DISTRICT OF COLUMBIA: Marijuana is already legal for medical and recreational use in the District of Columbia. Alaska, Oregon, Washington and Colorado also allow recreational use and possession of cannabis.
Twenty-three states from Maine to California allow medical marijuana use. But if you are taking a road trip from the NorthEastern shore to Southern California, there are a lot of states in between where marijuana is still illegal.
The laws are constantly changing and the laws for use and possession in those states is not uniform.
And ignorance of the law is not a legal defense.