I'm just going to join the safety debate as it's a popular "discussion" with my dad....
The LPG tanks are very strong pressure vessels so are significantly less likely to rupture in a crash than a plastic petrol tank.
However if there was to be a major leak, LPG will immediately vaporise and disperse, unlike petrol which will spill and linger. But you do not want to be within a hundred yards of it if it does burst or snap a connector off.... however unlikely that might be.
So whilst logic says a car in a crash carrying 60L of petrol and 30L of LPG is going to be less safe than just 60L of petrol, but I think the probability of there being an issue like that is very low.
Interesting watch :
https://www.youtube.com/watch?v=AG4JwbK3-q0