Washington:An estimated 35,200 Americans died in traffic accidents in 2015, an increase of 7.7 percent from 2014, according to government data. Nine out of 10 regions within the US saw more traffic deaths in 2015, shows the preliminary data released Friday by the National Highway Traffic Safety Administration (NHTSA), reports Xinhua news agency. The NHTSA is analyzing the data to determine what factors contributed to the increase in traffic fatalities. Improved economy and falling gas prices were believed to be among major reasons for the increased traffic deaths in the US last year. However, NHTSA head Mark Rosekind said improved economy only explains part of the increase as 94 percent of car crashes were caused by human behaviuor or error.