I don't consider CNG or propane to be any more dangerous under ambient conditions than regular gasoline. Ounce for ounce, their explosive combustibility is roughly similar. CNG and LPG are both usually stored under pressure, and with small volumes this may accent the explosive force some, but I've seen what happens to a 5 gallon container of gasoline when it is ignited, and I don't think it is gonna be any safer an explosion to be around. In greater volumes, my experience is the disparity is even less.

As to the spread of gases compared to vapors, I would reckon that ambient conditions can make a big difference there. Gasoline vapors are more dense than methane or propane, so will tend to settle and stay put in low places longer. However, they don't tend to concentrate like gases do, so it may be a tradeoff.

I wish there were a way to sequester CNG and propane like what can be done with hydrogen. I watched an experiment where a cylinder of propane, a can of gas, and a cylinder of hydrogen were each shot with a high powered rifle bullet. They all exploded in a predictable and violent way. Then a cylinder full of anhydrous pyrite saturated with hydrogen roughly equivalent to what was in the regular cylinder was shot, and it didn't explode, it had a small jet of flame at the hole, but did not burst into a fireball.

Someday, I expect we will go to hydrogen combustion vehicles, and the safety factor will go up.