Where I live it is not an issue, but it is theoretically possible that there might be a location on the planet right now where Fedex trucks crash into UPS trucks, and Dominos pizza cars are running Chinese delivery cars off the road, on a daily basis. However that weird situation existing would still be a workable system for perhaps 99.9% of the worlds surface.

There are also practical scaling issues. The local pizza delivery joint cannot produce fast enough to saturate a 2-D delivery system during the superbowl, just can't pump out pies fast enough. Naturally going to 3-D delivery with maybe 100x the delivery capacity because of 3-D isn't going to alter their delivery rate, the skies will be pretty empty. Even if you drive there to pick up, there will be a 2-hr wait for pizza. Drones and trucks are expensive and the finance guys would prefer the absolute minimum be owned and deliveries be made 24x7. That would imply we're not going to get drone congestion until we have deliveries at 3am. I'm just not seeing people order enough raw "stuff" to keep delivery services busy enough.