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.
Below. Side view of Aesculapian snake, round pupils, large scales on top and side of head, these features will be found on all Non Vipera snakes in France and will be harmless.
Below. Side view of the head of an Asp Viper, small scales & slit pupils are easily seen from a safe distance
Below. Top of the head of a common adder, small scales.
A quick guide to the major visible differences between the Vipera (Adders & vipers) and the other snakes to be found in France.The Vipera have vertical pupils, other snakes in France have round pupils.