[sparkeysteve]

As an Auto Electrician, i can tell you that the battery would not set on fire or give off gas in the event of being crushed in an accident.
the nature of the construction of lead/acid battery's is so when crushed the plates inside would short together and stop all electro-chemical reaction.
therefore no reaction = no heat = no fire.

as to the last comment on gas being produced, well that was due to being short circuit across the main terminals, which would cause a massive discharge of the battery, hence lots of heat generated and the gas given off by the battery, basically boiling sulphuric acid.

please rest assured that the normal car battery's will not set on fire in a collision.