Car tires tend to lose air pressure in colder temperatures, and tend to "flatten". Will the traction in my minivan be better if the tires are fully inflated or allowed to remain slightly flat?

I am assuming that because there would be slightly more surface area on a flatter tire, that the traction would be better, and that a harder, more inflated tire would tend to "ice skate" more.

While you may get better traction, mileage will go down and there will be a lot more wear on the tires as well as possible sidewall damage. The small increase in traction will not make up for the extra cost of having to replace your tires early.

Best thing to do is to keep the tires properly inflated. Invest in a good set of snow tires and chains. (consider studs if they are legal in your area)

tire pressure is intended to be measured at "normal" temperatures - you don't add air as the weather gets cold and remove it as the weather warms up (in the course of a day, anyway - seasonally, check your pressure in normal daytime temperatures)

that said, an underinflated tire only helps traction in soft surfaces: I.E. sand, mud, deep snow. otherwise underinflation actually reduces your effective "traction patch" as the natural flex of the tire actually makes it slip, or scrub, as it turns.

in argument to VW's recommendation, use modern studless snow tires, over studded tires. the new compounds used in studless tires enhance your ice traction without damaging road surfaces.

ok. so there are two ways a tire loses traction. one is over inflation, this causes the tire to bulge outward, leaving only the center touching the ground. the other is underinflation, leaving only the edges touching the ground. both cause loss of traction and uneven wear of the tire. so really, if you keep your tires properly inflated through out the year, you should have the same traction through out the year. like Lightworks said, check your pressure during the day, in what would be the average temperature. and use traction devices if you know you will be facing snow and ice.

I own a 1999 Subaru Impreza, and when I bought new all season tires in October, I was told, by the salesperson, that my Subaru, because it is all wheel drive, doesn't need snow tires.

A snow tire works better in snow than an all season tire. You have to decide whether your driving conditions merit snow or ice tires.

All wheel drive helps in the winter but only under acceleration, not braking or cornering.

it kinda depends on your climate: in m area it freezes or snows two or three times a year (enough to have bad road conditions) any normal person can safely get around with all season tires, or knows better than to try to drive before it warms up. in contrast, in the high country, there are frozen driving conditions most of the winter, and snow tires definitely make a difference

Traction will vary with road-surface. Under-inflated tires will typically provide more traction through deep snow, as it can conform better to the surface, however on ice it could go either way. Properly inflated the sidewalls and center should have equal load-bearing. Over-inflate and the edges lose contact, under-inflate and the center loses contact but the tire also "rolls out" more.

The problem with under-inflated tires is the loss of stability at speed and when cornering. (increased body roll)

I have tested pressure in the dry with a 500HP vehicle. With "street" tires, less pressure seemed better, but with "Performance" tires, (e.g. Pilot Sports) the traction was best at the rated inflation.