How does Aquinas's idea of determinatio explain rules about driving on a particular side of the road?

In the philosophy of law, St. Thomas Aquinas explains that valid positive law is derived from objective moral principles in two ways. While some laws are deduced directly from moral truths (such as prohibiting homicide), others are derived through a process he called determinatio.1

Determinatio refers to the determination or specification of how a general moral principle applies to specific circumstances to facilitate human coordination. Traffic laws requiring drivers to stay on a particular side of the road serve as a classic example of this process.1

Morality does not dictate whether one must drive on the left or the right; however, it does require that society avoids exposing others to undue risk and facilitates the common good through orderly commerce. Once a legitimate political authority makes a specific determination—such as choosing a side of the road—that rule becomes binding on citizens because of its indirect connection to these broader moral principles. If a law is not derived from moral principles through either direct deduction or determinatio, Aquinas argues it is unjust and fails to qualify as true law.1