First, as miles driven increases, driver experience increases. I suspect this effect may not be as significant as you think. Certainly, someone who has been dr

First, as miles driven increases, driver experience increases. I suspect this effect may not be as significant as you think. Certainly, someone who has been driving every day for 20 years will usually be a better driver than someone who has been driving every day for 20 days. I suspect the difference between 2 years and 20 isn't all that dramatic though. I further suspect that most driving is not practice, by which I mean that drivers are not actively thinking about their driving and how to be better at it. Most of us, once we're reasonably proficient stop actively thinking about the act of driving entirely unless an unusual situation occurs. If your second reason turns out to be correct, then it also means that a larger percentage of driving than before isn't useful practice for avoiding crashes since highway driving tends to be safe and predictable.