A large majority of those petrol tanks would've been obtained by simply moving forward along the path, and then moving forward again and finding yourself by the truck again. On the highway Duke had to quite literally move along a straight line, get into a little shed, then move back along that same straight line. You don't have to explore to find them, you can't change the order in which you find them, so if that's the best DNF has to offer in terms of keycards, it's no wonder it wasn't very well received as a Duke game. Ha, I just remembered even Duke Nukem 2, a
side scroller, required more exploration, was less linear, and had more keycards than DNF
In Serious Sam 3, you also had to get petrol tanks, but you had to actually search for them, and I think there were actually 8 of them that you could collect in any order, with enemies that spawned in as you collected them to keep the adrenaline flowing. People would say "it's serious sam of course enemies are going to spawn in", but lets not forget it's a fun mechanic, and Duke 3D had enemies spawn in after you collected keycards (most of the time).