This November, Massachusetts voters will have the opportunity to vote on whether to legalize recreational marijuana. The proposal is raising a lot of peripheral questions, including how motor vehicle laws would be enforced when it comes to drivers who have smoked or ingested marijuana before getting behind the wheel. WGBH All Things Considered host Barbara Howard spoke about the looming issue with Dr. Kevin Hill, director of the Substance Abuse Consultation Service at Belmont's McLean Hospital.