I'm feeling terribly smug right now- I proved the naysayers wrong and found a feral R Banksiae lutea growing along the I-5 here in Sacramento where the freeway follows the river. Of course there was only one naysayer and she was me, but I don't care! It's super exciting. There are Kerria japonica planted all along I-5 here and if you squint and are going fast enough it looks an awful lot like a Lady Banks.
I've seen this blooming for the last two springs but it took me until today before I figured out how to get to the darn thing for a closer look. I'm happy to say that the trip was about 95% legal and along the lovely paved river walk.
I just can't help but wonder how on earth it got there! I think it's unlikely someone planted it as the fence it's up against isn't strong enough for a Banksia which means it might be a seedling of one of the plants in the Old City Cemetery! The spot is about a mile "as the crow flies" from the cemetery so I guess it's possible.