The National Black Police Association wants UK forces and the government to monitor US cannabis legalisation.

Washington is due to follow Colorado in making sales of recreational marijuana legal to over-21s.

Campaigners and police in Seattle say they hope the legalisation will mean fewer young black men in the city are punished for minor drug offences.

The Liberal Democrats will speak to US politicians and police as part of a review of drug policy around the world.

A public vote to legalise the use and possession of cannabis was passed in Washington State in November 2012.

Alison Holcomb is a lawyer who led the campaign for legalisation.

She said: "We've not just changed policy in Washington State, we've blown open the debate nationally and internationally."

She said the changes are about more than just allowing people to get high.

"Really this campaign has only minimally been about marijuana," she said. "It's about how we use the criminal laws.

"In Washington, a black person is three times as likely to be arrested and convicted of a marijuana offence than a white person despite the fact that whites actually use marijuana at a slightly higher rate."

Kevin Siplin, 25, smokes up to 15 joints a day and claims he is rarely stopped by officers when he is with his white friends but says it's a different story when he's on his own.

"If I'm not with a white face, I know if they smell any of my weed I'm getting stopped and I'm getting a ticket," he said. "I'm just hoping that these changes coming in mean I'm left alone to smoke without any hassle."

The Seattle Police Department says arresting adults for possession of cannabis has been its lowest priority for the past decade.

However, it hopes the changes will improve relations with young black men in the city.

I'm just hoping that these changes coming in mean I'm left alone to smoke without any hassle Kevin Siplin

Sean Whitcomb, the department's head of public affairs, said: "With the passage of this new law we truly do believe fewer young black men will have enforcement related contact in regards to marijuana."

Nick Glynn is the Vice President of the National Black Police Association and also a police inspector with Leicestershire Police.

He said he believes the UK could learn important lessons from monitoring how legalisation works in cities like Seattle.

"We've had our current approach to drug laws for 20 years. If we can learn anything from the US I think we should to see whether we can get some better outcomes," he said.

"There about a million stop and searches carried out in England and Wales every year. Around half of those are focused on street possession of cannabis so there's a lot of time spent dealing with that very low level offence."

He added: "In the US they've done it in separate areas instead of across the whole country so maybe that's something we can replicate here."

The Liberal Democrats told Newsbeat: "Drug policy should be based on evidence of what works, not guesswork or dogma.

"That's why Liberal Democrats in Government are leading a Home Office review looking at drugs policy around the world.

"This includes looking at what is happening in Washington State and Colorado, including speaking to politicians, police and campaigners about the impact of these changes."

The legalisation in Washington means adults over the age of 21 will be able to carry up to an ounce of cannabis but they still won't be allowed to use it in public.

Medical marijuana was already legal in Washington State but it had no formal rules of its supply and distribution.

For the past few months authorities have been granting licences to those wishing to grow and sell the drug.

Shy Sadis runs the Starbuds dispensary chain in Seattle and said: "We're just going to check IDs and give people what they want.

"We're here to educate them. A customer is going to be dealing with a bud tender who is going to give them the best knowledge of cannabis."

Follow @BBCNewsbeat on Twitter and Radio1Newsbeat on YouTube