Authorities in two US states are wrestling with the practicalities of legal marijuana use after voters approved historic legislation for recreational use.

Seattle Police Department has come up with an online guide to marijuana use which has been viewed more than 120,000 times since it was published last Friday.

Under the new law, possession of an ounce or less of marijuana in Washington state will be legal for people aged 21 or older after December 6.

The guide answers burning questions, including whether you could smoke pot outside at a park or magic show. The guide says users would risk a fixed penalty ticket but not arrest.

Police spokesman Sean Whitcomb said the reaction from the general public had been very positive. He added that the response in police ranks had been more mixed but still favourable.

Meanwhile, officials in Colorado are trying to figure out what the safe limit is for drivers, and it is not as easy as measuring blood alcohol content.

Marijuana stays in the system long after its side-effects wear off, and there is no widely available test to determine whether someone is driving while impaired.

Politicians in Colorado, which also allows medical marijuana, have failed to set a driving limit for THC – the psychoactive chemical in marijuana. They say they will try again next year.

Washington’s law sets a driving-while-intoxicated limit for THC, but the standard is already being criticised as arbitrary.