These are actually two very relevant considerations for considering a tool to cut seatbelts. Being involved with racing as a marshal (think safety crew) I’ve had more than one occasion where I’ve had to extract a driver in an incident. While racing harnesses have quick release mechanisms, in an emergency, I have had to cut straps quickly to help extricate someone on a few occasions. I’ve also gone through training, where we not only practice removing someone else, but experience being strapped into the rig ourselves as the ‘dummy’. Here are my thoughts based on personal opinion and experience...
first and foremost, 99% of the time, if you’re cutting a belt, either for yourself or someone else, it is going to be a high stress situation, under less than ideal situations. Anything you have to manipulate to open (ie: fold) adds another level of undue complication to the scenario. Especially if it’s not a scenario you actively train for, with a tool you’re familiar with.
Size matters! All those cutters on the back end of ‘rescue knife’ handles are fine for cutting a seatbelt, or piece of cord, or other strap, if you aren’t under stress and are able to calmly line up the relatively small capturing area to draw it into the cutting blade. Of course, you also don’t want to try getting a 10” long Bowie under the belts of an already struggling, freaked out passenger as they see you coming at them with a large knife.
finally, if you’re worried about cutting your own seatbelt, remember, your pockets may not be easily accessible to pull out a clipped folder or fixie in a pocket sheath. Same holds true with something on your keychain, dangling from the ignition.
So for me, I always recommend one of three options: a 3-5” fixed blade, a dedicated strap/belt cutter, or a pair of medic/rescue sheers. I actually keep all three on me at the track, as they all will serve multiple, different purposes.
for cutting someone else out, your pocket knife will work, but a small fixed is better under stress of you edc one. Better is a strap cutter with a finger hole or a pair of sheers. Both are gonna be the safest for whoever you are cutting out as the tips will be blunt, and they are going to be less prone to be dropped with your fingers in the holes. The sheers aren’t necessarily practical to keep on you at all times (unless you’re involved in an activity that warrants them), but are going to give you more control and easier cuts, and will easily fit in most glove boxes or center consoles. The strap cutter is small enough to include with your EDC gear, be thrown in a pocket, or some even on your belt.
if your concerned with cutting your own belt, go with a dedicated, hook type strap cutter (and not a folding one). I’ve been stripped in, upside down, to a faux cage at training seminars, and let me tell you, if you think your gonna be able to grab your pocket knife, think again. Even if your not upside down, try grabbing it out of your pocket while sitting buckled in your driver’s seat, wearing a winter coat, with the steering wheel in the way, and potentially being in shock in an emergency. A strap cutter is going to be your absolute best option in this situation. Just make sure it’s somewhere accessible. Center console is ok, but if your upside down, it’s gonna fall out, and if you keep a lot of other stuff you’ll have to dig for it - neither are ideal.
So... for my personal use, I keep a small strap cutter in each vehicle, clipped to the sun visor. While I can’t predict every possible scenario, it is the most accessible place for the vast majority of any situation your likely to encounter. It doesn’t have to be expensive, or a big brand name, or anything fancy,
just sharp.
I know OP was looking for something on a knife, but a dedicated strap/rescue cutter is much more user friendly in an emergency.
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