Using a handheld mobile phone while driving will become illegal in the state of California from midnight on 1 July.
Drivers must use a hands-free device while calling, and those breaking the law will face a $20 fine rising to $50 for a second offence.
The California Highway Patrol will be enforcing the law rigorously over the forthcoming Fourth of July holiday.
"The simple fact is that it is dangerous to talk on your cellphone while driving. Data shows that cellphones are the number one cause of distracted-driving accidents," said state governor Arnold Schwarzenegger.
"So getting people's hands off their phones and onto their steering wheels is going to make a big difference in road safety. The Hands-Free cellphone bill will save lives by making our roads safer."
Stores selling Bluetooth headsets have reported a sharp increase in sales over the weekend, and some companies, such as the Walter Clark Legal Group, are providing headsets to staff and local townsfolk free of charge.
"With the new law taking effect, many people are forced to purchase hands-free devices if they want to continue using their cellphones while driving," said attorney Walter Clark.
"We want to help local residents comply with the law, and alleviate the costs of purchasing new headsets while promoting driver safety."
Another firm is getting publicity from the new law by giving away headsets, but to drivers who break the law.
Online retailer Headsets.com is promising a free Bluetooth headset while stocks last to drivers who can show they have fallen foul of the legislation.
"Frankly, we do not think driving while talking even on a headset is as safe as putting the phone down and focusing on your driving," said the company. "But if you are going to talk on your phone, we want you to be safe."
