An operator may not drive at a speed greater than __________ and __________ under the circumstances then existing.

The answer "reasonable, prudent" reflects the key concepts in traffic laws regarding safe driving practices. These terms are critical in establishing a driver’s obligation to adjust their speed based on the conditions of the road, weather, traffic, and other circumstances that may impact safety.

"Reasonable" indicates that the driver should maintain a speed that is sensible given the current situation, which may include factors like heavy traffic, pedestrian activity, or road construction. "Prudent" emphasizes the need for caution and judicious decision-making. Together, these terms underscore the principle that a driver must not only adhere to speed limits but also consider whether their speed is suitable for the given circumstances to ensure the safety of themselves and others on the road.

The other options do not encapsulate the necessary legal and safety-based framework for driving speed as effectively. While "safe, appropriate," "controlled, normal," and "lawful, safe" touch on aspects of driving, they do not emphasize the comprehensive necessity to adjust speed based on the specific context and conditions, which is central to prudent driving behavior.