In everyday matters, if you are stopped by the police, bear in mind that there is no system of paying traffic or other fines on the spot. Attempting to pay a fine to an officer is frowned upon at best and may result in a charge of bribery. For traffic offenses, the police officer or highway patrol will explain the options to you. There is usually a 30-day period to pay a fine. Most matters can be handled by mail.
If you are arrested, you have a legal right to an attorney, and you are allowed to remain silent. There is no legal reason to speak to a police officer if you don't wish to, but never walk away from an officer until given permission to do so. Anyone who is arrested is legally allowed to make one phone call. If you can't afford a lawyer, a public defender will be appointed to you free of charge. Foreign visitors who don't have a lawyer, friend or family member to help them should call their embassy; the police will provide the number upon request.
As a matter of principle, the US legal system presumes a person innocent until proven guilty. Each state has its own civil and criminal laws, and what is legal in one state may be illegal in others.
Bars and stores often ask for photo ID to prove you're of legal drinking age (21 years or over). Being 'carded' is standard practice; don't take it personally. The sale of liquor is subject to local government regulations – some counties prohibit liquor sales on Sunday, after midnight or before breakfast. In 'dry' counties, liquor sales are banned altogether.
In all states, driving under the influence of alcohol or drugs is a serious offense, subject to stiff fines and even imprisonment. A blood alcohol level of 0.08% or higher is illegal in all jurisdictions.
The states have quite different laws regarding the use of marijuana, and what's legal in one state may be illegal in others. As of mid-2017, recreational use of small amounts of marijuana (generally up to 1oz/28g) was legal in Alaska, California, Colorado, Maine, Massachusetts, Nevada, Oregon, Washington and the District of Columbia. Another 13 states have decriminalized marijuana (treating recreational use as a civil violation similar to a minor traffic infraction), while others continue to criminalize non-medical use, punishing possession of small amounts as a misdemeanor and larger amounts as a felony. Thus, it's essential to know the local laws before lighting up – see http://norml.org/laws for a state-by-state breakdown.
Aside from marijuana, recreational drugs are prohibited by federal and state laws. Possession of any illicit drug, including cocaine, ecstasy, LSD, heroin and hashish, is a felony potentially punishable by a lengthy jail sentence. For foreigners, conviction of any drug offense is grounds for deportation.