You mean there are people who wouldn't know that 75/mph is not good in a residential area?
That is widely known, although some will flout it.
Have you ever driven any long distances?
I have.
Speed limits vary across state lines, around curves, in construction zones, & thru towns.
Without the signs, one wouldn't know what's legal.
If one wants the 10 Commandments posted because of the one about murder, there are some problems with this argument....
- It wouldn't tell anyone anything they didn't already know about murder.
- It wouldn't inhibit anyone from murder.
- The 10 Commandments contains silly prescriptions for non-Christians.
- The 10 Commandments falls far short in listing important moral things,
eg, support yourself, pay your rent, don't abuse others, don't oppress others.