Federal floor — Title II of the ADA
Title II of the Americans with Disabilities Act requires public entities (including cities) to make reasonable modifications for disabled people. Refusing to accept a valid placard, or citing a disabled-marked vehicle in a properly designated accessible bay, can violate Title II in addition to state vehicle codes.
State placard rules
Each state issues its own disabled placard (and reciprocates other states' placards). A valid placard typically exempts the vehicle from meter time limits, residential permit restrictions, and most timed zones — but not from no-parking zones, fire-hydrant zones, or red-curb zones. Check your state's reciprocity and exemption list.
How to evidence the defence
Submit a photo of the placard showing the placard number, the expiry date, and the placard displayed from the rear-view mirror at the time of the citation (where possible). Attach a copy of the state-issued certificate or the placard receipt as proof of authorisation.