Final answer:
Signs instructing drivers to stop and inspect their brakes before a downgrade are regulatory signs, meant to enhance road safety by preventing brake failure due to overheating.
Step-by-step explanation:
Signs posted in advance to a downgrade, instructing drivers of trucks to stop and inspect their brakes, are considered regulatory signs. These signs are crucial for road safety, especially in areas where steep declines can cause significant wear and tear on a vehicle's braking system. Drivers are alerted by these signs to check their brakes to prevent potential failures that can lead to accidents. For example, as indicated by the statement regarding the smoking brakes on a truck being evidence of the mechanical equivalent of heat, it is essential to ensure that brakes are in good working condition to avoid overheating and possible brake failure.