When the car hits an object - the momentum is changed into pressure. This voltage will cause an arc across the switch that is interrupting the current. This will eventually burn the switch, but in the short term, it causes electrical noise, which can have all sorts of effects, such as making electronic equipment misbehave - or even fail. Clearly, what we need is some way of interrupting the current flow slowly - a method of letting the current die away slowly. Fortunately there is indeed an easy way to do this.
Waveforms Consider the diagram to the right.
When the switch is closed, current will flow and the coil will energise. When the switch is opened, the current will stop and the coil will de-energise. The drawing below shows how the current and voltage vary with time. At the first red line the switch is closed to energise the relay. At the second, it is opened to de-energise the relay.