L30 CTLR EXPRESSED CONCERN REGARDING TWR SEPARATION OF RNAV DEP ACFT AND APPLICATION OF VISUAL PROCS.

Narrative

THE TWR WAS USING VISUAL SEPARATION ON DEP. BOTH ACFT WERE GOING TO FLY THE SAME RTE ON DEP. THE COMMUTER CAME OFF RWY 25. I RADAR IDENTIFIED THE TFC AND WENT ON TO OTHER DUTIES. WHILE I WAS MAKING A MANUAL HANDOFF TO ZLA; THE ASSOCIATE POINTED OUT A B737 TURN INSIDE AND PASSED THE COMMUTER ACFT. I HAD NOT YET IDENTIFIED THE SECOND ACFT; BUT WHEN I DID SEE THE SIT; I ISSUED TFC TO THE COMMUTER WHO WAS NOW ABOUT A MILE BEHIND THE B737. HE HAD THE TFC IN SIGHT AND I INSTRUCTED HIM TO MAINTAIN VISUAL SEPARATION. THE OVERTAKE HAPPENED INSIDE OF 'ROPER.' THE FIRST ACFT WENT TOO FAR W BEFORE STARTING HIS TURN. THE SECOND ACFT (WHEN USING VISUAL SEPARATION) CANNOT KNOW WHAT THE FIRST ACFT IS DOING; WHICH IS WHY VISUAL SEPARATION IS NOT A GOOD IDEA WITH SUCCESSIVE RNAV DEPS. THIS HAPPENS TOO MANY TIMES TO COUNT OR CAPTURE. 80% OF THE TIME; DEPS OFF LAS ARE WORKED BY ONE CTLR WITH AN ASSIST. AT LEAST 60 MILE RANGE DUE TO THE SIZE OF THE SECTORS. DURING THIS SESSION; I WAS WORKING ASSOCIATE TAKING RUNDOWN FROM THE TWR. NUMEROUS AIR CARRIER ACFT CAME OFF WITHIN 2 MILES AND CLOSING/CONVERGING ON EACH OTHER. CTLRS ARE FORCED TO ISSUE TFC CALLS WITH ACFT CONVERGING ON EACH OTHER AND INSTRUCT THEM NOT TO OVERTAKE THE PRECEDING TFC. IT GOT SO BUSY THAT I STOPPED DEPS BECAUSE I FELT THE CTLR WAS OVERWHELMED DUE TO FREQ CONGESTION. I DO NOT BELIEVE VISUAL SEPARATION WAS INTENDED TO BE USED THIS WAY. USING RNAV; THE COMPUTER IS FLYING THE PLANE. DIFFERENT PLANES AND 'BOXES' PERFORM DIFFERENTLY.