Whatever the emergency, you will be better prepared for it if you are wearing a seatbelt. This will at least halve your risk of death and injury and also allow your airbag(s) to do their job better.
Your seatbelt will keep you secure in the vehicle - those thrown out of vehicles in accidents are often killed. It is estimated that you are up to five times safer if you are held in your seat.
A vehicle hitting an obstruction will lose speed almost instantly. An un-belted driver or passenger will continue moving at the cars impact speed until they hit something. This could be a wall or tree if you are thrown out of the vehicle, the steering column, dashboard or windows if you are thrown around inside. Perhaps even worse, un-belted rear passengers (even small children) can be thrown forward with enough ferocity to smash the seat in front of them, killing the occupant (and often themselves).
Going slow doesn't help much ... At 30mph the impact is approximately the same as hitting the ground when jumping from the roof of a three-storey building.
Seatbelts help drivers to regain control in a violently unstable vehicle by keeping them securely in the driving seat.
Car occupants form 64% of all road casualties. In 2010, 133,205 people were killed or injured while travelling in cars, of these 89,787 (67%) were drivers. Driver Passenger All Occupants
The table above shows the number of car occupant casualties during 2010, sorted by severity of injury, and seating position. Source: Reported Road Casualties 2010, DfT, London September 2011.
Over 90% of adult front seat passengers and drivers wear seat belts, as do 66% of adult rear seat passengers. Since the law to wear seat belts in the front was introduced in 1983, front seat belts are estimated to have saved over 50 thousand lives in Great Britain.
Some people worry about being trapped by their seatbelt - there is a risk that this might happen. But then there is a risk (a much greater risk) that you will be knocked down and killed crossing the road on foot. As with all things in life we need to weight the risks against the benefits - wearing a seatbelt is a 'no-brainer'.