When exit lights and way point signs are going to be used (as in during a fire, or such), they should be down near a skirting board hieght.
You can't stand up in a smoke/heat-laden atmosphere for long.
The number of Fire deaths caused by this annually would prove this.
Considering that smoke will spread along the upper parts of a compartment in a building and move downwards towards the floor, would suggest to me that our thinking of Escape lighting is backwards.
Just my opinion though.