Public washrooms can be quite challenging for children with special needs even after they are toilet trained. On the surface you wouldn't think it would be a problem transferring skills from your home washroom to a public washroom until you really consider all the differences between the washrooms. In a public washroom everything is becoming automatic and extremely loud. You have urinals that aren't typically in homes. All these things can add challenges to the task of toileting.
Public washrooms are extremely loud and if you have a child sensitive to noise this can be even more frightening. The toilets that flush on their own are scary for children, heck it can be startling for an adult if it flushes while you are sitting on the toilet. From that scare you move on to the automatic sink where you have to place your hands in just the right position to get the water to run. Finding the right spot can be a challenge for a child that is coming at the sink from a different angle than an adult. Then there is the automatic paper towel dispenser or the automatic hand dryers - some of these dryers have such force that it feels like your skin is coming off and the noise is horrendous.
Another issue we recently had is with urinals, having boys that trained extremely late in their life and with the need to sit (and their toileting mainly done with females) they haven't had many opportunities too see or use urinals. On a recent dinner outing I asked my husband to take our son to the toilet, he said "our son's oldest brother was in there he would be fine". Our boys come back to the table and our oldest, trying to keep a straight face says his little brother needs lessons on how to use an urinal. Somehow our son was able to climb himself up onto the (tall) urinal and was seated, preparing to do his business when his brother realized he was there. Fortunately big brother was able to relocate his brother to the toilet before the automatic flushing of the urinal began and his brother was sprayed down.
We have also had issues with the urinals in port a potty's. I just assumed that the children (especially the ones adopted at an older age) knew that one part was a urinal and the other part was the toilet. Never in my wildest dreams did I think a child would mistake the urinal and the odor puck as a sink and a bar of soap or a baby holder or a purse holder. (All true examples)
It wasn't until we started raising children with special needs that it became apparent that you need to teach things that you would commonly take for granted and you might need to reteach over and over.