Traffic is about to get a lot worse in Iowa’s largest city with the holiday crunch. Authorities are warning motorists to take it easy — or else. 14 law enforcement agencies in the Des Moines metro area are joining forces this week to broadcast a common message — relax, slow down and buckle up. Sergeant John Broderson of the Clive Police says Polk County commuters drove a total of three-and-a-half billion miles last year, with numbers that are increasing by 100-million miles every year.Broderson says the 20-percent of Iowans who don’t wear seat belts account for up to 65-percent of the state’s traffic deaths.