Collision safety between humans and robots has drawn much attention because service robots are increasingly being used in human environments. The design of a service robot usually requires reliable collision analysis based on appropriate safety criterion. Previous safety criteria are too restrictive or generous with respect to collision injury. This paper proposes a new safety criterion for physical human-robot interaction. Injury tolerance related to the fracture force of the thyroid and cricoid cartilage in the neck is more suitable to measure injury to humans from robots than criteria representing serious injury in car crash tests. To accurately evaluate robot collision safety, a novel collision model between a human and a robot is established which include the stiffness of the neck and covering, and the input torque of the robot. The injury criteria suggested in this paper were verified to estimate the safety of service robots. Various collision analyses based on this criterion are conducted, and thus the design parameters of robot arms can be adjusted to enhance safety.