Let's just clear up terminology first:
Vipers are a type of snake and belong to the group of snakes that have venomous bites. Other snakes are constrictors and kill their prey by stopping it from breathing by using their coiled body to squeeze it.
Venom is derived from the snake's saliva, and depending on the species kills in different ways. Not always by paralysis, as people who have been bitten could tell you.

 All snake are venomous. Just that some snakes, like the Viperous ones, are most venomous and therefore the most deadly of snakes. Snakes have evolved along with the prey that they kill. For the vipers, like rattlesnakes and cobras, they need to imbed their long grooved teeth vipers deep into their prey so that the venom can flow deeper into the prey. So they've needed to evolve by always taking their venom to be more powerful so as to paralyze and finally kill their prey. A snakes prey develops a resistance to their venom with time. But a snake always has a preference for a certain type of prey, and adjusts the power to kill it's choice prey accordingly.

 When they purchase special vehicles like da Porshe or VMW vrom Germany, they just as all other buyers are shown the control for the Vipers to clear the windshield, that's exactly how it happens!
Because of driver's requirements not being met by the Vipers, remember, no arms, or; no legs....never get proper training & licenses, leading directly to poor skills and.......inevitibly accidents occur. Enuf said!