Which principle applies when an officer stops a vehicle and detects the smell of illegal substances?

The principle that applies when an officer stops a vehicle and detects the smell of illegal substances is known as the plain smell doctrine. This legal concept allows officers to use their sense of smell as a probable cause for further investigation. If an officer smells something that indicates the presence of illegal substances, such as marijuana or other drugs, this sensory information is considered sufficient to justify a search of the vehicle without the need for a warrant or explicit consent from the driver.

This principle is based on the idea that if something is immediately apparent to an officer, such as the odor of illegal drugs, it may indicate that a crime has occurred, and therefore, the officer is justified in taking action, such as searching the vehicle. This is consistent with the Fourth Amendment, which protects against unreasonable searches and seizures, recognizing that certain circumstances may provide sufficient cause for law enforcement to take necessary action to ensure public safety.