[troad]

And my father-in-law is still upset about it.

It's led to some interesting outcomes - for example, I've seen people write "Princess Highway" a fair few times, presumably on account of the official spelling ("Princes") falling confusingly somewhere half-way between the more usual "Prince's" and "Princess".

In day to day use, I don't think most people actually use the apostrophe-free official spellings, at least for names comprised of ordinary English words (Prince's Hwy, Surfer's Paradise), but it might be more of a free-for-all with proper names (Wiseman's Ferry or Wisemans Ferry).

[caf]

I have never seen anyone write "Prince's Hwy" (or "King's Hwy" for that matter), the apostrophe-free version would be far more common.

The "Princess Hwy" thing is also common I think because the pronunciation most often used sounds a lot like "Princess", particularly when it's run right into "Highway" without a gap.

[troad]

> I have never seen anyone write "Prince's Hwy" (or "King's Hwy" for that matter), the apostrophe-free version would be far more common.

I don't know what to tell you; I've seen it often. I'm talking day to day here, text messages and handwritten directions. If you're thinking of things like road signs, those will obviously follow official usage.

> The "Princess Hwy" thing is also common I think because the pronunciation most often used sounds a lot like "Princess", particularly when it's run right into "Highway" without a gap.

Obviously that is so, but I would suggest that the reason 'Princess Hwy' is comparatively so much more common than other similar mistakes (e.g. 'sea' for 'see' etc.) is because the official name (Princes Highway) feels so unnatural in English that it fails to act as a meaningful standard for usage.