Traffic violations are designated as either "primary" or "secondary" offenses based on seriousness. This difference matters because police can stop a vehicle for a primary but not a secondary violation. This article gives some examples of offenses normally categorized as primary and secondary and explains how police enforce the laws differently.
Typically, an officer must have "reasonable suspicion" of criminal activity to lawfully initiate a traffic stop. In other words, it's generally illegal for police to pull someone over unless they've actually broken the law. (Sobriety checkpoints are one of the few exceptions to the reasonable suspension rule.)
But not all criminal activity justifies a traffic stop. Some minor traffic offenses categorized as secondary violations can result in a citation but only if the driver was first stopped for a more serious offense that's considered a primary violation. In other words, police can initiate a traffic stop for a primary but not a secondary violation. All traffic violations are categorized as primary or secondary violations.
Most traffic violations are primary offenses. Common primary offenses include:
Secondary traffic offenses are normally minor violations of laws meant to encourage safe driving. Common examples of primary offenses include:
However, each state gets to decide how to categorize traffic offenses. So, what might be a secondary offense in one state could be a primary offense in another state.