I got a pair of KETL outdoor pants and wore them over cycling undershorts last winter. The aim was to appear more 'normal' than I do in cycling tights. I think it makes a difference in how I am perceived by some drivers, some times. The Shenanigan pant in the slim style works well for cycling imo, but they are rather expensive at $100.
Plain old sweatpants are a cheaper option.
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Beneath the valley of the underbikers.