About as enforceable as the a sign posted on Lake Michigan that no waves allowed on shore.
Wisconsin Statute 348.10 (2) states:
No person shall operate a vehicle on a highway unless such vehicle is so constructed and loaded as to prevent its contents from dropping, sifting, leaking or otherwise escaping therefrom.
Additionally in (5) of that statute, it states that loads shall be “distributed in a manner that will prevent side sway under all conditions of operation.”
The Dept of Transportation advises this means that a load in a vehicle or trailer must be:
Tie it down with rope, netting, or straps
Tie large objects directly to your vehicle or trailer
Cover the entire load with a sturdy tarp or netting
Don't overload the vehicle or trailer
Always double-check your load to make sure it's secure.
Secure your load, https://wisconsindot.gov/Pages/safety/education/SecureYourLoad.aspx
At least under that statute you could get up to a $200.00 forfeiture. It can be argued by law enforcement that failure to follow this statute resulting in great property damage, bodily harm or death is criminally reckless or negligent conduct and seek to impose incarceration and a fine.