A SAAB 340 RECEIVED AN IMMEDIATE TKOF CLRNC AND CROSSED THE INTERSECTING RWYS LNDG ACFT BY 3000 FT AT DFW.

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CLRED INTO POS AND HOLD RWY 13L. 3 JETS LAND RWY 17L. WITHOUT WARNING 'CLRED FOR IMMEDIATE TKOF; TFC 2 1/2 MI FINAL RWY 17L.' WE QUICKLY FINISHED THE LAST 4 ITEMS ON OUR CHKLIST AND STARTED OUR TKOF ROLL. I WAS PNF. I KEPT THE TFC IN SIGHT AS WE CROSSED THE EXTENDED CTRLINE OF RWY 17L. I ESTIMATE THE TFC WAS BTWN 1/2 - 1 MI FINAL. THE OTHER ACFT LANDED NORMALLY AND SAID NOTHING ABOUT SPACING TO THE TWR OR TO US. THE TWR NEVER QUESTIONED THE SPACING EITHER. ONE FACTOR THAT CONTRIBUTED TO THIS EPISODE IS THE DFW ARPT. DFW ON A VFR DAY IS OPERATED ABOVE CAPACITY. WHEN ATC CTLRS ARE FORCED TO USE 2 1/2 MI SPACING TO AVOID BACKUPS WITH TFC; THE THRESHOLD OF SAFETY IS OCCASIONALLY CROSSED. SUCH SPACING FORCES CTLRS TO WORK AT OR ABOVE THEIR CAPACITY LEVEL; AND AS A RESULT WE DID NOT RECEIVE THE NORMAL 'BE PREPARED FOR IMMEDIATE TKOF' NOTICE PRIOR TO OUR ACTUAL TKOF CLRNC. HAD I RECEIVED SUCH NOTICE IT WOULD HAVE SAVED A PRECIOUS FEW SECONDS IN OUR TKOF ROLL; AND HELP AVOID THIS ENTIRE EPISODE.