That’s not how shadow map works. There’s no “scope” - shadow camera looks in a specific direction and renders things that it sees as light obstructions.
If clarity and contrast is what you’re looking for - I’d take a look at N8AO or any other ambient occlusion postprocessing. N8AO can create faux shadows quite well, and applies them only to objects that are close to one another, regardless if it’s tiny objects or cars. Properly configured, it can create a soft shade between the car and the floor, and leave the floors underneath untouched.
Layers are a only visibility switches - they don’t affect the final “logic” of rendering the scene, ie. even if a light / shadow source is part of a layer, it’ll still affect the entire scene and all other layers.
If you’d decide to go that way, I’d first try to define the logic behind what you’re trying to achieve - esp. in more complex scenarios.