I agree absolutely that risk is a sliding scale (or a matrix, to simplify things). I think we are in agreement here.
A hazard is a condition or action with the potential to do harm. A leak is an incident. As you note, the consequence of a (leak) incident can vary and we should consider the "worst reasonable case" as our concern. Also I agree that the probability is uncertain, and that's why we use our judgement to assess this. At the end of the day we end up with a characterisation somewhere between "insignificant" to "catastrophic" and seek to bring the perceived risk down to ALARP or at least "acceptable".
As you say, it's not helpful to make blanket unsupported declarations.
So, to clarify my statement that carrying fuel inside a vehicle is "unsafe", I'll quote the following Globe & Mail article in which this is discussed, with quotes from Calgary Fire Department and the Canada Safety Council:
""It's not safe. It's not recommended,
" said Carol Henke, spokeswoman for the Calgary Fire Department. If the lid is not tight or if there's any leak, the fumes can escape and when they reach a certain concentration any spark or heat source can cause them to ignite. And even if the fumes don't find a spark – from wiring or static electricity – you shouldn't be breathing them in. "The fumes and vapours can make a driver light-headed and nauseous
," Lewis Smith, manager, national projects with the Canada Safety Council, said in an e-mail. "[... cans] should always be stored in well-ventilated areas, which rules out the passenger cabin of vehicles for obvious reasons.
" Just as there are still warnings on gas pumps that static could cause an explosion, there's a long list of warnings on the sides of new [...] cans, including "keep out of reach of children and direct sunlight,
" "store in a well ventilated area
" and "do not store in home or vehicle
."
The Canada Safety Council doesn't recommend keeping gas inside your vehicle. So what's left? "Securing a [... can] to a roof rack is also a possibility, but given the relative weight of the fuel it's important not to overload the vehicle and throw off its centre of gravity,
" Smith said. "The roof-rack solution also comes with the added concern of a loose can flying off at highway speeds – obviously less than ideal.
" If you have a truck, you can safely keep a gas container in the bed of the pickup. And, if you have an SUV, you can get a metal [...] can mount installed on the back of your vehicle."
I don't think that outside storage is a significant risk, although obviously in a crash there exists the possibility of damage to a fuel can, with consequent loss of containment, and if there is either a spark or a flame this would be a concern (although I suspect that at this level of damage we would have other things to worry about.
We carry fuel in cans occasionally and we're not concerned about it, although I have to say that the way I did this many years ago was probably less safe than the way we do it now. I had a metal can mount (well made) which mounted to the back of the rear spare wheel mount (on a Nissan Patrol) - in the event of a rear end collision this would have resulted in diesel all over the place, which could have been bad news. We now use cans secured into an area of the roof rack with a crossbar and side bars, and this is probably a lot better, albeit a little less convenient.