Hi,
  as there is often a chance that I forget to switch the mobile phone off during driving, I find myself tempted to answer the phone during commuting. Hence, I would like to use a gadget of sort -- possibly bluetooth based that can switch the answer phone message automatically when I am sitting in the car. It also could switch it back once I leave the car.

  People calling me would not be so disturbed that I am not answering immediately and maybe inclinded to leave a message instead.