Personally I dont see a understand your lack of competition comment, especially for car users, many motorists look their routes online before. I look my routes up and estimate where I will need to stop/ fill up with diesel etc before hand, and find alternatives close to the motorway.
I Recently drove Cornwall to Heathrow, within 1/4 mile of motorway there is a supermarket at all of the following locations:
Exeter M5 J30 Morrisons
Cullompton M5 J27 Tesco
Taunton M5 J25 Sainsburys, and Asda
Avonmouth (Bristol) M5 J17 Asda and Morrisons
Swindon M4 J16 Asda
Reading M4 J12 Sainsburys
I refilled at Taunton Sainsburys where diesel was 8p a litre cheaper than at Taunton Deane services.
The only benefit of a service station is the convenience, and if more people realised how close the supermarkets are to the motorway, I'd imagine they'd use the supermarkets vs. Service station any day!