The nominal 12 volt supply (often 14 volts or so) in a car is generally very dirty with high current devices switching in and out (an electric cooling fan for example might take 25 amps) and high voltage spikes from the ignition perhaps some being negative being picked up.

I had some problems using Revolution Education Picaxe microchips in cars where on the road they would hang and need a power off reset which was not good. (Incidentally, I don’t recommend using home made circuit a mission-critical system say a home-built cruise control!)

I always use the above circuit which I copied from The Amateur Scientist in Scientific American. I don’t know whether it does any good but it probably doesn’t do any harm! Anyway, so far I haven’t had any problems with misbehaving Arduinos or Adafruit Trinkets.

Power supply filtering