My first line will always be polite.
If I have to repeat, I never give face already.
Most will stop eating after you tell them the first time.
My worst case scenario was this woman in AM peak.
Pax: wah why you take so long to come, I'm in a rush.
Me: there's a jam at the right turn at the junction outside coming in.
Pax: Gps say you 3mins away, but you take more than 5mins, so slow.
Me: Miss, I dont earn anything extra by delaying your time. I also wanna pick you up fast. To the gps, every traffic light is green colour.
2mins later..
Pax: I'm going to eat my bread in the car ok.
Me: pls do not eat in the car.
Pax: why cannot? I waited so long for you, I havent had my bfast. So many taxis drove past me earlier, but I waited for you to come.
Me: huh? What you mean why? This is my car, later cockroach come, then how?
Pax: I got 2 plastic bags, dont worry. So can eat or cannot eat?
Me: I already tell you nicely pls dont eat in the car.
Pax: FINE!
Me: if you insist on eating, I can stop the car now and we can cancel this trip. You can take a taxi and see if the td allow you to eat.
Pax: *diam diam*