Lee D
Been there, done that.
A knife with a sharp plain edge set at 20 to 25 degrees inclusive will glide through a seat belt. (and nylon rope) Although, truth to tell, a knife isn't the proper tool for cutting a seat belt. Mechanic's, Electrician's or EMT shears are better than any knife. Come to that, heavy leather shears or scissors would probably be better than a knife, too.
A serrated blade only cuts with the tips/points of the serrations. They rip, they don't cut.
Examine the "cut" edge of that seat belt after using a serrated blade on it, compared to a plain blade. The plain blade cuts the nylon thread. Serrated blade tears them. The cut edge is frayed when you use a serrated blade.
I understand that in an emergency situation where a seat belt has to be cut to save someone's life, it is more likely the person assisting the trapped person will have a knife (if he/he has any cutting tool ... seems a lot of folks including cops and ambulance personnel , don't these days), as opposed to shears.
Honestly though ... how often does a person who
isn't a first responder, have to cut someone' seat belt to get them out of a vehicle? In over 60 years of carrying a pocketknife daily, I've never had to. Not even during the years I was an over the road truck driver. (not to mention that moving the trapped person without using a back board and/or neck brace could cause a fatal or paralyzing injury if the trapped person has a broken/near broken neck or back. You can be sued or that ... and likely lose ...)
I don't know about you, but I base my daily carry on what decades of experience tells me I'm likely to need; not worst case "what if" fantasy.
As for "thick cardboard" to an inch or so thick, I prefer a Stanley box/utility knife.
Considering the silica, sand, and other contaminates in cardboard, I prefer the box/utility knife with easy to change blades, over any pocket knife, regardless of blade steel. Cardboard quickly dulls them all.
200 replacement Stanley blades costs a heck of a lot less than a good pocket knife. You can even get "long lasting" ceramic blades for the Stanley. Allegedly, the ceramic blades are "almost as hard as diamonds" and last noticeably longer when cutting cardboard.
