Possibles...
Did you see the James May programme last week on BBC2?
There is a fair bit of driving around test tracks, re-tests, driving around plant to dispatch point, driving onto trains, off trains, ports onto ferries (like one massive car park) and when at destination from port to presumably another delivery area in country of destination etc.
I've seen a batch of brand new Fiat 500s (about 30 cars) in a large convey being driven along the M4 to a destination. Don't know if Mercedes do this from one dealer to another dealer within the same network who has stock.
Maybe dealers allow test drives in cars that have already been sold?
Depending on where the car is bought e.g. Drive the Deal, the default method of delivery is dealer driven delivery unless extra is paid for transporter.