Based on preliminary data provided by the highway departments of all 50 states and the District of Columbia, motorcyclist deaths in the United States appear to have decreased 7 percent in 2013, according to an analysis released Tuesday by the Governors Highway Safety Association, a nonprofit organization. But motorcyclist safety has not improved over all in the last 15 years, the group said.

Should the final analysis confirm the 2013 data, the association said, it would be the second year since 1997 that fatalities were lower than the year before. Cool, wet weather — not ideal for motorcycle riding — was most likely a factor in the projected decrease in rider deaths, the group noted.

Motorcycle fatalities in the United States more than doubled from 1997 to 2008. A “substantial” decrease in the number of motorcycle deaths for 2009 followed, according to the association’s report.

“We had hoped that was the beginning of a trend and the fatalities were about to keep going down, but they haven’t done so,” James Hedlund, the author of the report, said in a telephone interview. He is a former senior official from the National Highway Traffic Safety Administration.