The downside of always 'filling up' is that the additional weight makes your car less efficient in the miles per gallon stakes.

And why, when there looks like there may be a shortage, do garages (US=Filling stations)start sprouting "£20 Max" signs? This encourages people to top up regularly, and so make more visits to the garage. Each time wasting fuel while sitting in a queue. It makes the garages look busier, and persuades more people to join the queue as they may miss out.

Just get the garages to act in concert, and display "Minimum Charge £20- (except motorcycles)". Cars would only queue if they had room in their tanks for £20- worth of petrol (or more) thus invalidating the vicious circle described in the previous paragraph.

Just my two-pennorth