There are two generally reasons to cut a car open to extract a victim:

- Possible neck and/or back injury, which requires the patient to be removed on a spine board.
- The victim is physically trapped in the car, due to collapse of the passenger compartment or intrusions in to the passenger compartment (steering wheel, pedals, dashboard(components)). This however has become less and less of an issue, with newer safer cars.

Cutting a car roof does not help in these situations. It does create a new hazard; the sharp edges and you are prying with a knife near a patient and well it’s also dangerous to yourself.

Note, sometimes you do not need to cut. Keep things simple. Sliding the seat back or tilt the seat can create space. Also note that cutting is not always the solution, spreading to create space is just as important in the rescue bag of tricks. (note, most cars have a jack, which in some situations could be used as an emergency tool. However most jacks lack proper bases, so they may slip off easily. Again first choice is professional rescue.)

Also if you pry, pry in smart places and sometimes combine it with other tools. For example, for door removal prying space on the hinge side, can open up enough space to access the bolts. The door can then be removed with a socket set.

What some rescue departments do with possible neck and/or spinal injury in car with back access (minivans, hatchbacks), is to remove the back seat, tilt the patient on its chair and slide spine board under the patient and slide the patient out the back. No cutting required, saving time, damage to the vehicle and issues with sharp bits. Tools required: socket set to remove the backseat.

As for a knife as metal cutting tool or pry bar. Well no experience with that use, but compared to a proper tool, a knife looks like a very very poor choice.

Extra note; with crush injuries. If body parts have been crushed and bloodflow stopped for longer periodes of time; beaware of crush syndrome.