At a recent sobriety check point in San Francisco, police looked for valid driver's licenses and made sure people weren't driving drunk.
San Francisco police Sgt. John Bragagnolo estimates one in four drivers they stop for driving under the influence is driving high on pot.
"Marijuana has become just as strong, if not stronger of a problem with driving as alcohol," said Bragagnolo.
He worries it will only get worse if Proposition 64 passes. That's the statewide ballot initiative to legalize the recreational use of cannabis in California.
"Thirty percent of our fatal accidents the driver has some sort of impairment from either alcohol or drugs," said Bragagnolo.
Oakland based Hound Labs may have the tool police need.
Mike Lynn, M.D., is the company's co-founder. "We are a marijuana breathalyzer company," said Lynn.
The company invented a device that measures THC, the psychoactive ingredient in cannabis that makes people high.
Determining when someone is too stoned to drive is currently a huge challenge for law enforcement.
"There is a smell test. You can smell the burnt marijuana in a car, but also you look at the impairment of the driver, they can have bloodshot, watery eyes, they could have the droopy eyelids, those are symptoms that we look for at which point we will have us go further," said Bragagnolo.
If they suspect someone is stoned, specially trained officers perform a more advanced field sobriety test. If the suspect is arrested, a blood test will confirm just how much of a drug is in a suspect's system.
The process is time consuming. "We've created the first marijuana breathalyzer that has ever been used at the roadside on actual suspected impaired drivers," Lynn said.
Lynn is also an emergency room doctor and reserve police officer.
He says drivers can breathe into his device, and it will tell you just how much THC someone has smoked or eaten.
"It only picks up people who have smoked in the last two to three hours, you can smoke all day long, all week long, but if you haven't smoked for a few hours, it's not going to be in your breath anymore, and you are probably not impaired at that point," Lynn said.
Police officers say the need for such a device would help them tremendously.
Since Colorado legalized marijuana in 2012, the state has seen an increase in the number of people driving stoned. Marijuana was a contributing factor in 59 traffic fatalities in Colorado in 2015.
Bragagnolo said every day without a device like Hound Labs marijuana breathalyzer could prove to be fatal.
"Fatal for you, and fatal for someone else," he said.
Colorado currently uses blood tests to measure THC content. Lynn says they are working on establishing a standard for testing and hope to get their product in the hands of police soon.
Written and produced by Ken Miguel.