Sorry but I think that a hammer in the car is a bit of overkill.
Sure, get one if you’re concerned.
How many times does a person in the UK need a hammer to get out of their car? (MG4 owners excluded..?)
Ok, once might be enough, but you’re far more likely to be mugged, should you carry a hammer just in case? Or crash in a plane, should you wear a parachute ?
And (thankfully) how often do you find yourself submerged in water with doors jammed? Most people (myself included ) would probably panic anyway.
I’m all for being prepared and safety is paramount but I would suggest their are many more areas to focus on that will be far more likely to cause incidents.
For example;
Improving your driving by taking extra courses
Being more aware at all times
Trying to be more courteous
Being more speed aware
Keeping your car and tyres etc roadworthy
Get swimming lessons…
Carry a full divers kit….