It is a variation on the classic trolley problem designed to focus on the ethics of autonomous vehicles, as well as the question of who gets to decide how they react in life-and-death scenarios.
[2] The premise of these thought experiments is that even with highly sophisticated self-driving-car technologies, the cars will face situations where harm cannot be avoided.
Because of that feature, Millar argues that the tunnel problem forces us to question whether designers/engineers have the legitimate moral authority to make the decision on behalf of autonomous car users.
Where design features provide "material answers to moral questions"[3] in the use context, Millar argues that designers must find ways to incorporate user preferences in order to avoid unjustifiable paternalistic relationships between technology and the user.
[5] Roger Crisp featured the tunnel problem on the Oxford University Practical Ethics blog.