ATCT CTLR WORKING LCL AND GND CTL POS COMBINED AND HAD JUST ASSUMED THOSE POS; CLRED A VEHICLE TO CROSS THE ACTIVE RWY AS A C130 WAS STARTING HIS TKOF ROLL. THE C130 ADVISED TWR HE WAS ABORTING TKOF DUE TO VEHICLE ON THE RWY.

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ON SEP/THU/96; I WAS DIRECTLY INVOLVED IN AN OPERROR THAT OCCURRED AT XA41 AM MDT. THE INCIDENT IN QUESTION OCCURRED IMMEDIATELY AFTER I ASSUMED CTL OF THE LCL/GND POS. C130 HAD JUST BEEN CLRED FOR TKOF ON RWY 35 BY THE PREVIOUS CTLR AND PRIOR TO THE POS RELIEF BRIEFING. AFTER THE BRIEFING; I ASSUMED CTL OF THE LCL/GND POS. A GND VEHICLE REQUESTED CLRNC TO CROSS RWY 35 AT MID FIELD. UNDER THE ASSUMPTION THE C130 WAS NOT YET CLRED FOR TKOF; I CLRED THE VEHICLE TO CROSS RWY 35. THE C130 THEN ADVISED OF THE ABORTED TKOF DUE TO A VEHICLE ON THE RWY. THE POS RELIEF BRIEFING THAT I RECEIVED WAS COMPLETE AND CONCISE. TFC WAS LIGHT; WITH ONLY 3 ACFT ON FREQ AND A GND VEHICLE. THE C130 BROUGHT THE INCIDENT TO MY ATTN BY ADVISING HIS ABORTED TKOF. MY PERCEPTION OF THE TFC PICTURE WAS THAT THE C130 DID NOT HAVE A TKOF CLRNC. A HEARBACK PROB DURING THE POS RELIEF BRIEFING PLAYED A CONTRIBUTING FACTOR IN THE INCIDENT. THE PERCEPTION OF WHAT I HEARD DURING THE BRIEFING; WAS IN ACTUALITY; NOT WHAT WAS COMMUNICATED. IN ADDITION; A BREAKDOWN IN VISUAL SCANNING TECHNIQUES ALSO PLAYED A ROLE. HAD I SCANNED THE RWY VISUALLY PRIOR TO ISSUING THE XING CLRNC; IT WOULD HAVE ALERTED ME THAT AN ACFT WAS IN POS FOR TKOF. LACK OF AWARENESS; RATHER THAN POOR JUDGEMENT WERE PRIMARY FACTORS IN THE INCIDENT. ON THAT DAY; PRIOR TO RPTING FOR WORK; I CONSIDERED REQUESTING SICK LEAVE DUE TO A PHYSICAL ILLNESS. MY DECISION WAS TO RPT FOR WORK. COGNITIVE AWARENESS WAS SOMEWHAT REDUCED BY THIS CONDITION AND I OVERESTIMATED MY ABILITIES TO FUNCTION UNDER THESE CIRCUMSTANCES. IN FUTURE; ESTABLISHING GOOD READBACK/HEARBACK TECHNIQUES AND INCORPORATING CONSISTENT SCANNING TECHNIQUES; I BELIEVE WILL HELP ELIMINATE FUTURE INCIDENTS SUCH AS THESE. UTILIZING SICK LEAVE THAT IS PROVIDED SHOULD BE ENCOURAGED AND UTILIZED WHEN PHYSIOLOGICALLY ONE IS NOT FUNCTIONING 100 PERCENT.