I am trying to do a project about car safety and i was wondering if it is possible to use the cars own crash sensors (getting the info from obd port) to detect that there is a crash(real time) or do you have to install your own sensors in the car ?
No you can't. The only things that you can do are listed in the wikipedia page:
That are only pids for engine information e something regarding fuel with mode 1 and trouble code with mode 3.
The other things are managed by on board computer (ABS, Airbag, ESP) and for a reason about security you can't access this information....
There is some device for talk with on board pc of car but they are very expensive and use something like that can damage the configuration of the computer of car a let him not reparable (my mechanic say this to me).
BUT you can install our own sensors and controll them with an Arduino for example.