In my opinion, the traction advantage of low tire pressure is limited on the street, even a snowy one. I would prefer the more stable cornering and predictable behaviour afforded by higher tire pressure. I would stay over 30 or 35 psi, but that's just my preference. Experiment a little. On my MTB, I can loose the bead of my 2.25's under 30 psi, YRMV depending on your tire, rim, and weight. With wider rims, you can safely go to lower pressure, but mine are pretty narrow.