In uk.transport, John Rowland wrote:
A solution could be to give the different styles of bollard different
asymmetrical shapes at the bottom, with matching differently shaped
metal frames set into the concrete - then it would be impossible to
switch them or put them in the right hole the wrong way around.

That seems rather inflexible. I'd have a sticker on the diffuser showing
the proper sign. And, for good measure, which way it should face. :-)

--
Mike Barnes