Taggent Material To Indicate Roadbed Digging / Road Side Bombs

Ordinary traffic performs a certain amount of "gardening" on a dirt road, meaning some of the taggant _will_ be mixed into the roadbed material. How deeply depends on the exact nature of the material (sand/ clay/organic dirt, salts, concrete, asphalt, etc.) the kind and degree of traffic, the weather _and_ climate, and so on.
Worst case you get washboarding which requires periodic repair, meaning deeper penetration by the taggant. It will however be more evenly distributed than the sort of localized dugskullery you're talking about.
Hence the sort of disturbance due to traffic will be easily differentiable from that due to digging.
Just off the top of my head, how about microscopic polymer chips like those used in dynamite. Rather than the complex layering used to indicate batch numbers etc. it would be infused infused with an additive which, when exposed to UV, fluoresces in the IR, not the visible. If laid down by ground vehicle or say Predator or other drone during low traffic periods it would have a fairly even characteristic distribution, and any disturbance will be immediately visible by inspection by personnel wearing IR goggles during the day, and at night with IR goggles and the assistance of a UV lamp. Inspection could also be done by suitably-equipped drones of course.
Attempts to "sweep up" and use the dust to cover new bombs will not replicate the dust distribution as laid down originally.
Mark L. Fergerson

Emplace dense minefields under the road and to both sides. Allied vehicles dynamically inert the mines with a short-range rolling encrypted digital signal. Everybody else enjoys virgins FOB allah ("Santized For Your Protection")

Scattered about. It would be impossible to mine all the roads.
It wouldn't be too bad an idea if civilians didn't get blown up.
Bret Cahill