How can I get my 16 year old son to drive more slowly?

George, ‘‘more input’’ please.
You want him to drive more slowly…in relation to what ?
The posted limit ?
The other traffic ?
Your opinion ?
Let’s put it in perspective. Just how slow is he NOT driving ?
Do you want him to be the slowest vehicle in the line, sandbagging the flow ?
Do you want him to co-ordinate and flow smoothly with the others ?
Is he now passing eveyone else and changing lanes back and forth ?
Is he racing toward a red light only to brake at the last hundred feet ?

Make it a vehicle use ultimatum.
Operating the machine is one thing.
DRIVING is another can of beans.