According to the National Highway Traffic Safety Administration, after four straight decades of decline, there was a huge spike in traffic deaths last year. The cause? The NHTSA is attributing the spike in fatalities to mobile apps. After steady declines over the last four decades, highway fatalities last year recorded the largest annual percentage increase in 50 years. And the numbers so far this year are even worse. In the first six months of 2016, highway deaths jumped 10.4 percent, to 17,775, from the comparable period of 2015, according to the National Highway Traffic Safety Administration. “This is a crisis that needs to be addressed now,” Mark R. Rosekind, the head of the agency, said in an interview.