Petrol is a fun liquid to deal with, not being particularly flammable as a liquid but having an extremely low vapour point and in turn the vapour having a very low ignition point.
As pointed out above, plastic jerry cans have some design flaws that make them not ideal for carting fuel long distances. The metal canisters also are basic to say the least and shouldn't be used inside a passenger vehicle. Not only is vapour an issue but even things like static electricity pose a level of risk.
There is a system called explosafe that is designed to be the safest way of transporting fuel
http://www.explosafe.ch/system.php?nav=system. Although they are over priced and slow to get liquid in and out of they do offer piece of mind.
More fun on this topic, google BLEVE (BOILING LIQUID EVAPORATIVE VAPOUR EXPLOSION.) and see what LPG has to offer.
