Your going to be competing with Vehicles or Pedestrians one way or the other, I highly doubt any city is going to blow tens of billions on a grade separated autonomous delivery thoroughfare. Most cities are barely willing to put paint down for bike lanes, let alone create a physical barrier so they are safe for cyclists to use daily.
Unless you are restricting deliveries to within 2 or 3 miles of where they start from (considering people walk at 3mph, and Segways were banned from sidewalks for going 10mph) you will probably be on the roadway.
Well 2-3 miles is fine, especially if you're considering graduated deployment of autonomous vehicles.
I'm a big proponent of refactoring the world to facilitate autonomous agents within our lives, so I think spending money to figure this out is part of what we should be doing.
Perhaps we need more people with similar opinions in government.
Unless you are restricting deliveries to within 2 or 3 miles of where they start from (considering people walk at 3mph, and Segways were banned from sidewalks for going 10mph) you will probably be on the roadway.