Six years ago, California began enforcing a ban on the use of cell phones behind the wheel. In the latest edition of the journal Transportation Research, a study found that issuing tens of thousands of $170 tickets each year for this new offense failed to yield measurable safety benefits.

"Our main result was that we found no evidence that the California cellphone ban decreased accidents," Colorado University economics Professor Daniel T. Kaffine, one of the lead autors of the study, said in a statement. "This is surprising, because a lot of prior studies had shown that people who talk on cell phones, while driving, are just as impaired as people who are intoxicated."

Along with Colorado School of Mines mathematician Bob Yu and Rand Corporation analyst Nicholas E. Burger, Kaffine looked at the six months from January 1 to June 30, 2008 as the "before" period and July 1 to December 31, 2008 as the "after period" to avoid overlap with a ban on text messaging that took effect on January 1, 2009.

The researchers looked at the average daily number of collisions, verifying that other factors such as the number of miles traveled, rainfall and gas prices did not affect the numbers. The final figures also accounted for holidays, as the numbers show accidents fell 15 percent because people drive less on those days. No matter how the numbers were analyzed, the results did not change.

"When we go to the data we just didn't see any evidence that accidents actually declined in the six months after this ban that was put in place," Kaffine explained.

The study did not attempt to investigate why the law failed to produce results, beyond suggesting a few possibilities for further research.

"Even if drivers fully complied with the law, it is possible that accidents would not decrease," the study found. "First, if hands-free cell phone use is as distracting as hand-held use, substitution from one method to the other may leave the number of accidents unchanged. Or, risk may vary across drivers, and drivers who use cell phones may simply be inherently more careless and accident prone."

The researchers believe legislators in Sacramento and elsewhere can learn from the outcome of California's cell phone law.

"One of the punch lines, I think, for policy makers in how we might think about these results is that simply passing a ban on something doesn't always get you the intended results," Kaffine said.