Why isn't 'neige' in the plural in the clause "n'oubliez pas d'installer vos chaînes neige sur vos pneus !"

As Jim notes, seems to be 'informal' usage for chaînes à neige 

https://www.pneus-online.fr/chaine-neige.html

Hi Jillian,

I can find no reference for the noun neige being employed as an adjective.

To equip a vehicle with snow chains we could use the verb chaîner.

My other thought is that "chaînes-neige" may be a compound noun or perhaps "chaînes à neige"

Sorry, but am unable to help further  --  I look forward to reading offers from other contributors.

Bonne Journée.

Jim