I'm glad to see others mention the CS Bushman as a primo, "forget it's in the trunk til you need it knife". There's one in each of our three vehicles as well as a smaller folding knife in each.
Just a quick word of caution: I don't know how it is in other states, but in NC, having a knife under ones seat or otherwise out of view, but capable of being readily accessed _could_ constitute grounds for a charge of illegal carry of a concealed weapon. Having a SAK in the console or glovebox isn't a problem, in most sane locations, but having a fighter taped to the steering column would definitely be a problem if one was ever the hapless victim of a random roadblock, illegal drug search, or trooper training day.
I don't want to warp the intent of this thread, but it's something to think about in case you've never thought about it. What one does with any knowledge so gained is obviously up to the individual. Clearly in many locales other considerations make such ridiculous interpretations of law less of a consideration than that next stoplight 'Squeegee Squeeze". One interesting side bar note is that I've never heard of a case where a defendant was convicted of having ready access to a large Maglite and/or a quickpop highway flare. ;-)
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Some other primo car knives are Frosts of Sweden or KJ Erikson Mora fixed blades, Ontario machetes, any SAK of choice, Rapala fillet knives, etc. My criteria for a vehicle knife is that it be useful, cheap and plentiful enough to make replacement easy and painless. My philosophy is that there are certain knives that just get stored in the vehicles and basically forgotten til needed.
With that as a mindset, putting your old Buck 110, or stockman, Boy Scout knife or whatever you've had since 'back when' but no longer use all that often, in the trunk or toolbox makes a lot of sense. That way, if you should ever break/lose/misplace your primary carry knife, you know that there's always another one in the toolkit in the trunk. Hey, who amongst us doesn't have enough 'spare' blades that we couldn't dedicate 1, 2 or 6 to that obscure area next to the jack or spare tire or something?
Some often overlooked items that ought to be in every vehicle next to the jumper cables, the hand tools, and the first aid kit are: an inexpensive multimeter that reads DC, some method of cutting a tree obstructing the road, (bowsaw, ax or machete), and a small non-folding shovel or spade and perhaps most important of all is a battery terminal brush and spare change to buy a Coke to clean said terminals. ;-)
You can't prepare for all that life throws at you, but you can fit a heck of a lot of stuff into a medium canvas mechanics bag.
mps