to inform the public that you are a provisional licensed driver and that they my need to give you more room/attention/patient as you are still learning etc thats why some states there is P1 and P2, so if you havent broken any traffic rules you are allow to drive on a provisional license without having to display p-plates... well thats my understanding of displaying p-plates
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also i did a similar essay on this subject awhile back, be careful when looking at data to see if they are including passengers (ie not just the driver in the results) as this affects the under 20 category for accidents/deaths etc?