Well, whether or not one wants them to simply rest on the ground, OR if one wants to interact with them, here is a list of ways to achieve this - and their proper uses:

---Maya i.e. 3D:

Particle Instancing - The particle intstancer can take a handful of randomly-shaped objects and randomly emit them over the entirety of a given surface with or without dynamics involved. Best solution for true 3D, light/shadow/dynamics etc.

Sprites - same deal as above, but not true 3d and thus cannot hold up at extreme off-angle shots, among other caveats such as clipping, loss of lighting control over a fake-3d surface, etc.

Scatter Melscript - Same as duplicate-special, but with randomized resulting positioning. Caveats include only being able to duplicate one rock at a time, so random looks with multiple objects are extremely difficult if not impossible.

Projection Mapping - Same as sprites, however, just one or more larger pieces of geometry (the road in this case) that is textured with a painting.


---In Post i.e. 2D:

Particles - If one has access to an advanced compositor with FBX-import ability such as Fusion, Flame etc. you can import FBX geometry and simply texture your geometry there. One advantage to this method is the ability to bake-out a Normal Map to control situations where you may need to completely relight your scene in post.

Sprites - Same issues as 3D above.

Projection mapping - Same issues as 3D unless you are using an advanced compositor like above and you can generate sphere particles there and color them with some rock material or project a texture on them.