The implied part is “within common conditions” it’s a law limited to a specific regime, much like Newtonian physics. We know it’s not universally true but we can see it’s often true in common scenarios.

In the extreme case, as road coverage approaches 100%, the city stops containing buildings so the traffic will drop towards zero, so there’s actually a balance point somewhere, but roads are quite inefficient for high density cities, so probably the balance point would be less about fulfilling traffic demand and more about reducing the demand by demolishing most of the city.