I certainly agree with the comments about Seat Belt webbing being difficult to sever. If slack, it is a bitch to slice. Plain edge knife has tendency to melt and bind the Nylon. Easier when under tension.
This is where a GOOD serrated edge shines through. Like a hot knife through butter. Spyderco serrated edges have proved the point for me.
I do cut my fair share of line (rope) both Polyester and Nylon and webbing as a fulltime Boater.
I have tried various cheap serrated steak/kitchen knives over years as emergency blades secured in spots about Sailboat. Always tried before hand and mostly crappy but considered disposable...