I've always thought that vehicle skills in Grand Theft Auto: San Andreas were just a factor of how much time you spent driving/riding and the amount of distance you travelled.
However I've been paying more attention of late, and I have noticed most of the time when I go up in skills is directly after I've just pulled off some massive feat, like jumping off a cliff on a motorbike and landing it perfectly, or sent flying in a car and land on all four wheels, facing the right way.
I'm not just talking about performing 'Insane Stunts' either, although they are not completely excluded, given the nature of most jumps
There are other things that seems to boost your skills quicker as well - entering (and completing) driving/riding school missions, car rallies.
Now it could just be coincidence, but it has happened a fair few times - both with bikes and cars. Is there any evidence to support this?