Green Street Car Park - class diagram

The class diagram is mostly self-explanatory, however, I will give clarification of these relationships:

At the entrance, when a car approaches it is sensed by the sensor which sends a message to the card reader asking it to validate a card. The card reader then validates and returns the driver's card. Once the card is validated, the card reader sends a message to the barrier asking it to raise. So there is an entrance sensor, a card reader and and an entrance barrier.

At the exit, when a car approaches it is sensed by the sensor which sends a message to the barrier asking it to raise. There is no card reader at the exit, only an exit sensor and an exit barrier.