We were not talking about a MuTe to make it soft as water/mud/sand, but a MuTe to change its shape. Using Rego to make something change its shape for most things will retain that shape when the spell ends. Using Muto to change the shape of something will return their original shape when the spell ends.
Your spell as designed would leave the dirt path when the spell expired. While it would lose its hard as stone effect and would quickly become less flat, it would still be there. The duration is long enough that someone chasing you would benefit from the hard surface for speed (plus running on stone is bad for horses). And of course unless you are very careful in how you use it there is a good chance of upsetting some group (peasants, merchants, lord, faerie, etc).
The spell we were discussing would be a a Mu(Re)Te(He) spell. Muto would be used to split the path down the middle and squeeze everything that was originally there into two narrow strips on each side of your now open path. The Rego requirement is to ensure that everything on the ground (rocks, leaves, grass, bushes, small trees, etc) handles the movement without falling over or being oddly displaced, both at the beginning and end of the spell. The Rego is to ensure smooth movement/placement, not the primary means of movement.
Some of the other changes that would happen with this version is the the duration could be lowered to Diameter, the Herbam Requisite would be +0, and you still need the +1 for a Complex (or fancy as you say) effect. It would not have the hard as stone effect, but that is not a big loss for horses running on it (I have horses, you never run on blacktop or stone).
You should really try for an effect width that is roughly double what you want your path width to be. I should have been clearer in my original breakdown, since what I meant was you want 2 or 3 paces cleared which requires 4 to 6 paces of effected width.
Depth required will vary dramatically on what is in the way of the spell. If it is mostly brush, smallish rocks (not boulders), and very small trees then half a pace depth might be fine. However if there are full grown trees then you will need noticeably more depth on average. I figured 1 pace deep in the original math even if the actual path is only about half a pace of depth, leaving 500 cubic paces of dirt to handle things like the roots of larger trees. That is on the high end, so when I wrote the spell I figured 250 cubic paces of extra would be enough.
The Temporary Path of Travel
Mu(Re)Te(He) lvl 35
R: Voice, D: Diameter, T: Part
This spell causes the top quarter pace of a 250 pace long by 6 pace wide strip of ground to split down the middle and folds to the sides leaving a clear 3 pace wide path down the middle. Any rocks and other debris on the cleared path are pushed down a quarter pace into the exposed dirt. As the spell ends, the ground folds back flat and returns everything to its original position.
Note: The spell has 250 cubic paces of unused effect to handle moving large tree (specifically their roots) and larger rocks.
(Base 3, +2 Voice, +1 Diameter, +1 Part; +1 Requisite, +2 Size, +1 Complex)
A few notes:
- The path produced is a little longer than what can be ridden in 2 minutes at normal travel speed.
- At the fast (unladen rider, not breaking horse) speed you will cover the length in 1.6 minutes.
- The spell would need to be cast again every 20 (normal) or 16 rounds (fast). Because of this, this spell would be best in an enchanted device if used under stress.