Laws and regulations can vary between different jurisdictions, so it's essential to consult local traffic laws and legal authorities for the most accurate and up-to-date information.
In general, law enforcement agencies need to adhere to legal procedures and requirements when enforcing traffic laws. Intentionally posting fake speed limit signs would likely be considered deceptive and unethical in most places. Additionally, such practices may undermine public trust in law enforcement and could potentially result in legal challenges to the enforcement action.
If you have concerns about speed limit enforcement or related practices in your area, it's best to contact your local law enforcement agency or a legal professional for guidance and clarification.