Bommer said the 3.58 percent figure represents the number of drivers with cellphones in hand at any one time.
"That's the percentage of people who are currently doing it," Bommer said.
The higher numbers from driver surveys reflect "whether they have done it across a long period of time," he said. "If you texted twice last month, you would show up in the survey number. But unless you happened to be texting while you were being observed by a roadside observer, you would not show up in the lower number."
So: The new study is suggesting that statewide, about one in 28 of your fellow motorists are texting or talking on their hand-held phones right this very minute. If you count hands-free devices, the ratio is about one in 13.
The survey found San Bernardino County drivers had the highest rate of hand-held cellphone use -- 8.2 percent. In second place: drivers in Contra Costa County, at 6.3 percent. Among other Bay Area counties surveyed: Alameda, 2.66 percent; Sonoma, 2.35 percent; Solano, 2.13 percent; and San Mateo, 1.05 percent.
Still, the study finds preliminary signs of a decline in the statewide percentage of drivers observed using phones behind the wheel.
Bommer said the best basis for comparison with previous years was the report's estimate of cellphone use including hands-free devices. The report concludes that accounting for both hand-held and hands-free phones, 7.64 percent of drivers used the devices at any given time in 2017, compared with 12.8 percent in 2016, 9.2 percent in 2015 and 6.6 percent in 2014.
"I do think there's been some impact of public awareness campaigns, et cetera, that are making people more aware of not actively texting and driving," Bommer said. "You see those numbers down with fines and enforcement up."
But Bommer added that more data are needed to understand whether the 2017 numbers really signal a downward trend. "Overall, it will take looking at this a couple more years to really see the impact," he said.
In part, that's because of a shift in the methodology used in the 2017 survey.
Earlier surveys targeted streets and highways Caltrans identified as the busiest in the state. The 2017 study, using federal guidelines, focused on counties that had the highest number of fatalities and used a variety of roads in each county -- not necessarily just the busiest ones.
"It's more shifted toward a real representation of the whole state and away from just picking some selected cities we're going to look at," Bommer said. "The new sample has to be reflective of your entire state, and it didn't use to have to be."
All use of hand-held cellphones while driving was outlawed by a 2016 law sponsored by East Bay Assemblyman Bill Quirk. Citations carry a minimum fine and fees of $162.