Road Safety Bill [HL]

Can my noble friend clarify how the alcolock works? The noble Earl, Lord Attlee, has suggested that someone in the passenger seat could blow into the machine, enabling the engine to start. Quite often occupants of a car decide to change drivers. The driver may get tired and decide he wants to go to sleep, or he may decide that he is too drunk to drive. They may then stop the car, keep the engine running and change drivers? What is it that requires someone to blow into the machine again? Is it to make the engine start or is it when someone gets out of the driver’s seat? This could lead to interesting debates as to who was driving and who was puffing into the pipe—especially if the engine is kept running, which I assume would avoid the necessity of having to recalibrate and retrigger the device. I should be grateful for an explanation from my noble friend.