Each commuter on buses that passed through an ERP has a special
personal device attached on their wrists like watches. Inside these
devices are mini mircrochips with stored value much like cashcards.
When the bus passes through a gantry, there is a beep thats very
much similar to the beeping you get when you drive through a
gantry. An amount is deducted from the store value of these
devices. Please be aware that should you fail to have sufficient
value stored in your personal device the bus driver will have to
throw you off the bus and that you have to walk afterwards.