DA10 CREW AT TYS WERE INSTRUCTED TO TAXI TO RWY 23R; HOLD SHORT OF RWY 23L. CREW CROSSED THE RWY WHILE A PA28 WAS LNDG.

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THE FALCON REQUESTED TAXI AND WAS TOLD 'RWY 23R HOLD SHORT OF RWY 23L.' THE PLT READ BACK 'RWY 23R; HOLD SHORT OF RWY 23L.' (TAPE VERIFIES THIS.) THE PA28 WAS CLRED TO LAND ON RWY 23L. THE PA28 LANDED AND WAS ROLLING OUT WHEN I OBSERVED THE FALCON XING RWY 23L. THE PA28 WAS TOLD TO STOP SHORT AND GIVE WAY TO THE FALCON WHICH HE DID. THE FALCON PLT WHEN ASKED SAID HE WAS CLRED TO CROSS (TAPE SHOWS OTHERWISE). A CONTRIBUTING FACTOR WAS THE HT OF TYS TWR. OUR TWR IS SO LOW THAT WE CANNOT SEE AND DIFFERENTIATE MOVEMENT ON THE RWY OR TXWY ON THE FAR SIDE OF THE FIELD WHERE THE FBO'S AND ACR RAMPS ARE. THIS HAPPENED DURING THE DAY. AT NIGHT IT IS NEARLY IMPOSSIBLE TO SEE MOVEMENT ON TXWY A AND RWY 23L. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE TWR IS 84 FT TALL TO THE FLOOR OF THE CAB. HE SAYS CTLRS CANNOT SEE SMALL ACFT AT VARIOUS TXWY LOCATIONS ACROSS THE FIELD. RWY 5 IS 26 FT LOWER THAN THE TWR AND SMALL ACFT CANNOT BE SEEN NEAR THE APCH END OF THE RWY. AT NIGHT IT IS VERY DIFFICULT TO TELL VEHICLES FROM ACFT ON THE OTHER SIDE OF THE FIELD. HE ALSO SAID THAT RWY 23L; EXCEPT FOR THE FIRST 3400 FT; HAS BEEN CLOSED FOR ABOUT 1 YR FOR CONSTRUCTION. IT IS EXPECTED TO STAY CLOSED FOR APPROX ANOTHER YR AND ONCE COMPLETED THEY WILL WORK ON RWY 23L/5L. HE BELIEVES THAT A LITTLE MORE HT ON THE TWR WOULD GREATLY IMPROVE SAFETY AND HELP REDUCE THE OCCURRENCE OF RWY INCURSIONS.