Cellphone distraction causes car crashes but it isn't clear what effect state laws are having on distracted driving, a U.S. study says.

A study by Temple University researchers published in the American Journal of Preventive Medicine is the first comprehensive collection and coding of state laws attempting to address the public health risk posed by distracted driving, a university release said Wednesday.

The researchers analyzed distracted driving laws passed between 1992 and 2010 and found large variations in laws from state to state based on type of mobile device involved, such as cellphones, laptops and tablet computers, the categories of drivers by age or by driving permit type.

Enforcement and penalties also varied from state to state, the researchers found.

As of November 2010, 39 states and the District of Columbia had one or more laws restricting use of mobile devices while driving, 11 states had no laws and no state outlawed the use of cellphones completely.

Researchers say comparing legislation from state to state could identify what provisions within a given law make it particularly effective.

"We know that distracted driving is dangerous, yet despite the diffusion of distracted driving laws, there is evidence that driver use of mobile devices is increasing," said Jennifer Ibrahim, an assistant professor of public health in the College of Health Professions and Social Work. "Our study is the first step toward understanding which laws really do reduce distracted driving, and thus can reduce related crashes and associated injuries and fatalities."