Slow Down. It's the Law.
We see signs like this everywhere we drive either fixed posts or lighted billboards. Whether it's Slow Down, Fasten Your Seat Belts, Move Over, Don't Litter or the multitude of other "It's the Law" commands.
It isn't only through driving where we are introduced to these commands. We are given this reason by government jurisdictions with many things we do daily. If you inquire from many official bodies why you need to be doing one thing or not doing another they will commonly reply "because it's the law".
I seem to remember this from my childhood except it wasn't the state or police telling me this, it was my mother or father. Except their reply was "because I said so". Of course when we are very young these responses, while many times a convenience for our parents, can be attributed to our lack of decision making capability as we mature and later on we tend to expect a little more information from our parents.
This particular insight doesn't seem to come from the government, though in announcing its authority to us masses, which I tend to see it as an insult to my critical thinking ability, it does project their attitude toward what they consider lesser beings; you and I.
Common sense laws are written to give information and warning about things that aren't either safe, tolerated, or acceptable to society in a general sense and in a specific sense indicates impinging upon another's rights. Or in the case of taxation to theoretically take from people resources for a determined collective good.
While we can all certainly debate on the merits of many laws and the result of their enforcement I am increasingly disturbed by the attitude of the government class that it feels the only explanation that must be given for enforcement is "because we said so".
In the case of your daily commute or other travels wouldn't it be more reasonable to have signs to enlighten us? Such as "Slow Down. Unsafe Conditions" or "Move Over for Officer Safety".
Decisions of what is correct cannot be imposed by laws, but if necessary fostered by them to bring enlightenment to our thinking process during our busy days.
Authoritarian presumption by government which first seems an annoyance to most of us is becoming a habit and eventually they won't just presume authority, they will assume it. So watch your complacency with what you might consider trivial items such as road signs.