I didn't say "WOULD" I said "COULD" under worst case conditions.
Also, the level of filtering/protection required depends on what electronics is used and how/what outputs are implemented.
An air-bag system has no outputs at all and I'd be very surprised if it has a microprocessor. Hence, very little filtering required.
An incandescent light-bulb needs no filtering because the filament has a large enough thermal lag to absorb the transients.
How long does it take the varistor to react compared to what is the shortest time a digital input can react to? (typ. = 3nS by the way)
How much of a 10nS 5kV pulse will a varistor suppress?
Filtering elecrical noise is comparable to filtering sound.
Consider the VERY loud car stereo.
A lot is 'muffled' by the fact it's in a car. (suppressor on alternator)
A lot is suppressed when you are indoors (local regulator)
A lot is suppressed if you wear headphones (decoupling caps)
The big 'bassey' bits are dampend by solid concrete flooring (local varistor)
Nearly all of it goes if you put your under the duvet (local choke).
Do all of that and you probably wouldn't know the car was there.
Like I've said several times, the choice is yours.
Odds are, you'll get away with it with that last circuit, but it is NOT gauranteed if your car gives you a worst case scenario.
I work mainly in the semiconductor manufacture industry. One of the processes involves producing a beam of ions to impregnate the silicon with impurities. That is done by accelerating the atoms with high potential at voltages up to the best part of 1MV with 10kW behind it. That sometimes 'cracks' over and all the electonics must cope with that. We can't use any memory devices such as micros or even flip-flops because the EM pulse could reset them.
Now, heres the sad but true part.
When designing a circuit that can save a life, the investment in that circuit is valued at £1M per life! That is, it's only worth spending an extra million if it saves a life or (more to the point) if it costs more than a million just to save just one life, it's not worth doing
A down semiconductor plant can cost £10M/day.
Hence, the failsafes I deal with are comparable (financially) to those that would cause the death of 10 people/day. A very sad fact!
Maybe that has made me over cautious over the years?
How crucial is it that your circuit works every time?
What is the risk and what are the consequences?
How much is a cut corner worth?
Compare that to the original design you are replacing.
What is the life expectancy of a regular car bulb?
Is your circuit more/less reliable?
If it does fail, how quickly/easily can the failed part(s) be replaced?
These are all basic fundamentals that the electronics design engineer must answer as part of the design process. They can only be answered by having a full understanding of both the design and the working environment. If any aspect is not fully understood. Then get in a qualified consultant and pay for a litigation waving certification and peace of mind.