They don’t stop speeding just like harsher penalties don’t stop most kinds of crime

If you want people to slow down, you paint the lanes smaller, make them narrow regularly, add roundabouts and plastic bollards between the lanes - you have to actually address the problem, which is the speed people feel natural driving at on any given road

The ones you can go after are the people weaving between traffic and racing, but I doubt a letter in the mail is going to do much