FO OF A B757 SWITCHED TO GND CTL PRIOR TO LEAVING THE ACTIVE RWY AND BEFORE BEING INSTRUCTED TO DO SO BY LCL CTL.

Narrative

WE LANDED ON RWY 16R AT SEA. THE TWR TOLD US TO CONTINUE ON THE RWY TO TXWY P AND I BELIEVE HE SAID; EXIT ON TXWY P AND CONTACT GND CTL XING; OR WORDS TO THAT EFFECT. INSTRUCTIONS ON WHEN TO CONTACT GND SEEMED AMBIGUOUS; OR INCOMPLETE. AT THE TIME; I DID NOT REALIZE MY COPLT SWITCHED TO AND CONTACTED GND CTL WHILE I EXITED RWY 16R ON TXWY P. WE ARE NOW BTWN RWYS 16R AND 16L AND ON GND CTL. DURING HIS CALL TO GND CTL I REALIZED WHAT HE WAS DOING AND VOICED MY OBJECTIONS; BUT IT WAS ALREADY DONE. GND CTL ASKED WHERE WE WERE; BUT DID NOT SEEM TOO UPSET ABOUT OUR POS BTWN RWYS AND ON HIS FREQ. HE TOLD US TO CROSS RWY 16L AND CONTINUE TO THE GATE. PRIOR TO XING RWY 16L I NOTICED THERE WAS AN ACFT TAKING THE RWY 16L. THAT WAS THE END OF THE SIT. I AM A RESERVE CAPT AND I FLY WITH A DIFFERENT COPLT EVERY TRIP. ALTHOUGH THIS WAS THE MOST BLATANT EXAMPLE I HAVE ENCOUNTERED; I FIND THIS TO BE NOT THAT UNUSUAL. IT SEEMS THERE ARE MANY COPLTS THAT WILL SWITCH TO GND FREQ AND CONTACT THEM WHILE STILL ON AN ACTIVE RWY. EMPHASIS NEEDS TO BE PLACED ON REMAINING WITH THE TWR FREQ UNTIL CLRING THE ACTIVE RWY; OR PARALLEL RWYS. THE TWR NEEDS TO BE SPECIFIC WITH THEIR INSTRUCTIONS.