If the relative speed difference between the two vehicles is below 15 km/h, the system can help the driver avoid the collision entirely. Between 15 and 30 km/h, the focus is instead on reducing speed as much as possible before the impact.
The difference between City Safety and advanced radar-guided cruise control systems such as those found in the new Mercedes Benz S-class is that City Safety is designed to work in stop start traffic and only monitors vehicles up to 6m ahead of your car.