How does 非机动车 (fēijīdòngchē) mean "bicycle"?
It doesn't only means bicycle.

"非机动车" is non-motorized vehicle, NMV, including tractors,human cargo vehicles, animal-drawn vehicles, etc., according to the explaination of "非机动车" in Baidu Baike. 

So bicycle is only one sort of NMVs and is used to reprensent NMV on the sign because it's very common.