We all know it is more difficult to drive at night for the simple reason you can't see as well as you can during the day.

But you may not know that the reduced vision associated with nighttime driving is playing a more prominent role these days in highway fatalities.

Overall, the roads are getting safer.

From 1999 to 2008, the number of people who died annually on the nation's highways fell 10.7 percent from 41,717 to 37,261. And the number of crashes in which someone died declined 8.4 percent from 37,140 fatal accidents in 1999 to 34,017.

But another trend is going in the opposite direction.

According to a study released Thursday by the Texas Transportation Institute, a growing proportion of fatal crashes is occurring after dark.

The trend is particularly pronounced among teen drivers.

Back in 1999, the proportion of fatal crashes caused by drivers age 20 and up that occurred at night was 38.6 percent for the nation as a whole and 42.2 percent in California. Nine years later, those percentages had increased to 41.5 percent and 45 percent, respectively.

When it came to fatal accidents caused by 16- to 19-year-olds, the proportion occurring at night increased from 45.1 percent nationwide in 1999 to 49.7 percent in 2008.

In California, the proportion of teen-triggered fatals that took place after dark had already topped 50 percent a decade ago. The proportion of teen-caused fatal accidents at night increased only slightly here, from 51.4 percent in 1999 to 51.6 percent in 2008, according to the study's data.

At any rate, nighttime driving conditions are increasingly becoming major factors in fatal crashes.

But why?

Visibility is one factor, the Texas Transportation Institute researchers say.

"The average person's field of vision is smaller without the aid of light, and glare from oncoming headlights can further limit the ability to see clearly and avoid hazards," their report states.

And the reduced visibility often proves deadly when mixed with alcohol. The study found that drivers age 20 and older who caused fatal accidents were under the influence 6 percent more often in 2008 than in 1999.

But, interestingly, the proportion of teen nighttime fatal crashes involving alcohol fell 1 percent.

So what gives?

The researchers say the driving factor behind the rise in deadly nighttime teen-caused crashes is the cell phone.

They say the trend correlates with a National Highway Traffic Safety Administration finding that the proportion of young drivers pressing phones to their ears doubled to 10 percent between 2002 and 2005.

And other research indicates teens are texting more than ever before.

View the report at http://t-driver.com/files/2010/05/Nighttime_Risk.pdf.

Call staff writer Dave Downey at 951-676-4315, ext. 2623.