The idea evolves me on an urban bus, while i am sitting at the bus a lot of people try to use the smart card (with prepaid i suppose), which fails a lot. The tech-idea is to use a bluetooth device (like headphone device) to authenticate with bus receiver and let the user pass the bus .
This method would be more reliable and more standard than the smart card, accelarating the entrance of users on the bus, and stop delaying because smart card beeps on error.

Technically:
On the hardware side, we need a bluetooth devices that may be with power on, at the time to arrives to bus. The first thinking is to use a bluetooth mobile phone that a lot of people alredy have.
The Software may be a java application on the phone (or on the bluetooth hardware) that are capable of communicating with an authentication gateway