits seem to increase travel times in a very obvious way, and the congestion relief they provide is pretty abstract. It's understandable that people aren't entirely convinced.
That's when you get things like 6 lane divided roads with a 50km/h speed limit. Just excruciatingly slow.