Abstract: A locking unit for immobilizing road vehicles to prevent their unauthorized removal comprises a sealed body which houses a movable locking member projecting from the housing and adapted for connection with a brake or clutch operating foot pedal or hand lever or with the control valve of an air-powered parking brake system. The sealed body also houses a key-released lock of the rotary barrel type having its plunger movable into locking engagement with the locking member manually without use of the key. The locking member may be a ratchet wheel engageable by a pawl which is pushed into engagement by the lock plunger acting on a spring. Further springs ensure the pawl being held clear of the wheel when the lock plunger is withdrawn. A modified form of the unit has a fixed jaw and a movable jaw by which the unit can be clamped around the stem of the draw-pin of a trailer vehicle and locked without using the key, thereby to prevent the hitch of a tractor from being coupled thereto.