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How do traffic laws illustrate Aquinas’s idea of determination or specification?

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Painting by Rembrandt showing Aristotle in rich dark clothing with a gold chain, resting his hand thoughtfully on a marble bust of Homer, illuminated against a dark background. — natural law *Aristotle Contemplating the Bust of Homer*, oil on canvas by Rembrandt van Rijn, 1653; in the collection of the Metropolitan Museum of Art, New York.

In the philosophy of law, St. Thomas Aquinas explains that positive law is derived from objective moral principles in two ways. While some laws are direct deductions from moral truths (such as prohibiting homicide), others are derived through determinatio, or determination/specification.1

Traffic laws serve as a classic example of this process. Morality does not dictate whether humans must drive on the left or the right side of the road; however, it does require that society coordinates to avoid harm and facilitate the common good. Because a specific rule is necessary to achieve this moral end, a legitimate political authority must "determine" or "specify" one side of the road over the other.

Once this determination is made, the resulting traffic law becomes binding. It is not derived from a specific moral command to drive on a particular side, but rather from the general moral obligation to coordinate behavior to prevent undue risk and ensure safety.1