The number of drivers involved in fatal accidents from distracted driving rose 42 percent from 2005 to 2008, according to a new study of highway deaths.

But another way to read the study is that "inattentive" or distracted driving was recorded as a "primary" factor for just 7 percent of the 50,430 drivers involved in fatal accidents in 2008. Total road fatalities dropped 22 percent from 2005 to 2009. That's the fastest rate of decline in traffic deaths in peacetime since the beginning of automotive mass production in 1913, according to the Wall Street Journal.

The reasons for the decline in highway fatalities in the U.S. since 2005 could be better vehicle safety technology, less reckless behavior, smarter strategies for easing teens into the responsibilities of driving, or even an unexpected positive side effect of a slumping economy.

A new study by two University of Michigan researchers of detailed federal crash statistics from 2005 to 2008 suggests all these reasons could be behind the reduced death toll.

The federal highway fatality data analyzed by University of Michigan Transportation Research Institute researchers Michael Sivak and Brandon Schoettle suggest that talking and texting behind the wheel are a smaller problem than motorcycle deaths. Sivak and Schoettle found that in 2005, 2,369 fatal accidents were blamed on "inattentive" driving, including eating, talking or using a phone. By 2008, inattentive driving was blamed for 3,366 deadly crashes. But the number of fatalities involving motorcycles grew by 14% to 5,129 deaths in 2008 from 4,492 in 2005.

A Transportation Department spokeswoman said Dec. 14 that a broader definition of distraction-related fatalities grew to 16 percent of all traffic deaths in 2008 from 10 percent in 2005. The rate leveled off in 2009, around the time Transportation Secretary Ray LaHood's national campaign against distracted driving and enactment of tougher state laws on texting and phoning while driving.

The department has just launched a new effort called "No Refusal" that encourages police officers to call judges who will quickly authorize warrants to obtain blood samples from people who refuse a breath-alcohol test in an effort to avoid prosecution.

Technology deserves some credit for reducing deaths, according to the data. Deaths in side-impact crashes declined between 2005 and 2008 at a faster rate than the decline for deaths overall. That suggests that side airbags are helping more people survive crashes, the researchers found.

Fatalities and injuries have tended to drop during economic slowdowns, but when the economy recovers, they have not rebounded all the way to pre-recession levels, the Transportation Department says.

Fatal accidents during rush hours also declined more sharply than overall deaths.