I'm just curious as to why DC? They're not based there nor is DC considered to be one of the most densely populated cities.
San Francisco, by contrast, is almost too perfect. It would no doubt be successful there, but it doesn't give them as good of a feel for how this service would fare outside of Silicon Valley where these types of ad-hoc delivery services are far more common.
I get the sense that they're trying to gather data without drawing a lot of attention to their experiment.