Not defending the airlines for a minute, but this kind of thinking can lead to calculations as to how many towns' and cities' worth of pollution would be saved if everyone drove their car with half or even a third of a tank of gasoline/petrol.
E.g. Should the good *EC citizen(ess) really be thinking, "Fill 'er up at the local garage, coz it's cheaper than on the motorway!" or "Not sure when I'll find another Tesco 100 RON, or Shell V-Power station, so better fill it to the brim now." ???
*EC environmentally conscious