Yes - I can identify. I have a fear of being stuck on queues on the motorway, either as a driver or a passenger - and part of this is being unable to escape (I also feel this in crowded cinemas or concert halls) and part is as soon as I find myself in a queue, I need a wee!!
One time I was stuck in a motorway queue on the M25, and drivers were getting out of their cars and talking, we had been there so long. I too got out, and mentioned to the driver in front I was desperate for the loo. He said - "go up the bank, behind a bush" so I did. Next thing I know, the cars start moving - you have never seen anyone move so fast! My car was in the middle lane. It's a funny story now, but it wasn't at the time!
The worst place was in Paris, some years ago - me and my husband went for a long weekend, we thought it would be romantic - but there were NO public toilets - at least none I could find and it was a nightmare.