Do cars have a fuel and electricity cut-off system which kicks in after a collision? Can such a device be retrofitted? Some cars do have an inertia switch designed to cut off at least the fuel pump in the event of a collision.
The idea is to eliminate the possibility of fuel spray - and fire - should any of the fuel lines puncture during the incident. In modern cars, especially those with direct fuel injection, the pump pressurises the fuel lines to between 150 and 200 bar. The average saloon car's tyre pressure is around 2.5 bar.
At over 150-bar pressure, a small rupture in the fuel pipe or the fuel hose would cause a spray in a manner similar to liquid shooting out of a jet nozzle. Even if the volume ejected may be small, the spray can contaminate various parts of the car that are in close proximity to the leak.
This is a potentially disastrous condition, particularly if one of the components is a hot catalytic converter.
Power cut-off or battery isolation systems are also designed for safety. In combustion cars, it prevents electrical sparks after a collision, which might trigger a fire. In electric vehicles, it prevents electrical shocks to occupants and rescue workers.
Both fuel and power cut-off systems work with a force-activated mechanism. However, this mechanism is not as simple as perhaps the inertia reel seatbelt. You surely do not want the fuel or electricity supply to be cut off when you hit a pot hole or ride a speed hump a little quickly.
Like the airbag, it requires a set of sensors not just to detect impact, but also to evaluate the nature of the impact. For this, the engine control unit that commands the cut-off device must receive the necessary signals from a network of sensors. It analyses the car's speed, deceleration and even the dynamics of the car body just milliseconds before the impact, in preparation for the final decision to trigger or not.
As far as we know, such a complex system cannot be retrofitted.