For my entire driving experience, from the North East of the US to Northern California, to specificially the portion of the road this car was driving on, this is false. People drive, very reliably, ~5mph over the speed limit. When you get to 35/40+ mph speed limits, ~10 over isn't uncommon, ~15 for people going uncommonly fast. I can't recall the last time I drove under a speed limit (minus exceptional road/traffic/weather conditions) and I'm slower than most drivers I encounter.

More generally, urban streets, even arterial streets like El Camino, would really benefit from more strictly enforced speed limits, and lower limits wherever possible. A pedestrian or cyclist hit by a car going 20–25 mph is likely to sustain only minor injuries. A pedestrian or cyclist hit by a car going 35+ mph will probably die. Drivers also have dramatically less reaction time and much worse road awareness at 35mph compared to 20–25 mph (and realistically the speed of traffic is probably 40–45 mph on a road with 35 as the posted limit).

Cars are scary killing machines, one of the leading causes of death, and drivers are often poorly trained, distracted, or just idiots. The slower and more carefully they drive in urban environments, the better.

I’d love it if armies of 25 mph self-driving cars were constantly cruising around town, restricting other drivers to a safer speed. If they could supplant the taxi/uber/lyft industry whose drivers speed along my residential street at 55+ mph every night from 12–6 AM, that would also be dandy.

This is one of those examples of an explanation rather than a justification.

If someone else is going slow, that's no excuse to start acting like an idiot, and if you do and you get in an accident, that's on you, not the slow driver.