That being said, yes, it is likely that average walking pace was slower at the beginning of the 20th century. We have evidence that just in the last couple of decades, average pace of walking has increased around 10% in large metropolitan areas.
The cars were apparently staged but it's not clear that the rest of the traffic was as well: the reason for the former may have been to make SF look like a more advanced city than it actually was.
I walk to work. Briskly, it takes me 20 minutes, leisurely 40+. Because of heavy traffic, it's pretty much impossible to drive there in less than 15 minutes, so that's the distance it's assumed to be.