Which action indicates a failure to maintain minimum speed on Iowa highways?

Driving below the flow of traffic is an action that clearly indicates a failure to maintain minimum speed on Iowa highways. In Iowa, drivers are required to adhere to minimum speed limits established to ensure the safe and efficient movement of vehicles. When a driver operates their vehicle at a speed significantly lower than the flow of traffic, it can create hazards, as it disrupts the normal speed pattern, potentially leading to collisions or forcing other drivers to maneuver around them unpredictably.

In contrast, speeding in a construction zone primarily pertains to exceeding the posted speed limit rather than failing to meet a minimum speed. Using high-beam headlights improperly is related to vehicle operation and safety, but it does not pertain to speed limits. Parking in a no-parking zone concerns vehicle positioning and compliance with parking regulations, irrelevant to speed maintenance.

Thus, the correct action that directly illustrates a failure to maintain minimum speed is indeed driving below the flow of traffic.