Left: Common Adder - head with small scales - eyes with vertical oval pupils.
Left: Asp Viper - head with small scales - eyes with vertical slit pupils.
The strike range or distance for a snake to bite is generally reckoned to be between one third and one half of total body length.
What this means in practice is that you would be completely safe at a distance of 1 metre from even the largest fully grown Viper in France, in fact 50cm would be sufficient distance.
To be clear we can see from this that a 40cm snake can't bite at a distance of more than 20cm and as such most bites in France result from a snake being brushed against or touched accidentally in some manner.