PLT IN BE90 AT TYS INSTRUCTED TO CROSS RWY 23L AND TAXI TO RWY 23R. PLT TAXIED ONTO RWY 23R IN FRONT OF A DC9 PREPARING TO DEPART.

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A KING AIR WAS TAXIING OUTBOUND FROM FBO. HE WAS INSTRUCTED TO 'TAXI RWY 23R; CROSS RWY 23L AT TXWY A6.' THE ACFT TAXIED ACROSS RWY 23L AS INSTRUCTED AND THEN CONTINUED VIA TXWY B5 OUT INTO THE MIDDLE OF RWY 23R AT TXWY B5. THEY TAXIED JUST BEHIND A CLRING BARON AND OUT IN FRONT OF A DC9 THAT WAS IN POS AWAITING THE BARON TO CLR. THE ONLY THING THAT SAVED IT WAS THAT I WAS WORKING BOTH GND AND LCL AND I OBSERVED THE KING AIR'S TAXI SPD TO BE TOO FAST. THE KING AIR ENTERED THE RWY JUST AS I OPENED THE MIKE TO CLR THE DC9 FOR TKOF. IF THE CTL TWR WOULD HAVE BEEN OF A SUFFICIENT HT FOR ME TO SEE THIS SOONER; I COULD HAVE INTERVENED AND KEPT THE PLANE FROM ENTERING THE RWY. HOWEVER; I COULD NOT TELL IF THE PLANE WAS GOING TO ENTER THE RWY UNTIL IT WAS TOO LATE TO STOP IT DUE TO THE SHORTNESS OF THE TWR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT IT IS DIFFICULT TO CLRLY SEE TFC ACROSS THE FIELD FROM THE TWR; ESPECIALLY AT NIGHT. TXWY A6; FOR EXAMPLE; IS DIRECTLY ACROSS THE RWYS FROM THE TWR LOCATION. IT IS DIFFICULT TO DETERMINE IF ACFT ON TXWY A6 ARE HOLDING SHORT OR MOVING. THE E SIDE OF THE FIELD IS LITTERED WITH LIGHTS MAKING IT MORE DIFFICULT TO DISCERN BUILDINGS AND VEHICLES FROM ACFT DURING NIGHT OPS. THE PRESENT TWR STRUCTURE WAS CONSTRUCTED IN 1988 AND THE RPTR BELIEVES FUTURE ARPT EXPANSION WAS NOT SERIOUSLY CONSIDERED AT THE TIME. HE ALSO SAID THERE HAVE BEEN QUITE A FEW RWY INCURSIONS AT TYS THAT MIGHT HAVE BEEN PREVENTED IF THE TWR WAS TALLER.