Yeah. I think when you see three corners of a 4-way intersection with gas stations it obviously involves zoning as mentioned and traffic volume enough to sustain the three service stations. Where you have lots of volume in multiple directions, there is enough to sustain multiple stations --they in turn eliminate U-turns and crossing traffic in multi-lane roads/highways.

I don't often see consecutive stations on the same side of the highway/road --tho I do occasionally (19th ave in SF, for example).