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?
Because there is no indication in your question that you are a student or practitioner of economics, I am writing an answer for a lay audience. Let me know if you would like more technical detail.



A fairly general prediction from economic models of competition between firms is that the price that maximises their profit is higher the less sensitive is demand to that price (this sensitivity is measured by the "price-elasticity of demand"). Intuitively:


if demand is very sensitive to price then a cut in price will cause demand to increase a lot. The firm receives less for each unit it sells, but sells very many more units and is better-off overall.
if demand is not sensitive to price then the firm can increase price without causing its sales to fall very much and will therefore wish to do so.


There are lots of things that affect the sensitivity of demand. One example, as DornerA mentioned in a comment, is geographic location. If the nearest competing seller is very far away or inconveniently located then buyers will be reluctant to shop elsewhere, meaning their demand is likely to be less sensitive to price increases at locations that are geographically isolated in some way. This seems like a reasonable way to think of highway fuel stations.



A little thought should convince you that the same principle (that prices are higher when demand is less sensitive) holds for other cases where demand is price-insensitive too. For example:


why is food so expensive on trains/airplanes?
why are branded goods more expensive than generic ones?
why are the photographs from theme park rides so expensive?
etc.




Edit: A comment by AndrejaKo reminds me to add that the other very common reason why firms might increase their price is that they have higher costs. However, economics theory predicts that only unit costs (i.e. those that increase when you sell more units) matter for optimal pricing. Thus, for example, the fact that land near a highway is expensive would not seem like a very convincing explanation for high fuel prices (because the land costs the same regardless of how many units of fuel are supplied). But if, for some reason, supplying each litre of fuel was more expensive at highway fuel stations (because, for example, transporting the fuel there is more expensive) then we would indeed have an explanation for higher highway fuel prices.