I agree – while could have been driving is not altogether impossible (in other contexts), I’d also use could/would be driving here. Whether to use could or would is up to you and the context:
If I had the money I would go to Ireland for my holiday.
If I had the money I could go to Ireland for my holiday.
There’s not much difference. The former sentence says that you’ve made your choice; the latter is not so definite.
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