I second Crandell's suggestion about the garden soil. I have used water fractioning
to extract really fine material. You need 2 large buckets for this method. Put
a spadeful of soil in on bucket, then add water. Let the bucket
overflow to get rid of light floating bits - keep vigorously mixing the soil while
doing this. Then let the bucket settle for a few seconds. This will allow the
larger particles to settle to the bottom. Then decant the water to the second
bucket. Let it settle for a minute or so, then pour out the water (discard). You
should now have a fine silty mud on the bottom of the second bucket. You can
use the pantyhose method to get some of the excess water out. Now you have
perfect "sand" for your road

It looks very realistic and if you're modeling a
local road to you, it will be prototypically correct
Adjusting the times above allows you to extract any fraction according to
particle size (settling speed really). With mutliple buckets you could pull out several different fractions at the same time.
You sift the stuff on the bottom of the first bucket for various coarser sizes
for ditches, forest floor etc.
-ik