What color flag must be attached to the end of a load extending more than four feet over a vehicle during the daytime?

When a load extends more than four feet beyond the rear of a vehicle, safety regulations require a red flag to be attached to the end of the load during daylight hours. This red flag serves as a warning to other drivers about the extended load, helping to prevent accidents by improving visibility. The use of red specifically is standardized because it is easily recognizable and universally associated with caution and danger.

The choice of other colors, such as white, blue, or yellow, does not meet the established safety norms for indicating an extended load. White might be confused with unrelated signals or objects, while blue and yellow are not standard for this purpose and may not convey the necessary urgency and visibility required for safety in such situations. Thus, the requirement to use a red flag maximizes safety on the roads.