HRS §291C-85
When you must use turn signals instead of hand signals
This law says you can use hand signals or signal lamps for turns and stops, but some vehicles must use signal lamps. If your vehicle is too wide or too long, you must use signal lamps. The law gives specific measurements for when lamps are required.
The statute, as written — Signals by hand and arm or signal lamps
(a) Any stop or turn signal when required herein shall be given either by means of the hand and arm or by signal lamps, except as otherwise provided in subsection (b). (b) Any motor vehicle in use on a highway shall be equipped with, and required signal shall be given by, signal lamps when the distance from the center of the top of the steering post to the left outside limit of the body, cab, or load of such motor vehicle exceeds twenty-four inches, or when the distance from the center of the top of the steering post to the rear limit of the body or load thereof exceeds fourteen feet. The latter measurement shall apply to any single vehicle, also to any combination of vehicles.
LawTrove is not legal advice. The summary above is a computer-generated restatement — the authoritative text is the official version linked above.