Self-driving cars will bring mankind some benefits like increased traffic flow efficiency, the ability to sleep whilst your car is driving for you and, of course, laughing at 3 self-driving cars reaching a 3-way roundabout at the same time and spectacularly failing to navigate it. Society would argue that the major benefit of self-driving cars is a cut in road traffic collisions and, in particular, fatal accidents. However, this may also be its greatest cost and create a severe external cost.
The problem is, if there are less car fatalities then there will be less organ donations. According to Lipson & Kurman (2016, p.263), roughly 20% of organ donations are from victims of vehicular accidents. If car fatalities reduces drastically then hospitals will face a fall in the supply of fresh blood and body parts.
Organ donations are low enough already, if they fall any lower there is going to be an unmanageable excess demand for spare body parts. Its inevitable that people will turn to the black market for these body parts, and criminals may be tempted to kill or maim people to get their hands on these artificially high priced parts. So self-drive cars may lead to lower traffic fatalities, but also a higher a murder rate.
Lipson, H. & Kurman, M., (2016), Driverless: Intelligent Cars and the Road Ahead, MIT Press.