It starts with a smell.

A police officer approaches the window of a car during a motor vehicle stop and, as soon as the windows are rolled down, knows there's marijuana in the car. Maybe it's raw cannabis, an ounce of bud tucked somewhere in the vehicle. Maybe it's the odor of burnt marijuana, from a joint that was just extinguished.

In any case, 99 percent of marijuana busts in New Jersey start with an officer reporting the smell of marijuana, Freehold criminal defense attorney Richard Lomurro said. The smell can lead to a search, often the precursor to the nearly 33,000 annual marijuana arrests in the state, according to the FBI's Uniform Crime Report.

"They have a golden ticket for a free search of your car," Lomurro said. "The smell of marijuana has been used as a tool to support police investigation for a long time."

That would change under the New Jersey marijuana legalization bill pending in the Legislature. In addition to legalizing, taxing and regulating marijuana possession and use for recreational purposes, the bill would officially remove the smell of marijuana — or sight, in most cases — as probable cause for police to search a vehicle.

Instead, officers would be required to assume that the driver is using marijuana in a legal way unless they can provide evidence proving otherwise. Unless it's on prison or school property, the odor of cannabis will no longer "constitute reasonable articulable suspicion of a crime."

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According to marijuana legalization advocates, the change is seen as key to addressing a social justice justification for legalizing weed — by narrowing the circumstances where law enforcement could legally invade the space of a motorist and their property.

The change is seen as particularly benefiting African American users, who are three times as likely to get arrested on marijuana charges as their white counterparts, despite similar usage rates. Gov. Phil Murphy has said that legalizing weed would be a "monumental step" toward bridging racial disparities in those arrests.

While the change has caused controversy in states that already have legalized weed, law enforcement has not been left toothless.

Police officers could still conduct a search if they have clear evidence that the motorist has more than the legal limit — 1 ounce for recreational users or 3 ounces for a medical marijuana patient, based on pending measures — such as a garbage bag filled with marijuana.

And the bill states that marijuana would be subject to alcohol-like "open container" laws. Users would be subject to a $200 fine if an officer sees an unsealed container of marijuana in the vehicle. The New Jersey legal weed bill outlaws the consumption of marijuana in any form while driving.

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The bill's probable cause language states that police must presume that any cannabis observed in a vehicle is being used in a legal manner. That includes so-called exigent circumstances, such as if the marijuana is found in proximity to cash.

“The smell has been indicative of potential illegal activity. But we can't jump to that conclusion" under the proposed New Jersey marijuana legalization law, Lomurro said. "They have to give you the benefit of the doubt that you’re using it legally. And they can’t jump to the conclusion, which they’ve jumped to for years, that cash plus weed equals drug dealer.”

In the 10 states with legal marijuana, all but Michigan and Nevada have determined that the odor of marijuana isn't enough to search a vehicle.

Nonetheless, legalizing weed has opened up a can of legal worms in the everyday interaction between users and police, and complex questions about probable cause is often one of them — leading to numerous lawsuits argued all the way to state Supreme Courts.

Language in the New Jersey bill would let warrantless searches continue under certain circumstances. The bill — which still hasn't received a vote in the Legislature, as it doesn't have enough votes to pass the Senate — states that marijuana odor is still enough probable cause to search a vehicle if the officer is investigating whether the motorist is driving while intoxicated by or under the influence of marijuana.

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Wayne Blanchard, New Jersey State Troopers Fraternal Association president, said the smell of burnt marijuana is enough to launch a DUI or DWI investigation — and conduct a search.

“The odor of burnt marijuana is very distinct. If I smell the odor of raw marijuana, I might not conduct field sobriety tests on that driver,” Blanchard said. “But if I pull the car over with that distinct odor of burnt marijuana? I’d assume they recently smoked it.

"If that individual passed a field sobriety test, I don’t think you’re searching the car. You’re not conducting an arrest,” Blanchard said. “But there’s a lot of uncertainty – and that’s what’s concerning.”

Lomurro interpreted the bill to mean that the odor of marijuana is an additional proof factor — similar to slurred speech by someone suspected of drunken driving — "as opposed to right now, when the smell alone can lead to a search and an arrestable offense."

“This bill changes everything,” Lomurro said. “They’ll have to ground their suspicion on more reliable information.”

Mike Davis writes about the seemingly never-ending push to legalize marijuana in New Jersey, including the effects it would have on the economy, the black market and regular people. He's also a drone pilot for the USA TODAY NETWORK, co-hosts the Jersey Storytellers Project and in a lifelong love-hate relationship with the New York Mets. Contact him at 732-643-4223, mdavis@gannettnj.com or @byMikeDavis on Twitter.