I'm having trouble understanding their statistical work. How is the "differences in differences" framework different from having two experimental groups and no control?

I'm also not sure that extrapolating to Chicago makes sense. I live there. Since anecdotal observations seem to matter, it seems that the people who would order an Uber car would call a cab if Uber wasn't available. It's hard for me to imagine Uber providing such an addition in service that it would change DUI rates.