A portable electronic device includes a safety feature configured to prevent at least some forms of use of the device when the user is traveling beyond a speed threshold. The device may detect its linear speed, compare that speed to a threshold, and provide a response or blocking function upon detecting that the speed has exceeded a threshold. In one embodiment, the device is a cell phone configured to disable the transmission and reception of voice and text, conceal its display screen, and disable incorporated games, cameras, internet access, etc., if the cell phone is moving faster than walking speed. The blocking function in a cell phone may be partially overridden by the introduction of hands-free equipment. The introduction of a hands-free earpiece and microphone may serve to override the blocking of voice transmission and reception, and may also enable a voice activated dialing and/or answering function.