In an enclosed space, yes. But in open air, after a fairly short time the hydrogen has cleared off to the upper atmosphere somewhere, removing the risk altogether. You can achieve the same effect in a room by ventilating it. The diesel will still be there waiting for a suitable ignition source until you go in and physically clean it up.I think you are seriously overlooking the explosion risk associated with hydrogen?
I know where I sooner strike a match, given the choice of a room swimming in diesel or one with hydrogen gas present.
The issue with hydrogen isn't massive spills, but the fact that it's almost impossible to contain it and the fact that a small leak from a pressurised system is liable to create a very hot, almost invisible, blowtorch-like flame.
The risk profile of hydrogen is very different from that of liquid fuels, making comparison difficult. And it's quite natural as humans for us to readily identify the increased risks without recognising the reduction in other areas.
To stir the pot: has anyone looked at ammonia as an energy source? It's getting a lot of attention in the maritime industry at the moment because it's much easier to handle than hydrogen and has much better energy density. A recent study indicated that ammonia-fuelled ships are the most credible way to decarbonise shipping. Obvioulsy it's got the same fundamental issue as hydrogen in that it's a store of energy generated elsewhere.