PHILADELPHIA — When New Jersey lawmakers debated earlier this year whether to legalize recreational use of marijuana, the Garden State’s police organizations were adamantly against it. The cops said legal weed might lead to an explosion in the numbers of impaired drivers operating under the influence. And the police would be caught flatfooted trying to tell whether drivers they pulled over were high. “With alcohol, if you have over 0.08% in your blood, there’s the presumption that you’re intoxicated,” said Christopher Leusner, head of the New Jersey State Association of Chiefs of Police. But because marijuana stays in the bloodstream...