As the push to legalize marijuana gains momentum, so is evidence that more permissive policies on the drug are putting motorists at risk. The Insurance Institute for Highway Safety found, in a study to be released on Thursday, that traffic crashes are rising in states that have legalized recreational marijuana. That followed stark warnings from the National Transportation Safety Board, which on Tuesday issued several recommendations to combat drug-impaired driving. “The last thing in the world that we want is to introduce another legal substance where we may be adding to that toll and to the carnage on our highways,” David Harkey, president of the Insurance Institute, said.