AH wrote:

What is the deal with chains? I'm from Australia, but live in London, and
as it rarely snows in both places, I have never really driven in snow..

At what point should you put them on? If there is 2 inches of snow on the
ground is it really necessary to fit them at that point?

In California, unless you have a 4WD, you put on chains when the
highway folks tell you to. Here's the state's Winter Driving page:

http://www.dot.ca.gov/hq/roadinfo/wntrdriv.htm

Note that every vehicle must carry chains, even 4WD with snow tires.
I've been driving in the California mountains for years in a Jeep
and have never put on chains. I still carry some, to show the
Highway Patrol that I have them.
--
terry morse Palo Alto, CA http://ski.terrymorse.com/