It's been on the books for some time that if you're in the front seat of a vehicle, you need to buckle up.
The law now includes those in the back seat.
Many drivers KHON2 spoke with had no idea of the new rules and that there's no warning or grace period.
Honolulu Police and transportation officials praise the move, but not everyone is on board.
"I didn't even know about the law, but I think it's a good one. I've always had my passengers put a seat belt on front seat or back seat," driver Gerald Chong said.
"For kids yeah, of course. But for parents, we need to grab stuff. If we're in the back we grab stuff, so I hardly have my seat belt on in the back," driver Mia Arroyo said.
Emergency vehicles including ambulances and police cars, as well as mass transit buses, are exempt from the law.
Those in taxi cabs and limousines are required to buckle up.
The fine is $92 for each violation. If a driver is caught with two passengers not buckled up, he or she will face double the fine, or a $184 ticket.
The state Department of Transportation is putting out messages this week on radio and TV to educate drivers.
They also sent out flyers to schools and rental car companies.
About 40 students got sick and 25 of them, along with two adults, were taken to the hospital. Officials are now taking a look at what they ate for lunch.
Honolulu police are investigating two armed robberies that may be connected.
An 18-year-old man was arrested in connection with the death of an elderly man in Chinatown.