Whenever I see a new bridge or a sliproad being constructed, I often see 
lines of wooden posts. And I have no idea what their purpose is. Each 
one is approximately upright, with a smaller horizontal piece of wood 
nailed to it.

You'd think this is to do with some kind of measurement or layout 
process. However, the posts are all wonky and carelessly arranged, 
they're not all the same height, and the horizontal piece is only at 
approximately a right angle. In short, rather than being carefully and 
precisely measured, this stuff seems to be haphazardly thrown together. 
So... what's it for?

Similarly, you often see much larger wooden posts, painted with red and 
white stripes. What do those do?

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