Transporting Gas Cylinders - Safely.

I did a search on this and came up with pages of not very much, so please can an expert answer my question?

I had to remove my propane gas cylinders from my van last week so the dealer could take it from their storage compound into their workshops for a small repair.

I then took them back home with me, a distance of about 15 miles. The only way I could carry them in the car without them moving around was on their sides. With nothing to wedge them in, upright was not feasible as they would have fallen over, risking damage.

I read somewhere that it is not very safe to do that, even if they are securely packed in. But surely if they leak they will leak upright or lying down won't they?

Can someone explain why please?

Thanks

Peter C