SAN FRANCISCO - On July 1, California will become the largest state to ban unlimited cellphone use by drivers. The law prohibits drivers under 18 from talking on the phone, and it requires older drivers to use a hands-free headset.

A similar law goes into effect the same day in Washington state. Legislation has already been passed in New York, Washington, D.C., and elsewhere - and more laws are pending. What's a chatty driver to do? USA TODAY reporter Michelle Kessler asked insurance and driving experts to explain the hodgepodge of new restrictions, and what they mean for car- and cellphone-loving Americans:

Q: Is it legal to drive while talking on a cellphone?

A: It depends on where you are. In the USA, states - and some cities - set cellphone driving laws. They vary widely.

Idaho permits all cellphone use. Washington, D.C., prohibits learner's permit holders and school bus drivers from talking on the phone at all - and requires other drivers to use a headset or other hands-free device.

Laws also vary outside the USA. Germany and Australia are among countries that restrict cellphone use in cars.

Q: Just how dangerous is driving while talking on a cellphone?

A: The risk of having an "injury crash" increases fourfold when a driver is on the phone, says Anne McCartt, a vice president with the Insurance Institute for Highway Safety, a research group funded by auto insurers.

Q: What makes talking on a cellphone more dangerous than talking to a passenger in the car, or listening to the radio?

A: Researchers are still investigating precise brain functions of cellphone-chatting drivers. Early evidence suggests the problem is "cognitive distraction from the conversation," McCartt says.

A driver and a passenger who are having a conversation can both see the road. The conversation will likely stop if something unexpected happens.

But if a driver is having a conversation over a cellphone, the person at the other end of the line can't see the road - so he or she keeps talking when something dangerous occurs. That prevents the driver from completely focusing, McCartt says.

Talk radio doesn't cause the same problem because it's passive, she says. Drivers just listen and aren't expected to respond. That frees up more brain power for focusing on the road.

Q: Is using a hands-free headset safer?

A: Surprisingly, no, says Jonathan Adkins, a spokesman for the Governors Highway Safety Association, a trade group representing state highway safety departments. Studies have shown that drivers using headsets are just as distracted, he says. Although hands-free laws sound good on paper, "We think it's best for states not to pass more legislation," he says. "Let's focus on the research and the data. We need a good highway (cellphone) safety law, and we don't have that yet."

Q: What about text messaging or dialing when driving?

A: Research on texting and dialing is still in the early stage. In general, "it's dangerous" to do anything distracting while driving, Adkins says.

Q: Are cellphone driving laws effective?

A: It depends on the law. Handheld cellphone use among drivers fell 50% in Washington, D.C., after it required drivers to use hands-free devices, an Insurance Institute study says. But a similar New York law had little impact. And cellphone use among teenage drivers in North Carolina went up after the state prohibited it, McCartt says.

"The law has to be enforced, and the enforcement has to be publicized," McCartt says.

Q: How will travelers find out what the cellphone law is in a particular area?

A: They probably won't. Although new laws get lots of publicity, it usually fades over time. And travelers are likely to miss the latest news.

California has launched a media blitz about its new law, for example. But it hasn't specifically asked rental car agencies to warn out-of-state renters, says Jan Mendoza, spokeswoman for the California Department of Motor Vehicles.

To play it safe, stay off the phone no matter where you are.

Q: Can someone get in trouble for talking on a cellphone in a state that doesn't specifically prohibit it?

A: Yes. Most states have reckless driving laws that allow police to pull over anyone doing anything that they think is dangerous. If your conversation is causing you to drive erratically, you can get a ticket, Mendoza says.

Q: How much are the fines?

A: It varies. In California, the base fine ranges from $20 to $50 - and it can triple if certain penalties are applied. In Washington, fines start at $124.

Other restrictions may apply. The Washington law does not allow police to pull over drivers strictly for cellphone violations.

They must be pulled over for another infraction, but can then be hit with a secondary cellphone offense.

Q: How do driving laws affect the cellphone industry?

A: They cause sales of headsets and other hands-free systems to go up. "We definitely see an impact in a positive way," says Larry Rougas, a vice president at Pioneer.

The electronics-maker builds hands-free systems for cars that range from about $150 to more than $1,000. Sales rose after New York passed cellphone driving restrictions, and the company expects a similar boost in California. In response, Pioneer is broadening its product offerings.

"As more and more states are coming on board, we're starting to offer systems at a higher price point," Rougas says.

Q: Are more cellphone driving laws expected to be passed?

A: Probably. Hawaii and Massachusetts are among the states considering new restrictions.

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