Your comment does nothing more than reinforce my claim.
Paying attention in class is actually hard work. Some people cannot do it even if their life depends on it.
Paying attention in class is just the biggest indicator (IMO), but it is still just an indicator. Of course other aspects have an impact too. Having the time, space, and resources to actually be able to do homework at home is huge too. But homework is still at most 2 hours per day (on a particularly homework-heavy day). Kids spend ~8 hours at school.
That's why I believe that what you do in school has the most impact on your school performance. At the end of the day, the place where kids spend most of their time is at school. If they make the most of that time, they will most probably at least pass the class.
You either make driving fast less safe, or you put a speed camera on every corner with the appropriate punishment. Cameras are expensive, and drivers will complain about them. If you make driving fast less safe, only the ones that remember how the road was before can complain about it.