The installation of loop detectors is primarily done:

The installation of loop detectors is primarily done below the asphalt surface. Loop detectors are embedded in the pavement to monitor vehicle presence and improve traffic signal control. By being positioned beneath the asphalt, these detectors are shielded from environmental factors and physical damage that could occur if they were exposed above ground.

When placed below the surface, the loop can detect changes in inductance caused by the presence of a vehicle over it, signaling the traffic management system accordingly. This placement is ideal for creating seamless vehicle detection that ensures efficient traffic flow and proper signal functioning without the interference of weather, debris, or roadside activities.

The other scenarios, such as placing detectors above ground or within buildings, would not effectively serve the purpose of traffic signal control, as they would fail to accurately monitor vehicle movements or presence on the roadway. Therefore, the sub-surface installation of loop detectors is the most effective and standard practice in traffic signal management.