I received a nice email back from the inventor:
Thank you for your interest in our product you
I developed this product in a similar situation as yours.
We backed up to state park land on a hill and I had moles, gophers
and ground squirrels. I had to extend a hose (275 ft) to get to
some of the holes.
It will work on both gophers and voles but the instructions are
little different because they burrow differently. Inside the box
are directions that are specific to each rodent.
Gophers - look for the freshest mounds of dirt and the two or
three mounds in a given area which are only a few feet apart.
They stay underground. Treat each hole for about 30 min.
Voles - leave their holes open and come in and out of their
burrow usually at night or am to feed and then return.
Treat each hole for about 20 min.
Also, I am here to answer any questions you may have
as you do this - you are not in this alone.
I am very happy he responded, and the thing is so cheap we'll bet a couple. One for each car!
We'll also try some of the other options in this thread. This gives me hope!
Suzi