Since federal regulators and the trucking industry got serious about safety nearly four decades ago, thousands of lives have been saved on U.S. roads. But last month’s horrific high-speed crash west of Denver, in which an apparently out-of-control semitrailer plowed into more than two dozen stopped vehicles on Interstate 70 — igniting an inferno and killing four people — is part of a worrying trend. Over the last decade, fatal crashes involving large trucks have been on the rise again. The number of fatal crashes involving large trucks in the United States increased by 42 percent between 2009 and 2017, according to National Highway Traffic Safety Administration data, a trend reversal that came as trucking traffic levels recovered from the Great Recession. Nearly 4,800 people were killed in 4,237 wrecks in 2017 — most of them in neighboring vehicles.