Final answer:
A red triangle on the rear of a vehicle indicates it is not designed to exceed 80 mph, informing other drivers to expect slower speeds.
Step-by-step explanation:
A red triangle on the rear of a vehicle indicates that the vehicle is designed not to exceed a certain speed. Specifically, this symbol means option 3: Vehicles not designed to attain a speed of 80 mph or more. This is a safety feature that alerts other drivers that the vehicle ahead may be moving slower than other traffic, especially on high-speed roads. It's an important signal on vehicles such as agricultural equipment or construction machinery, which are designed to operate at lower speeds and may create a potential hazard if not identified by other drivers. Understanding such symbols can contribute to safer driving practices, as it allows drivers to anticipate the behaviors of such vehicles on the road.