According to preliminary statistics released Thursday by the National Highway Traffic Safety Administration, there were 1.6 deaths per 100 million vehicle miles traveled last year, a slight increase over 1.5 deaths in 1999. This was the first time in 24 years that the rate at which U.S. motorists and passengers died in traffic accidents. The increase represents a reversal of a decades-long trend in overall highway safety. The statistics could change when final results are compiled in May. In the meantime, researchers at the agency and highway safety advocates are searching for answers. Overall, 41,800 people died in traffic accidents in 2000, up from 41,611 traffic deaths in 1999. Some analysts blame a spike in the number of motorcycle-related deaths. Other experts cite an ongoing shift among U.S. consumers from cars to sport-utility vehicles and other light trucks, which are more prone to rollover accidents. Seat-belt use is a continuing concern. And some experts blame higher speed limits and an increase in unfavorable motorist behavior and driving habits. The biggest increase in traffic fatalities in 2000 came among motorcycle riders. There were 2,680 motorcycle deaths in 2000, up from 2,472 in 1999, an 8 percent increase and 27 percent spike since 1997. The number of deaths involving SUVs, vans and pickup trucks increased 1.7 percent last year to 11,439. Alcohol use remained the largest single factor associated with traffic deaths. There were 16,068 alcohol-related deaths in 2000 -- or 38 percent of all traffic deaths.