I don’t want to make the OP any more neurotic regarding his son’s driving than he already is, but I feel that I have to point out that there is actually something that is statistically far more dangerous than speeding, and that is texting while driving.
While all adolescents believe that they are indestructible and that no bad consequences will ever befall them, perhaps the image of this young man from my county–dressed in a prison jump suit during his sentencing to 5 years in the NJ State Penitentiary for vehicular homicide–might have a deterrent effect on your son:
Tell your son that you will be checking his cell phone when he comes back from driving, in order to determine if there were any texts or phone conversations that took place while he was driving. If you find that he was–as my friend, Judge Reed says–“driving blind”, then you have to protect him (and other people on the road) from injury, and protect your assets by taking strong disciplinary action in order to make him change his ways. And, with someone of this age, the only strong disciplinary action that is likely to work is to restrict his driving privileges in the event of violating your rules.