So do roads, slow downs happen when cars get on or off of highways.
Going from a road with a high speed to one with a lower speed, if those roads are at capacity, will always result in a backup. Off ramps serve this purpose of course, they basically convert cars going at 60mph into traffic going 30mph through stop lights or some other sort of flow control, but if the roadway is at capacity (as many major cities are), that off ramp as a flow control mechanism falls apart and cars pile up onto the freeway.
The math is unfortunately independent of self-driving cars. If every car is self driving capacity does increase, cars can self coordinate and some of those stop lights become unnecessary, but if 100 cars want to get onto a road that has room for 20, someone is going to have to wait!
Subways naturally don't have this problem!
Roads also allow for multiple lanes. This allows for somewhat degraded service when something goes wrong; a train stopped on the tracks blocks everything. With multiple lanes, roads can be maintained while still in service. Every subway struggles with maintenance windows.
I'm not saying one is always superior to the other - trains have their niche, but it's a niche that will never be as broad that of autonomous cars.
What I don't understand is why do autonomous self driving cars and subways have to be either or. Why can't we do both? After all, we can't use the subway for ambulances and fire trucks so we still need to build some kind of roads even if we had full confidence in the subways...
Just thinking out loud... Unless we had helicopters for emergencies?
Can we have a town with no paved road? Only sidewalks? That's still paved, right? Can trash collection also take the subway?
Not necessarily.
Rail is cheaper to maintain, and there is a huge opportunity cost of paving over a cities most valuable real estate to make roads.
The math on how large the streets would need to be in downtown Seattle to alleviate traffic end up with a giant % of downtown Seattle being torn down and paved over.
The Urbanist has a good article (https://www.theurbanist.org/2016/10/12/brt-is-not-cheaper-th...) on why light rail is cheaper than a huge freeway.
Rail can also have multiple tracks, it costs more, but amortizes really well.
Not having to intermix pedestrian, bicycle, and cars is another huge win for, well constructed, rail projects. Autonomous cars still have to stop at crosswalks! In urban cores, stop lights seriously cut into average MPH for a trip, and start shifting things into favor of rail.