I had one nearly zip between my legs once in Cameroon many years ago.
We were pushing a pickup truck out of a giant hardened elephant footprint and I leaped into the back of the truck in the nick of time. They are FAST. Like greased lightning.
If you were in Cameroon you must have either seen or heard about the Gabon Viper (and its cousin the Rhinocerous viper). Those two snakes, together with the Puff Adder, are my favorite African nightmares (apart from the hippo ...I have a very interesting hippo encounter story btw). The Gabon/Rhinocerous/Puff are heavy slow snakes, even though they have one of the fastest snake strikes in the world, with huge fangs (the Gabon has the longest fangs of any snake), totally invisible to the eye (in foliage Gabon vipers and Rhinocerous vipers are literally invisible), and have copious amounts of very nasty hemotoxic/cytotoxic venom. A really horrible way to die as your cells swell and breakdown, making you look like a balloon, with enough pain to make one laugh. I'd take a Mamba any day ...it will probably try to get away unless I am messing around with it (and I know enough about snakes to know which ones not to mess with), and were it to bite me and I am far from medical help I will simply stop breathing as the neurotoxins shut down my nervous system and go to sleep. A Gabon viper or Rhinocerous viper, on the other hand, will be hell on earth.