As a rule of thumb, motorists are not permitted to drive while under the influence of alcohol or other impairing drugs. But can a passenger drink alcohol in a car? Basically, No, but it depends on the state laws.
Open Container Laws
Most states have laws prohibiting the presence of open bottles, cans, or other unsealed containers of alcoholic beverages (even if empty) on sidewalks, streets, and inside vehicles. So, a passenger drinking an alcoholic beverage also is in possession of an open container. In some states, unsealed containers of alcohol are allowed but in secure locations only, such as a locked glove compartment or trunk.
43 states have open container laws in place, 40 of which conform to federal standards outlined in the Transportation Equity Act for the 21st Century (TEA-21), which mandates open container laws and other traffic safety measures.
Even if only the passenger is in possession of an open container, both the driver and the offending passenger may be cited for a violation of the open container law. In any case, the blood alcohol concentration (BAC) of an offending passenger is irrelevant.
There are seven states that do not have open container laws, including Missouri, Arkansas, Mississippi, West Virginia, Virginia, Delaware, and Connecticut, even if local ordinances in some of those states do in fact prohibit open containers of alcohol. Three states have open container laws (Alaska, Louisiana, and Tennessee) that do not fully comply with federal TEA-21 standards.
But can a passenger drink alcohol in an automobile in the absence of laws banning open containers?
Apparently, yes. As long as there are no local ordinances banning open containers in vehicles, passengers in states without such laws can in fact drink alcohol in a moving vehicle. In Mississippi they are even allowing drivers to drink as long as they remain under the legal BAC limit.
Some municipalities doesn’t conform to open container laws in an effort to boost tourism, such as the French Quarter in New Orleans, but the majority of such ordinances do not allow open containers in vehicles. The French Quarter, infamous for its drive-through frozen cocktail vendors, allows both drivers and passengers to have open containers of alcohol in vehicles, as long as the driver is not drinking.