I saw a couple of posts regarding the importance of lights on your bicycle but never saw the need to add them.
Got two Knog Frog China ripoff lights from Makro, one white one red. On tuesday I did my first commute with them - Huge impact!
I commute from Queenswood to Irene, using the Gautrain in between. In the Hatfield area there are enough sidewalks to easily keep to the first rule of Cycling: Don't get DEAD!
I keep to the side-walks, seeing that your odds of being hit by a car somehow decreases dramatically when you are not riding in the road

(much like how your odds of being attacked by a shark decreases dramatically if you stay away from oceans...)
Anyway, my route through Centurion does not have many side-walks and I need to navigate the notorious 900m stretch on Nelmapius Dr through the S curve near the Denel Office park. With the lights mounted to my bicycle I can see a clear difference in the attitude from the motorists around me. Someone mentioned an average of 0.5m more clearance when they pass you and I can confirm this.
I think driving has become too easy and routine. You are most alert when the roads are notorious, how many people have fallen asleep while driving through something like the Long Tom Pass...? Our brain switches to other issues like work or relationship issues when the roads become too predictable and we no longer concentrate on what is going on in front of you. Think about it, have you ever arrived at an off-ramp while driving on the highway and ask yourself "Am already here, can't remember the last 5minutes of driving..."
The flashing lights are something your brain is not used to and it draws your attention to it. People notice the cyclist and act in a much more responsible manner. When you don't have lights on, the brain of the motorist probably knows you are there but it doesn't focus enough attention on you and that is when people pass a cyclist only to turn left into the street 30m up ahead...
Just a theory, but at least I can say that I am never commuting without lights again...
Safe riding out there!