What motor vehicles may enter an environment zone that has this sign?

The sign indicates an "Umweltzone" (environment zone) where access is restricted based on vehicle emissions. Vehicles are allowed entry if they have a green particulate emissions sticker, which signifies that they meet the required emission standards. In this case, motorcycles are also permitted entry.

The critical regulation is that a vehicle must either have a green sticker or meet its requirements even if the sticker is not affixed to enter the zone. This system aims to reduce pollution in urban areas by restricting access to more polluting vehicles. Understanding these rules helps ensure compliance and contributes to environmental protection.