Many of our ignition systems need a coded transponder response from the key to operate. All you would need to do is build an interlock in the cell phone that would prevent it from working when within, say, 1 meter of an ignition that was turned on. It could be designed to allow the "ringer" to work, but not allow communication until the ignition was turned off.
BTW: I am not crazy, just dreaming. Such a rule wouldn't have a ghost of a chance of being implemented in the US.
John