I've been aware of the SLF epithet for many years and it has always brought a little smile to my lips. I suspect it is a term of endearment at times and a curse on others. I do not have to deal with the general public in my job, only healthcare professionals, so I don't get any of the grief that the public can and does hand out.
My friends in the health service are endlessly thoughtful and caring for their patients, yet I know many radiographers who have been assaulted. Beaten up in one case. So, 'the general public' - and that includes your passengers - can and do behave like oafs sometimes and at others I bet they can be charming and fun. So SLF probably covers it.
I love the 'Tonka Backseater' being referred to as 'Talking Ballast' and I bet many of them used it to describe themselves.
I am an MRI and CT Scanner Field Engineer - some of our colleagues refer to us as 'Tractor Mechanics'.
Roger (Time to get back to a bit of tractor mechaniking)