You're assuming a lot there. Mainly that the motorist will clearly see and understand the numbers, then act in a reasonable manner.
I did say "if", so I'm really not assuming that it will make people act in a reasonable manner. The jacket does have "My speed" written above the numbers, which should make it clear what the numbers are.
I did say "if", so I'm really not assuming that it will make people act in a reasonable manner. The jacket does have "My speed" written above the numbers, which should make it clear what the numbers are.