The meaning of "would have been a while ago" in context

Question

It was in 6 minutes from BBC Learning English. It is at 27 seconds. Here it goes:

Neil: Now Rob, Can you complete this saying: “Love and marriage go together like ….”
Rob: Love and marriage go together like… a horse and carriage!
Neil: That’s right, and when was the last time you saw a horse and carriage?
Rob: Well that would have been a while ago – they are quite rare now. Not an everyday sight.

Why did not the speaker say It was quite a while ago? By using the third conditional does he mean that he did not see a horse and carriage at all, but if he had seen it in the past, then it would have been quite a while ago?

Answer 1

Among other uses, the auxiliary verb, would, is used to express an assumption, presumption or expectation in the past. The "would (and in some cases should) + present perfect" is formed with the present tense of have, and the past participle of the verb

would/should + have + past participle

• Someone called after you left but didn't leave a message.
  ~ That would have been Cathy, probably.

• I would have thought that it was rather difficult

• The letter should/would have arrived by now. (I expect it has arrived.)

Despite the appearance of would, this is not the past subjunctive because there is no "if-clause" (protasi) present in the sentence and none is implied. The subjunctive is used to indicate conditions that aren't true, that might have happened in the past, but didn't. In the OP's example, however, the speaker is speculating as to when they last saw a horse and carriage (many years ago). Knowing that horses and carriages are a rare occurrence, the speaker assumes that it happened a long time ago.

When was the last time you saw a horse and carriage?
~ Well, that would have been quite a while ago - they are quite rare these days.

Compare that sentence with this one:

He would have seen many a horse and carriage (but he hadn't), if he had been born in the late 1800s (but he wasn't, the speaker was born in the 1980s).

In the if-clause the subject wasn't born between 1880 and 1899; therefore, he has never seen carriages pulled by horses (except maybe in the movies). The subjunctive mood tells us the hypothetical result of a contrary fact in the past.

References

Advanced Grammar in Use Martin Hewings (units 7, 8, 99)
Advanced Language Practice Michael Vince (Unit 12 Modal auxiliaries 2: past, page 66)
A Practical English Grammar A.J. Thomson A.V. Martinet (160 will and should for assumptions; 231 C should/would have expected + infinitive construction)
_{Adapted from an old EL&U answer}

Answer 2

This use of would conveys a sense of vagueness or tentativeness. It implies that he is not sure when it was. The meaning is the same as Well, I think that was probably quite a while ago.

For events probably in the more recent past would is used in the same way, eg That would be last week = I think that was probably last week.