You might recall the concepts of “Thinking distance” and “Stopping distance” from your driving test. Consider this: in a typical city environment, while driving with low beams at 30 mph (50 km/h), a car will move 150 feet (50 meters) on average after noticing an obstruction, applying the brakes, and coming to a full stop. This is approximately the same distance from which it is possible to notice a person who is not wearing anything reflective. So, basically, should a person slip and fall in the middle of the road, he or she is lucky if the driver manages to bring the car to a full stop just shy of hitting the pedestrian.
Wearing a reflector will double the distance from which you become visible to drivers.
Here is a cool simulator by the Finnish Road Safety Administration, Liikenneturva, to demonstrate this concept:
Get your reflectors at http://www.visibelreflective.com