There's a million social elements here we haven't explored. Imagine driving through a ghetto and lots of people rush over to your car to rob you (or worse). Your car will happily brake for them, when a human would have recognized the situation and accelerated and veered off to avoid them. Or weird edges cases like garbage on the road or somesuch and the car not being able to outmaneuver it because it may be illegal, but a human with a more lax acceptance of traffic laws will veer into the right lane close to another car to avoid it.
Or just the daily grind of speed limits. On the expressways I drive the posted speed is 65-70 yet most everyone does 80-85. Doing a steady 60-65mph because of how laws were written to get speeding ticket revenue might be annoying. Especially if you're the only non-human driven car and everyone is speeding past you and honking because you're not going the speed of traffic.
http://www.vox.com/2015/1/15/7551873/jaywalking-history
All of this happened by the mid-1920s. As of 1925, the number of cars per household in USA was 0.6. (It would later peak in the 1980s at 2.2.)
http://en.wikibooks.org/wiki/Transportation_Deployment_Caseb...
Is your hypothesis that the poor folks got cars first?