Emma drove, since she had the car. She had dropped Mark off at work that morning, because he was going for a drink afterwards and expected to be having enough that he shouldn’t drive home. She had offered to go and collect him when he was ready, but he had told her not to worry. He would get a bus or taxi instead.
She had the car, and they needed groceries, so she decided she might as well go and do it, that afternoon, while she was up and willing and dressed. It would save going another day, when she might have to walk. She walked down twice or three times a week, for something to do, but driving more pleasant, since walking meant going through the roadworks and dust where new subdivisions were being built, and also meant carrying heavy bags home.
It was better to take advantage of the car.