Molecular weight of methane around 16, air around 29, propane around 44. For the most part true.
I worked as a fireman for the emergency crew of a natural gas company, and for the most part the leaking methane would sit and haunt if there wasn't a wind, or if it was somewhat confined.
Trenches and bell holes took out their share of engineers and welders. A dangerous mix of air and gas stayed in place if it was being replenished by the leak... I saw yard leaks take down houses, but have also watched welders cap leaks in live lines without mishap. I think 6-15% was the threshold. With a blowing leak, the % was higher than the combustion mix.
I got to watch from the side of the trenches with the nozzle of a huge CO2 extinguisher in one hand, and the rescue line strung down in the trench around the welder's waist in the other. The bigger worry was him passing out from lack of O2.
I can see that, a trench can be an issue, as can certain situations under a house and such, I have run into a couple of situations where the conditions were just wrong when I was looking for a leak (while the gas was still on), but I could always stop the gas at it's source for my work.
I had training for LNG ships. One of the things we learned was the slow flame speed of natural gas, a man could out run it they claimed. We all took up jogging on our lunch breaks :)