Oh the sensors work - they make an earsplitting beeping when the door fails to close. But sometimes the drivers just take off anyway. This is a proble, with some of the older drivers who are only thinking of their pension in my view; there are a lot of hardworking and safety-minded drivers and station crew, but it only takes one irresponsible person to ruin the good work of many others.
I've ridden Muni only a handful of times. On one of them, a homeless man threatened to cut my throat with a knife. Admittedly, it was because I stepped up when he began screaming at a non-white person (Middle-Eastern, perhaps, with some variant of head scarf) and her toddler.