The 'ladder' superstition, apart from the practical safetly element, comes from the previous centuries when law and order - and punishment - was a lot more instant. A lynch mob could decide to hang some poor indikvidual, and the quickest way to do so would be to bang a ladder against a wall, throw the rope over, and swing the individual. Therefore, anyone walking under the ladder could be seen as trying to save the poor wretch, and may suffer the same fate - thus, walking under ladders is seen as unlucky.
I have no superstitions what so ever - I do walk under ladders if no-one is up them, just because I can.