Everyone riding in an automobile in Hawaii must now use a seat belt, under one of two traffic safety bills Gov. Neil Abercrombie signed into law Monday.

The second new law bans drivers from holding cellphones and other electronic devices, in effect replacing various county ordinances.

Abercrombie said of the seat belt law, "This measure closes the gap in protecting all passengers riding in a motor vehicle."

Previous law required occupants in front seats and minors in back seats to use seat belts or child safety seats. Adults in the back seat had the option of using seat belts. The amended law means everyone must buckle up.

The new law took effect Monday. Though police sometimes break in a new law by providing a grace period — with warnings but no citations — officials on Monday said there may be none this time.

Honolulu police spokeswoman Michelle Yu said there is little chance of a grace period for the seat belt law because of the widespread availability of information.

Caroline Sluyter, state Department of Transportation spokeswoman, said there have been extensive public outreach campaigns for Senate Bill 4 in radio and television ads, posters in public schools and tourism industry and rental car notifications.

Passengers in back seats who don’t wear seat belts are three times more likely to suffer serious or fatal injuries than those using seat belts, state Health Director Loretta Fuddy said at the news conference in the governor’s office.

David Manning of the National Highway Traffic Safety Administration said although data show occupants are 47 percent more likely to survive a crash by wearing a seat belt, only 19 states, including Hawaii, require all vehicle occupants to use seat belts. Manning said traffic accidents are the No. 1 cause of death for Americans between the ages of 2 and 34.

Honolulu police Capt. Darren Izumo said buckling up, or not, can affect the effectiveness of a car’s airbag system.

"You have to understand that in a modern vehicle, your computer detects all that," Izumo said. "If it thinks you’re wearing the belt, it deploys the airbags based on that or (doesn’t) deploy the airbags based on that. You try to trick the system, bad things will happen."

House Bill 980, which is effective July 1, bans the use of handheld electronic devices, such as cellphones, while driving.

Adults may use hands-free devices such as Bluetooth headsets, but drivers under 18 may not.

"While all counties have some form of a distracted driving ordinance in place, this measure establishes a state law that creates consistent requirements across all counties for the use of mobile electronic devices while driving and will simplify enforcement," Abercrombie said in a news release.

Nearly one-third of all Hawaii traffic collisions in 2007 were caused by distracted driving, according to a state transportation department study.

Using a cellphone while driving puts drivers at four times the risk of an accident with injuries, Transportation Deputy Director Jadene Urasaki said.

"In 2011, the national annual statistics showed approximately 3,300 people (were) killed in distracted-driving crashes," Urasaki said. "To put that into a local perspective, that’s all the people that can be seated at the Neal Blaisdell Concert Hall, plus 1,000 more."

The law carves out a few exceptions, such as allowing people to dial 911. Medical responders, work-related users of two-way radios and drivers with radio operator licenses are also exempt.

Violators caught using a cellphone in school zones and construction areas will face double the usual fine, which is $100 to $200 for a first offense.