It all seems so simple...

"Then one day I suddenly thought, since I spent most of my time in the taxi, why not install a karaoke machine in the cab?"

Fan says the passengers' singing doesn't distract him from driving, since he already knows all the songs and it's just like listening to the radio."

I think it is safe to say he is deaf. I mean, the average karaoke singer sounds like the radio about as much as Metallica sounds like well-harmonized, talented music. Which is to say...not at all. 

I guess the karaoke singers kind of look at it as payback. "You terrorize me with your maniacal driving, I will scar you for life with my horrific voice."

Great idea, Fan. Let's hope it catches on. Maybe it can be the next hit "reality" show. A contest between cab drivers and karaoke singers to make the other quit first. If the passenger quits singing from fear, tiredness, or whatever, the cab driver wins. If the cab driver stops the cab or orders them to stop singing, the "singer" wins. 

I better patent that idea. Can't be any worse than Nanny 911 or Wife Swap.