US-FAA14 CFR 91.305
Flight test areas
Read the official ruleOfficial FAA regulation text (public domain)
No person may flight test an aircraft except over open water, or sparsely populated areas, having light air traffic.
Verbatim text of a US Federal Aviation Regulation (public domain), reproduced for reference — not legal advice and not for navigation. A plain-English explainer may be added later; always rely on the current official rule linked above.