Guys, (and gals), marine
flares generally have flash points that are way up over 100C, well beyound the temperature any car gets inside.
For example the Pains Wessex flares, at least common in Australia
have a flash point on their MSDS sheet of 160 C or 320 F.
They are however not guaranteed to be useful as a safety
device if kept in heat and should be kept in a cool dry place. Max temp for keeping is 70 c. That's still much hotter than the inside of a car boot.
Orion road side flares (as an example) have a flashpoint of 167 F in contrast to the marine
The other thing to consider is that marine distress
flares are quite different than the roadside flares used in the US. If you set off a marine flare on a road side, you will risk blanketing the road, causing other problems for motorists. Where as purpose designed emergency
flares for the road have a small, but bright area which they light up.