A parking brake apparatus for an autonomously drivable vehicle having components of a parking brake system for applying a parking brake, the parking brake apparatus comprising: a first controller arranged to provide one or more control signals to be applied to components of the parking brake system to apply the parking brake in response to a signal requesting the parking brake to be applied; and a second controller arranged to provide one or more control signals to be applied to other components of the parking brake system to apply the parking brake in response to unavailability of the first controller to cause the parking brake to be applied, characterized in that the parking brake apparatus further comprising: a parking brake valve controllable by the first controller; a first 3/2 normally-open solenoid valve disposed between a primary compressed air supply and the parking brake valve; and a second 3/2 normally-open solenoid valve disposed between a secondary compressed air supply and the parking brake valve; wherein the second controller is arranged to provide a first control signal to the first normally-open 3/2 solenoid valve, and a second control signal to the second normally-open 3/2 solenoid valve, a first relay valve disposed between the first 3/2 normally-open solenoid valve and the parking brake valve; and a second relay valve disposed between the second 3/2 normally-open solenoid valve and the parking brake valve. a first controller arranged to provide one or more control signals to be applied to components of the parking brake system to apply the parking brake in response to a signal requesting the parking brake to be applied; and a second controller arranged to provide one or more control signals to be applied to other components of the parking brake system to apply the parking brake in response to unavailability of the first controller to cause the parking brake to be applied, characterized in that the parking brake apparatus further comprising: a parking brake valve controllable by the first controller; a first 3/2 normally-open solenoid valve disposed between a primary compressed air supply and the parking brake valve; and a second 3/2 normally-open solenoid valve disposed between a secondary compressed air supply and the parking brake valve; wherein the second controller is arranged to provide a first control signal to the first normally-open 3/2 solenoid valve, and a second control signal to the second normally-open 3/2 solenoid valve, a first relay valve disposed between the first 3/2 normally-open solenoid valve and the parking brake valve; and a second relay valve disposed between the second 3/2 normally-open solenoid valve and the parking brake valve. a parking brake valve controllable by the first controller; a first 3/2 normally-open solenoid valve disposed between a primary compressed air supply and the parking brake valve; and a second 3/2 normally-open solenoid valve disposed between a secondary compressed air supply and the parking brake valve; wherein the second controller is arranged to provide a first control signal to the first normally-open 3/2 solenoid valve, and a second control signal to the second normally-open 3/2 solenoid valve, a first relay valve disposed between the first 3/2 normally-open solenoid valve and the parking brake valve; and a second relay valve disposed between the second 3/2 normally-open solenoid valve and the parking brake valve.