But if we cared about the people in the cars (wich I do), winter tires (studded or not) is number one priority.

If you use non studded ones they don´t increase road wear, but you could reduce the use of salt massively - alongside the frequence of plowing the road (also road wear).

If the risk of getting killed in an accident (in Sweden that is) decreases with 43% when using studded winter tires instead of non studded, what is the number if you use summer tires I wonder.

I agree with kombat, security for the people in the cars is the most important thing. If you really care about the enviroment - why drive in the first place?

And as I said above, it's a lot more than just the trees -- when you look at the impacts on aquatic ecosystems and drinking water reservoirs it becomes a bigger question of balancing the safety of people who demand that our taxes pay for road treatments that allow them to drive in icy conditions versus the public expense of removing salt contamination from drinking water, and the longer-term negative impacts on society from the degradation of natural ecosystems.

It's easy to get lulled into thinking that human systems operate independently from natural ones, but all you have to do is consider, for example, the impact of the crash in bee populations on agriculture to see that it's not true. Most fruits don't grow without pollinators (and by fruits I'm using the botanical term, so we're talking tomatoes, beans, peas, etc. as opposed to simply oranges and apples), and now farmers have to pay to have their plants pollinated because bee populations have been decimated. Those costs get passed on to consumers. I'm not drawing a connection between salting roads and dying bees, just illustrating that it's easy to say "why care about a few trees" when in fact the potential costs and ripple effects could be greater than that.

Another datum for the conversation. I grew up and learned to drive in upstate New York. I drove for four years in/around Anchorage, Alaska with studded tires on my Honda Prelude, and then another couple years in northern Vermont with it. No accidents or wipeouts. Then my ex-wife got the Prelude in the divorce, and since then I've had a series of steadily-better all-wheel-drive small SUVs with good all-season tires on them, mostly in southern New Hampshire. Also no accidents or wipeouts.

My observations:
#1 - the first significant snowstorm of the season, no matter whether it is in serious-winter country or not, and whether it is really early for the area or really late, most people seem to forget how to drive in it.
#2 - Even in serious-winter areas, there's only a day out of every ten or twenty that counts as winter driving conditions. For that reason, I prefer local road maintenance crews to use salt and plows (they've started using a slurry liquid to pretreat roads in some circumstances and that's worked well too) rather than me leaving studded tires on my SUV for four months out of the year, if it has to be an either-or decision. If I lived in the middle band of the US where they get a storm or two each year, I'd probably get the studded tires just because the local infrastructure isn't built to handle snowstorms. I just spent a week in Little Rock during their ice/snowstorm, and after two days their roads were still worse than two hours' after the same storm left New Hampshire.