Has a pilot committed a runway incursion if they taxi across a runway that ATC did not clear them to cross, but no aircraft were using that runway?

A pilot has indeed committed a runway incursion by taxiing across a runway without clearance from ATC, regardless of whether there were any aircraft using that runway at the time. Runway incursions are defined as any unauthorized presence of an aircraft, vehicle, or person on a runway, which can lead to potential hazards, even if no other traffic is involved.

The purpose of requiring ATC clearance to cross a runway is to ensure safety and maintain organized traffic flow, preventing any unauthorized movements that could lead to dangerous situations. In this case, the pilot's action of crossing the runway without clearance constitutes a violation of air traffic control procedures, thus classifying it as a runway incursion.

Understanding that safety protocols are in place to protect all operations on the airfield is critical. ATC is responsible for maintaining situational awareness and managing the flow of traffic, which involves issuing clearances and ensuring that all personnel on the ground adhere to established procedures. This helps prevent incidents that could arise even in seemingly quiet situations.