People - maybe I'm missing something here, but don't we all (well most of us) fill our *petrol* tanks with that highly flamable stuff every few days ? What's it called again ? Oh yeah, petrol. And then drive around with the stuff sloshing around like it was only water . . .
Given a proper strong container which is properly restrained and protected from puncture there shouldn't be any more risk than filling your tank up. I guess there is a potential risk of asphyxiation or explosion if the fumes enter the body of the car, but that's where the importance of a proper container comes in - one that doesn't leak.
Having said that, I have a feeling there is some law from the dark ages making it illegal to carry petrol in anything other than the tank.
I carry diesel in a can around in my campervan ever since almost running out on a very, very long road with almost nothing on it for I don't know how many miles, but it was a long way. I understand diesel is less flamable & volatile.
z