The payback from state transportation investments will yield a sixfold payback: $42 billion in economic benefits from $7 billion invested. That's a key finding from a study released by the Washington Roundtable Tuesday morning, intended to encourage Washington state lawmakers to pass long-delayed transportation funding. "What we're trying to do is to restart some positive discussion in the next session," said Steve Mullin, president of the Washington Roundtable, an organization that represents…