Jennifer Ryan, AAA’s director of state relations, cautioned in a phone interview on Thursday that, “hands free is not risk free.”

Ms. Ryan said the California measure matched a larger trend of states bringing legislation up-to-date with contemporary phones. But while the state’s provisions were particularly broad, she said, it was important for motorists to take responsibility for themselves and pay attention.

The law goes beyond what federal authorities recommend to prevent distracted driving, which remains a significant cause of traffic fatalities. The latest statistics from the National Highway Traffic Safety Administration show that more than 3,400 people died in 2015 in crashes that involved a distracted driver. Of those, 272 were teenagers.

And while the use of smartphones is certainly part of the problem, many are unconvinced that keeping them out of drivers’ hands will be a panacea.

Some have speculated that the California law may encourage enthusiasm for self-driving cars. When Gov. Jerry Brown signed the measure into law in September, he approved of another bill that allowed for the first tests of autonomous driving vehicles on public roads.