CTLR AT GJT CLRS ACFT COMMUTER TO DEPART E AND CLRS AN F18 TO USE THE W RWY. BECAUSE OF A DIFFERENCE IN SPDS OF THE 2 ACFT; A CONFLICT OCCURRED APPROX 2 MI E OF THE ARPT.

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AT APPROX XA40 LCL; ACFT Y (DH8) IFR TO PHX; VFR CLB; WAS CLRED FOR TKOF RWY 11. AS THE ACFT WAS TAXIING ON A2 FOR TKOF; ACFT X (F18) CALLED 15 MI E OF THE ARPT. I INSTRUCTED ACFT X TO RPT INITIAL FOR RWY 11; HE THEN REQUESTED RWY 29. I INSTRUCTED ACFT X TO RPT INITIAL FOR RWY 29 AND ISSUED TFC TO ACFT X AS ACFT Y WAS TURNING ONTO RWY 11 FOR TKOF; ANTICIPATING ON THE F18 TRAVELING AT 250 KTS AND SLOWING DOWN TO 200 KTS IN CLASS D AIRSPACE IN ACCORDANCE WITH FAR 91.117. WHEN THE DH8 (ACFT Y) WAS ABEAM THE TWR I NOTICED THE F18 TURNING INITIAL ON THE BRITE SCOPE. I LOOKED SEVERAL TIMES TO THE E TO VISUALLY OBSERVE THE F18 WITHOUT SUCCESS. AS THE F18 WAS TURNING INITIAL I NOTICED HIS TARGET TRAILS TO BE APPROX 1/2 MI APART; OR 450 KTS. I AGAIN ADVISED THE PLT OF THE F18 OF THE DH8 TFC DEPARTING RWY 11 OVER THE THRESHOLD OF RWY 11. PLT ACKNOWLEDGED BOTH TA'S; AND ISSUED THE TFC TO THE DH8 PLT ALSO. I EXPECTED THE F18 PLT TO BE EXPERIENCED AND INTELLIGENT ENOUGH TO SIDE STEP TO THE L OR R OF THE INITIAL COURSE AND NOT PROCEED STRAIGHT IN OVER THE CTRLINE OF THE RWY. ALTHOUGH I ATTEMPTED TO MAKE VISUAL CONTACT WITH THE F18; I DID NOT OBSERVE THE ACFT UNTIL THE F18 WAS OVER THE THRESHOLD. HOW CLOSE THEY CAME I DON'T KNOW. BASED ON THE RADAR DATA; AND A REMARK BY ANOTHER CTLR THAT THE F18 WAS HEAD ON WITH THE DH8; I INSTRUCTED THE F18 PLT TO PROCEED SBOUND TO AVOID THE TFC AND INSTRUCTED THE DH8 PLT TO PROCEED STRAIGHT AHEAD; BUT BY THE TIME I FINISHED THE XMISSIONS; THE F18 WAS OVER THE THRESHOLD OF RWY 29 AND THE DH8 WAS 2 MI E OF RWY 11. THE SUPVR ACCUSED ME OF NOT BEING AWARE OF MY ENVIRONMENT AND INDICATED THE SPD OF THE ACFT WAS A FACTOR IN THIS CONFLICT; BUT I COULD HAVE PREVENTED THE CONFLICT. ALTHOUGH I COULD HAVE PREVENTED THE CONFLICT THE SPD OF THE ACFT WAS NOT A CONTRIBUTING FACTOR. THE CONFLICT WAS A DIRECT RESULT OF THE SPD OF THE ACFT. NO ONE HAD TIME TO REACT BECAUSE OF THE SPD OF THE ACFT. THERE WOULD HAVE BEEN PLENTY OF TIME TO REACT IF THE F18 PLT WAS OPERATING THE ACFT IN ACCORDANCE WITH FAR 91.117.