Painting yellow cross hatchings on the left side of each step would
be self explanatory, though expensive.
Huh? That ought to mean "Do not use the left side of each step."
Over here cross hatchings on the road mean "do not enter unless your exit is
clear" so the principle is the same. It's also used in pedestrian areas to
stop people waiting where crowds could build (I think the forecourt of kings
cross is currently so painted and it seems to work, I've seen it somewhere
anyhow).

D