Driver assistance systems in today's cars assist drivers when the car is in motion. However, there is relatively less, to almost no work on assistance systems when the car is stationary. This paper focuses on such a system, and describes the design and realization of a collision avoidance system for a novel car door assistance system, the smart car door. An omnidirectional camera is attached to each side-view mirror of a car, and the fold-in fold-out movement of the mirror is used to generate 3D information about static obstacles next to the car based on a motion stereo approach. The car door controller uses the sensor data to compute collision-free door opening paths, and the two-hinge kinematic system of the smart car door enables to move the door around static obstacles. The latter increases the door entrance area in tight parking lots, which drastically improves the ingress/egress to/from a vehicle. These are enhancements to currently available systems that only indicate the presence of obstacles next to the car. Experiments on a fully realized prototype demonstrated a robust generation of 3D information about obstacles next to the car. A study with 20 test users showed that our system strongly supports the users to avoid collisions when operating the door.