Act Utilitarianism:

Whereas rule utilitarianism focuses on the relative social benefit of rules, act utilitarianism concerns specific ethics of actions rather than the rules governing those actions. In many respects, act utilitarianism can be considered an extension, or a refinement, of rule utilitarianism, because it provides a method of incorporating useful or beneficial exceptions to general rules that benefit society more than rigid or absolute adherence to rules.

Using the same example of vehicular rules of the road, there are situations that sometimes justify violating those rules, notwithstanding the fact that those rules are generally useful, beneficial, and necessary. In emergencies, the value of violating a rule in specific instances...

In matters of life or death emergencies, such as fires or the need to deliver a baby, the value of delivering firefighting resources to extinguish fires as soon as possible and of delivering a baby in a hospital setting instead of in a car justifies suspending the ordinary rules of the road. Conversely, in those situations, rigid adherence to the rules of the road, even where those rules are generally beneficial and preferable to their absence, would cause social harm rather than preventing social harm.
Therefore, one could argue that act utilitarianism complements rule utilitarianism in principle, by enhancing their usefulness and beneficial application in society.