DEPARTING HVY JET OBSERVES CONFLICT WITH PREVIOUS DEPARTING MD80. L30 REQUESTS IF IN TRAIL ACFT CAN PROVIDE VISUAL SEPARATION FROM MD80. PIC OF IN TRAIL ACFT BELIEVES THIS WAS POOR CTLING.

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MD80 (ACFT Y) TOOK OFF AHEAD OF US (ACFT X) ON RWY 25R AT LAS BY APPROX 1 1/2 MINS AND CONTINUED ON RWY HDG TO THE W FOR UNKNOWN DISTANCE. WE WERE CLRED FOR TKOF AND FLEW OVETO 3 DEP SID. ON FIRST CALL; DEP CTL CLRED US TO 15000 FT. WE OBSERVED THE MD80 APPROX 4 MI TO THE W OF US STARTING A R TURN AND APPEARED TO BE DEPARTING NBOUND. APPROX 1/2 WAY THROUGH OUR R TURN TO 070 DEGS; WE SAW THE MD80 CONVERGING ON OUR FLT PATH. DEP CTL ASKED IF WE HAD MD80 IN SIGHT. WE REPLIED 'AFFIRMATIVE.' DEP CTL ASKED US TO 'MAINTAIN VISUAL SEPARATION' AND TO CONTINUE CLB TO 15000 FT. WE INCREASED RATE OF CLB AND THE MD80 CONTINUED TO TURN (AND CLB) TOWARDS OUR FLT PATH; AFTER WE WERE ON A 070 DEG HDG AND CROSSED APPROX 3000 FT IN FRONT OF US AND 800-1000 FT BELOW US. WE WERE CLBING AT A VERY RAPID RATE. WE RECEIVED A TCASII TA AND AN RA BRIEFLY (ADVISING 'MONITOR VERT SPD') AS WE WERE CLBING. I DO NOT UNDERSTAND HOW THIS CONFLICT WAS ALLOWED TO DEVELOP AND WAS NOT RESOLVED EXCEPT FOR THE REQUEST TO 'MAINTAIN VISUAL SEPARATION' BY DEP CTL. THIS WAS IN A FULL RADAR ENVIRONMENT. APPARENTLY THE MD80 FLEW WELL BEYOND THE 4 DME TURN POINT; AND DEP CTL DID NOT NOTICE. IF WE HAD NOT BEEN LIGHT ENOUGH TO INCREASE OUR CLB RATE AS MUCH AS WE DID; THIS WOULD HAVE BEEN A VERY CLOSE CONFLICT.