Which of the following conditions makes driving more dangerous when at the posted speed limit?

Driving in conditions of poor visibility significantly increases danger on the road, even when a driver is adhering to the posted speed limit. Poor visibility can be caused by various factors such as fog, heavy rain, snow, or darkness, all of which reduce a driver's ability to see other vehicles, pedestrians, and traffic signals clearly. This impaired vision can lead to delayed reaction times and increased chances of collisions, as drivers may not be able to accurately gauge distances or speeds of other road users.

In contrast, heavy traffic, while it can create stress and require careful navigation, does not inherently impair visibility; drivers can still see and be seen but must be vigilant about their surroundings. Flat road surfaces generally do not present hazards as these promote smooth driving, and dry weather tends to provide better traction and visibility, making driving conditions safer. Thus, the impact of poor visibility is far more pronounced in creating a dangerous driving scenario, even at the posted speed limit.