I just had the worst thought! What if Mary really is in labour in that car scene and there is a car crash (there were some hints earlier about something like that happening in this episode, i think?) and the baby dies (and maybe John also gets hurt) and bc

I just saw @queerjawn has a very similar thought to you here! That would definitely be dramatic and tragic. But, I’m still speculating that the possible car crash scenes are part of a car racing/Silver Blaze adaptation. (x)

As for why Sherlock is driving a car, I think it might be part of a nightmare/dream/mind palace sequence taking place immediately after TAB as I speculate here. I’ll copy the bit about the ‘how and why’ of cars underneath:

Okay adding my screaming to @waitingforgarridebs: What if Sherlock has fallen asleep during the car ride at the end of TAB and this is the result, his nightmare?! That’s why he’s dreaming of driving a car, because he’s hearing the noises of one…

Although perhaps Sherlock’s nightmare ends in a similarly tragic way before bizarrely ‘restarting’ to show John and Sherlock greeting Mary at her house with the baby and dog, and then they head off to Borough Market etc…with an increasingly disorientated Sherlock…?