According to the recently passed initiative, beginning December 6th, adults over the age of 21 will be able to carry up to an ounce of marijuana for personal use. Please note that the initiative says it “is unlawful to open a package containing marijuana…in view of the general public,” so there’s that. Also, you probably shouldn’t bring pot with you to the federal courthouse (or any other federal property).
Much like having an open container of alcohol in public, doing so could result in a civil infraction—like a ticket—but not arrest. You can certainly use marijuana in the privacy of your own home. Additionally, if smoking a cigarette isn’t allowed where you are (say, inside an apartment building or flammable chemical factory), smoking marijuana isn’t allowed there either.
If an officer believes you’re driving under the influence of anything, they will conduct a field sobriety test and may consult with a drug recognition expert. If officers establish probable cause, they will bring you to a precinct and ask your permission to draw your blood for testing. If officers have reason to believe you’re under the influence of something, they can get a warrant for a blood draw from a judge. If you’re in a serious accident, then a blood draw will be mandatory.