Really, anyone who is at fault or negligent in causing an accident can be held responsible for death or injury to a motorcycle rider. For example, if someone's riding their motorcycle through an intersection, and a driver ran the red light right in front of them and caused the motorcycle rider to drop their bike and hit the road and cause significant injuries, they can make a claim against that at-fault driver. Or, if the motorcyclist is hit from the side, due again to someone say running a stop sign, they can bring a lawsuit against that driver. Or, if a car negligently merges into the motorcyclist's lane, then they can bring a claim against that at-fault driver. Really, against anyone who was negligent in causing an accident.