Why is fuel more expensive on the high way in Europe?

In Europe it's usual, that fuel prices are 10-20% higher on the highway, than in the city. This is not only my personal observation, but also mentioned on travel sites (eg here and here ). I don't have much expertise in economics, but as I understand this should not be possible in a competitive market, since what would stop a petrol station to lower their prices to the same level as the ones in the city, and get all the customers? One solution could be, that they are different companies, but even the big ones like Shell and OMV have higher prices on the highway. My question is: what is the reason for this situation, and what stops petrol stations from lowering their prices?