[crazygringo]

You're right in that the, say, 20 min saved per person is going to go to some other activity.

And certainly some of those activities can involve driving, like going to visit friends. But many other activities will just be spending more time at home -- e.g. coming home straight from work instead of running errands, or staying home Saturday afternoon to read or play video games.

To your point: in areas where traffic congestion is already extreme, it may not make a difference because there's so much pent-up demand for car trips. But in most of the rest of the country (suburbs, rural, etc.) that doesn't suffer from congestion, I think package delivery would have to result in reducing traffic to some measurable degree.