I´m not quite sure what you want to do but it sounds like you want connect a car battery to an Arduino. In that case just use a fuse to be save. But it´s very impracticle.
You don't think you fully understand the question... Car 12V rails can have destructive inductive spikes (well above the nominal 13.8V max) and will drop-out when the starter motors fires up. Bad grounding connection or a dying battery can allow the alternator circuit to put out more higher voltages for extended periods of time too (bulbs tend to start failing then!) The problem is to cheaply and simply protect an Arduino and associated circuitry in such an environment (brings peace of mind after you've spend 100's of hours on a project!)
Yes, it seems that most devices designed for vehicle use have to individually solve this problem, rather than a power-conditioning unit being provided to drive all the sensitive electronics in the vehicle on a separate 12V bus. In the past there was nothing sensitive to worry about so it was never a feature of vehicle design. There are in-car USB chargers which ought to provide a good regulated 5V (avoid the very cheap ones, of course), but if you want a nice regulated 12V as well its tricky.
A combination of surge-protection circuit and a 10V regulator would be handy for some things (potentially little more than a series resistor, TVS diode and 10V regulator chip).
12V low-drop-out regulator would also probably fare well most of the time (car batteries should be fully charged and more like 12.5V than 11.5V).
DC-DC converters, while somewhat electrically noisey, will isolate completely and not be affected by the battery voltage changing between charging and discharging. A 12V -> 6V converter followed by low-drop-out 5V regulator could be a reasonable approach. Go for the converters with a large input voltage range (8..24V or something like that).