What is the relative vapor density of Liquefied Petroleum Gas (LPG, UN 1075) compared to ambient air, and what operational hazard does this create during a road transport leak?

1. LPG vapor is heavier than air (density ratio ~1.5 to 2.0); it sinks, spreads along the ground, and accumulates in low-lying trenches, drains, and ditches to form explosive clouds
2. LPG vapor is lighter than air (density ratio ~0.5); it rises vertically at high speed and dissipates harmlessly in the upper troposphere
3. LPG vapor has the exact same density as air and immediately turns into an odorless liquid at any temperature
4. LPG vapor is non-reactive and settles permanently as harmless crystalline snowflakes on asphalt