Here’s a dispatch from the hazy frontier of law enforcement: On an afternoon in June, Joe Golec, a 32-year-old cop from Northampton, Mass., was making a mock DUI evaluation in a sunny classroom at Salem State University with a group of other cops from around New England. Dressed in khakis and a T-shirt, he introduced himself to Kristina, a 39-year-old waitress dressed in jeans and a white top, one of several volunteers who had spent the afternoon drinking so that Golec and the other cops could practice their sobriety-test skills. Golec’s first task was to take Kristina’s vital signs.