I can’t offer any academic cites, but it seems to me that the advantages of clustering depend on whether the item being purchased was the primary reason for the trip or if that purchase is secondary. If I were shopping for a car, I would definitely prefer to go to an area that offered several choices. But purchasing gasoline is rarely the primary motivator for a trip. I would most likely choose a gas station based on its proximity to the primary destination of my shopping trip – a grocery store, a drugstore, whatever. So gas station clustering may be an effort to be close to a primary destination.