So, a typical snail moves at around 0.0048 km/h, and are around 1 cm tall. The snail in the image appears to be more than 3-4 stories tall, let's say 15 m. If the speed scales linearly with size, this snail could move at around 7.2 km/h.

So yeah, a very brisk walk for your life should just about keep you out of reach if you're in decent shape.

Spread salt on the road.

People can survive a Snailpocalypse.

The Turtlegeddon, not so much, because turtles are actually surprisingly fast.

Glad to see medieval era humor still alive and well

At least I can stop by the store and get some groceries while I’m fleeing.

This reminds me of "day of the trifids" where the world is "invaded" by evil, aggressive,s walking plants. Not very frightening on its own, but at the same time a meteor flew over earth that blinded most of humanity leaving only a hand full sighted. That changes the premise quite a bit.

Invader Zim did it first

Genius

A snail of that size would likely be much faster than the fastest human.

Source: my brain processing the thought in 3 seconds and spewing out this probability as "meh, speed scales with size in some ways"

A world where all humans are immortal until they get touched by the giant genius snail who knows their location at all times.