the speed of the forklift is controlled by the input from the accelerator pedal.
what you are asking is for it to 'sense' a pre-chosen area via sensor and to slow down truck speed automatically?

This would be an operation outside of the function of the truck capability and is not possible with current technology on any truck that is built in.
You would need an outside device wired into the truck system but again i am not aware of any such device without having to go to a drive 'on wire' type of setup which would be very expensive to install and setup.