The point is that there are marginal costs for the use of a car over and above the fuel cost. (BTW, 45mpg = 10 miles/litre = at least 12p/mile). Mileage-related maintenance and depreciation, tyre wear, extra insurance for higher mileage; the problem is that because we don't pay them at the time, like purchasing a rail ticket, we underestimate them even if we count them at all.
We've all driven further to buy something cheaper, without working out that it would really have been cheaper to buy it from a local store.
Those of you who drive to the airport or into town should be eternally grateful to those who use the bus or the train and so leave space on the road or in the carpark.