I am a wheelchair user myself so I know the problems - albeit here in
the UK we have nothing like the winters you have!
I have had a look and there are three main products. They range from
mats, grippers, to heated rubber tracks which are obviously the most
expensive. Mats are sometimes suitable, but the deeper they are, the
harder it is to propel the wheelchair over it. Here are examples of
the products but not necessarily in your location.
Grippers. (explore each of the sites to see the different types on offer)
(include all the links on the bottom right> Non-Skid Information.
Mats. These seem to be laid over the snow?s surface so may not be
suitable in your situation.
This is a heated mat called Heat Trak. Costings etc. are supplied on
the page. There is an example of a wheelchair user.
Similar product from Martinson-Nicholls
This site on ramp construction suggest that, ?on wooden ramps,
treatments that are used include commercially-available "grit" tapes,
strips of rolled roofing or shingling, or laying down coats of
polyurethane into which sand is sprinkled.? But they do not make any
suggestions about dealing with severe snow or ice.
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.