Which government agency is primarily responsible for enforcing vehicle emissions standards in the United States?

The Environmental Protection Agency (EPA) is primarily responsible for enforcing vehicle emissions standards in the United States. This agency plays a crucial role in regulating air quality and ensuring that vehicles meet specific emissions regulations established under the Clean Air Act. The EPA sets the standards that limit the amount of pollutants that can be emitted by vehicles, thus helping to reduce air pollution and protect public health.

The EPA conducts testing, monitors compliance, and implements programs aimed at reducing emissions from various sources, including automobiles. Their authority allows them to enforce penalties against manufacturers and vehicles that do not meet the prescribed emissions standards, emphasizing the importance of maintaining environmental integrity.

In contrast, the Federal Aviation Administration (FAA) is focused on civil aviation, while the National Highway Traffic Safety Administration (NHTSA) is primarily concerned with vehicle safety standards but does not enforce emissions regulations. The Department of Transportation (DOT) oversees infrastructure and transportation policies but does not specifically regulate vehicle emissions. Hence, the correct answer reflects the dedicated role of the EPA in environmental protection related to vehicle emissions.