What signal must all vehicles with air brakes have?

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All vehicles equipped with air brakes must have a low air pressure warning signal. This signal is essential because it alerts the driver when the air pressure in the brake system falls below a safe operating level. Air brakes rely on a certain level of air pressure to function correctly; insufficient pressure can lead to brake failure, posing a significant safety risk on the road. Therefore, the low air pressure warning signal serves as a crucial safety feature, ensuring that drivers can be informed about potentially dangerous conditions before they result in an emergency. It allows drivers to take corrective action, such as pulling over to address the issue before continuing to operate the vehicle.

The other options, while relevant in different contexts, do not serve the same critical function in relation to safety for vehicles with air brake systems.