Winter bike / Snow bike thread

I loved ridding around at the weekend. This weather makes the bike worth it. I dropped my tyres down to about 40psi. I'm a bit worried about punctures in the city.
I've got 28c schwalbe marathon tyres that say min inflation 85 psi (max 115 psi). I've got them at 90 and there's very little give. Is it safe/worthwhile to run lower than that? I don't weigh much.