I too have 15" Agilis'.  I wouldn't trust them in real snow and I would
recomend cable chains over link-chains, as the clearence to the rear
trailing arms is very tight.  Best to get chances that are rated for limited
clearance.

Last winter I drove through a bit of snow on my way home from Colorado on my
Agilis' and while I carreid chains I never used them.  I went through a
couple areas that where they were requireing tractor trailers to chain up in
eastern Utah, but never passenger vehicles.  A lot of 2wd passenger vehicles
were putting on changes anyway.  Regardless, with 4x4 and the good sense to
slow down you'll rarely if ever "need" chains on a major interstate.  That
said, you "might" be required to at least carry a set during bad weather.
Local roads are a different story, in which case you might legitamately NEED
chains on steep and icy local roads which are not regulalry plowed and are
likely steeper than anything you'll see on the interstates.

One last thought, if you drive through areas that salt thier roads, be sure
to wash it off when you get the chance including a through washing of the
underside of your van.

-Jon