You can walk around Chicago and Seattle with marijuana in your pocket and not a care in the world.

But if you fly nonstop from O’Hare to Sea-Tac, you should probably think twice about what you’re bringing with you.

Twelve U.S. states and the District of Columbia have legalized recreational pot since 2012, when Colorado and Washington became the first two states to make the move. (Pennsylvania is not among them.) And most people understand that if you’re crossing state lines with weed in your car, you’re asking for trouble.

But what if you’re flying between two marijuana-friendly states and not setting foot inside one that still bans pot sales and use?

Airports sit at the nexus of contradictory state and federal marijuana laws, creating a potentially confusing situation for travelers – and law enforcement.

“There are all kinds of mixed messages being sent, but that’s because we have mixed messages in the legal system,” David Bannard, an attorney who consults with airports on marijuana and other regulatory issues, told Forbes.

U.S. airspace is regulated by the federal government, which still considers marijuana to be an illegal drug. If you get caught with weed as you go through airport security screening, Transportation Security Administration officers will turn it – and you – over to local law enforcement.

But if that happens at Portland International or Detroit Metropolitan Wayne County airports, for example, police are unlikely to do anything other than send you on your way (provided you’re carrying the legally allowed amount) because Oregon and Michigan have legalized marijuana.

In Portland for instance, if the travelers is of legal age and carrying a legal amount, and has a boarding pass for a destination within Oregon, the traveler will be allowed to proceed through the screening process. If the traveler is of legal age carrying a legal amount to a destination outside Oregon, the traveler will be asked to return the marijuana to their vehicle or leave it with a friend or family member, airport officials said.

“Additionally, if asked, we advise travelers to check with their airlines for the air carrier’s policy related to traveling with marijuana,” said Portland airport spokeswoman Kama Simonds.

Some airports in states with legalized pot are employing an inventive solution: the marijuana amnesty box.

Amnesty boxes are secure, monitored containers that allow travelers to dispose of their marijuana before boarding commercial flights, when they would technically be breaking federal drug laws. That risk doubles when passengers fly to a state where pot is still illegal. The moment they step off the plane, they are committing a criminal act.

The Chicago Department of Aviation, which operates O’Hare International and Midway International airports, is the most recent agency to turn to amnesty boxes, which were placed in terminals on Jan. 1, when Illinois officially legalized recreational marijuana. City police empty the boxes and file reports on the contents.

The boxes sit on the airside of security, after travelers have passed through screening.

“We’re not encouraging people to bring cannabis through the airports at all,” Maggie Huynh, a spokeswoman for the Chicago Police Department, told the Chicago Tribune. “But if for some reason you have it on you, we have those amnesty boxes out there so that you can dispose of it prior to getting on the airplane.”

Colorado legalized recreational pot in 2012, and the Colorado Springs Airport installed amnesty boxes in 2013. But Denver International Airport, less than two hours away, has not, because it forbids marijuana to be carried on airport property.

“It is federally illegal to transport marijuana in a plane,” spokeswoman Emily Williams told Denver’s KMGH-TV last year. “So, you’re coming out here to the airport, you’re flying in, you’re flying out, there’s really no reason for you to have [marijuana] here at the airport.”

Weed is also forbidden at McCarran International Airport in Las Vegas even though Nevada decriminalized marijuana in 2016. Still, the airport installed 20 amnesty boxes in 2018.

Other major airports in marijuana-friendly states, such as California’s LAX, have not installed amnesty boxes for a variety of reasons, officials told Blue Sky. Some must abide by state laws that would make the containers difficult to install. Others are leery of the boxes becoming hiding places for explosive devices.

Elsewhere in North America, the Ottawa International Airport Authority installed amnesty boxes following the legalization of marijuana in all of Canada in 2018. The boxes serve as a reminder for passengers that there are limits for traveling with cannabis, even domestically, and crossing an international border with any marijuana products is prohibited, said OIAA spokeswoman Krista Kealey.

Kealey said the airport authority, along with Transport Canada, used social media to share information, rules and tips for travelers. “As a result, our passengers paid attention and we have had some deposits in the cannabis bins, but nothing major.”