About half a million people, or 3 percent of passenger vehicle drivers, are talking on a hand-held cell phone while behind the wheel at any given time on U.S. roads, according to a National Highway Traffic Safety Administration study released yesterday.The research was the first major study by NHTSA of cell phone use and driving. While the research did not address the relationship between cell phone use and accidents, a study earlier this year by the American Automobile Association (AAA) found that cell phones played a role in an estimated 1.5 percent of accidents.NHTSA watched more than 12,000 vehicles at 640 intersections in October and November 2000, recording cell phone use by drivers of passenger cars, vans, sport utility vehicles and pickup trucks.The research covered use of hand-held cell phones, not "hands free" sets that allow drivers to keep both hands firmly on the wheel.The highest rate of hand-held cell phone use observed, 8 percent, was by van and SUV drivers during non-rush hour periods.Use rates by drivers of all types of passenger vehicles were almost twice as high during non-rush hour than rush hour, according to the study.Women drivers used cell phones more frequently than male drivers. This was especially true for female van and SUV drivers, whose cell phone use rate of 6.1 percent was nearly twice that of male drivers, according to NHTSA.There was little difference in cell phone use by drivers between the ages of 16 to 24 and 24 to 69. However, the use by seniors, 70 and over, was 1.4 percent.While the study did not evaluate the role cell phones play in accidents, government data indicate that driver distraction is a contributing factor in 20 percent to 30 percent of all accidents.The controversial AAA study found that wireless phones were not among the top five contributing factors in auto accidents. From the more than 32,000 accidents analyzed, wireless phones contributed to 1.5 percent.However, highway safety and consumer groups suspect the devices play a larger role in crashes than AAA found because they believe there is an understandable reluctance among motorists to admit they were using them at the time of an accident.There are about 118 million mobile telephone subscribers in the United States, according to the Cellular Telecommunications & Internet Association, a leading industry lobbying group.