One out of five Fortune 500 companies that responded to a recent National Safety Council survey has a total ban on cell phone use while driving that covers all employees.

More than half of these policies were implemented since 2008, the survey indicates.

Two years ago, the National Safety Council urged motorists to stop using cell phones and messaging devices while driving.

"In January 2009, NSC called for a ban on all cell phone use while driving because research identified the behavior as dangerous. A driver is four times as likely to crash while talking on a cell phone while driving," said Janet Froetscher, president and CEO of the Council. "Now, in 2011, our call to action is getting results, and our nation's top employers are taking steps to protect their employees and communities in which they operate by implementing total cell phone bans."

Preliminary survey results indicate productivity either increased or remained at the same level for 40 percent of companies with total bans. (About half of the companies with total bans implemented their policies recently and do not yet know if productivity has been affected.)

NSC estimates at least 23 percent of all motor vehicle crashes each year involve cell phone use, and more than 50 research studies show talking or texting on a cell phone while driving is a dangerous behavior.

However, there are also studies that show for commercial truck drivers, hands-free use of a phone may not pose the danger that hand-held versions pose. Late last year, the Federal Motor Carrier Safety Administration proposed a ban on the use of hand-held cell phones for truckers behind the wheel, but said it was "not clear if simply talking on a mobile telephone presents a significant risk."