I wear thin, windproof pants from Costco over my cycling shorts if the temp is below 60F, maybe I'll push it a bit lower before switching. When temps get down to around 40F I switch to fleece-lined windproof pants (also Costco). These are good down to the low 20's, colder than that I don't ride (feet and hands get too cold).

But I hate wearing these pants, and all spring I'm looking forward to weather warm enough that I can start wearing shorts again.

Mark