15 miles/hr is a realistic average speed in many cities. With its dedicated lane it could be quite competitive in terms of travel speed, just like bicycles.

But really the competition for these systems aren't cars or bicycles. They are mostly shuttle systems, so the competition is either walking, or a car that can't find a parking spot. Maybe public transit. With the highest speed being 5 times average pedestrian speed there are plenty of useful use cases.

Inner city Paris is limited to 30km/h, I don't think going 25 km/h would be a problem.

There's an autonomous shuttle in testing near where I live (goes between a high-density living/shopping area an a metro station) which only goes about 10 mph. So not entirely vaporware...