A B757 GIVEN LNDG ON SDF RWY 35R BUT EXPECTING RWY 35L EXITED AT TXWY 'D' WITH CLEARANCE TO 'P' BUT TURNED ON 'D' BECAUSE SIGNAGE WAS CONFUSING.

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EXPECTING RWY 35L GIVEN RWY 35R. AFTER LANDING AND CLEARING RWY 35R WE WERE GIVEN TAXI INSTRUCTIONS; PAPA TO GULF THEN CHARLIE TO THE RAMP. AFTER READING BACK THE TAXI INSTRUCTIONS; I LOOKED UP TO SEE THE SIGNAGE AND SAW PAPA AND TURNED LEFT (ONTO DELTA) AND TAXIED TO GULF WHEN THE GROUND CONTROLLER POINTED OUT THAT I HAD NOT USED PAPA. I TOLD HIM ABOUT THE SIGNAGE AND HE AGREED THAT SOME OF THE TAXIWAY SIGNS WERE NOT VERY USEFUL. I CONTINUED TO THE RAMP WITHOUT FURTHER ISSUE. NO CONFLICTS RESULTED FROM MY WRONG TURN.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT THE SIGNAGE IMMEDIATELY VISIBLE AFTER EXITING 35R AT 'D3' IS 'P' WITH AN ARROW RIGHT AND LEFT. THE SIGNAGE FOR 'D' AT 'D3' IS NOT VISIBLE. THIS LEADS THE PILOT TO BELIEVE THAT THE FIRST TXWY IS 'P' WHEN IN FACT IT IS 'D'. THE REPORTER STATED THAT AFTER TELLING HIM OF HIS ERROR THE CONTROLLER SAID HE HAS BEEN TRYING TO GET SOMEONE'S ATTENTION FOR YEARS IN ORDER TO CORRECT THIS SIGNAGE THAT LEADS TO MANY ERRORS.