Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Commercial; Flight Crew Multiengine
Flight Crew Last 90 Days 101.3; Flight Crew Total 381.1; Flight Crew Type 308.5
Confusion; Communication Breakdown; Training / Qualification
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Procedure; Human Factors
I was with my student holding short at taxiway D for a Runway (12R) crossing clearance to go back to the ramp after the last landing on 12L. It was a busy morning and soon there was another aircraft pulled up behind us. We had to wait for sometime; about 7 minutes or so while multiple aircraft taking off and landing on the runway. But then the Ground Controller started transmission 'No need to respond; I acknowledge your patience.' While I didn't see any more traffic on the final; I started moving before realizing it wasn't a crossing instruction. I pulled up and heard the controller stopping the aircraft behind us then subsequently instructed us to expedite crossing the runway. I stopped on the taxiway cleared of 12R until the Ground Controller called us again. He asked what happened and was answered with apologies that we misunderstood. Then we got the taxi instruction to the ramp. I may have got little complacent with the operations at this airport and didn't pay much attention to different instructions. The phrases like 'Thank you for patience' were with runway crossing clearance in many cases. And often I heard 'no need to respond' whenever a controller makes multiple aircraft crossing a runway at the same time. But they never should be confused or assumed to be a clearance.
Pilot thinks he has clearance to cross an active runway; then while crossing; questions if he really did.