ACR X TCASII RA AFTER TKOF WITH DH-8. EVASIVE ACTION TAKEN. VISUAL SEPARATION IN USE.

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ACR X HEARD THE TWR CLR THE NOW AIRBORNE DHC-8 TO TURN R TO CROSS THE DEP COURSE OF RWY 18. THEN TWR CHANGED HIS MIND AND SAID SOMETHING TO THE EFFECT OF 'OH; THAT'S NOT GOING TO WORK; TURN BACK TO YOUR ORIGINAL HDG' TO THE DHC-8 CREW. TWR CLRED X FOR TKOF. I WAS WATCHING THE DHC- 8 AND REMEMBER HEARING TWR ASK THE DHC-8 CREW IF THEY COULD MAINTAIN VISUAL CONTACT ON US. AFTER AN AFFIRMATIVE REPLY FROM THE DHC-8 CREW. TWR CLRED THEM TO MAINTAIN VISUAL SEPARATION FROM X AND CLRED THE DHC-8 TO MAKE THE R TURN. WHILE X WAS STILL ROLLING DOWN RWY 18; TWR ADVISED US DURING THIS CRITICAL PHASE OF FLT THAT TFC WOULD BE XING OUR DEP COURSE; MAINTAINING VISUAL ON US. I SAW THE DHC-8 BEGIN HIS SECOND R TURN JUST AS WE LIFTED OFF FROM RWY 18. THINGS WERE HAPPENING RAPIDLY. IN MY JUDGEMENT THE DHC-8'S INITIAL RATE OF TURN AND CLB MIGHT HAVE PRESENTED A HAZARDOUS SIT. AS WE CLBED THROUGH ABOUT 500 FT AGL I WAS NOT COMFORTABLE WITH THE RELATIVE POS OF THE DHC-8 SO I BEGAN TO REDUCE OUR RATE OF CLB. WITHIN A FEW SECONDS; PASSING 700 FT AGL; OUR TCASII ISSUED 2 'REDUCE CLB' RA'S; IMMEDIATELY FOLLOWED BY AN RA 'DSND.' I WAS FLYING X. AS I BEGAN TO REDUCE OUR RATE OF CLB DURING SECOND SEGMENT; MY FO PICKED UP HIS MICROPHONE. WHEN WE RECEIVED THE TCASII RA; HE IMMEDIATELY NOTIFIED THE TWR THAT WE WERE RESPONDING TO AN RA; AT WHICH POINT THE TWR CTLR SAID IN A VERY ANNOYED VOICE; 'DISREGARD; THE DASH 8 HAS A VISUAL ON YOU.' I MAKE NO ACCUSATIONS; BUT I BELIEVE THAT TOO MUCH WAS HAPPENING IN TOO SHORT A TIME. BELIEVING THAT THE DHC-8 HAD ORIGINALLY BEEN TURNED BACK L TO AVOID OUR CLB; I ACCEPTED THE CLRNC TO TKOF. VERY SOON THEREAFTER I FOUND MYSELF THINKING; 'OK; THE DASH 8 DRIVER IS GOING TO MAINTAIN VISUAL ON US; BUT THIS LOOKS LIKE IT WILL BE AWFULLY CLOSE TO ME.' I BELIEVE THE ATC TWR AT DCA COULD HAVE EASILY AVOIDED A POTENTIALLY HAZARDOUS SIT BY 1) NOT ALLOWING A SIT TO DEVELOP THAT WOULD REQUIRE ROUTINE TA'S TO ACFT IN CRITICAL STAGES OF FLT; E.G.; TKOF ROLL. 2) NOT REQUESTING THE CREW OF AN ACFT TO MAINTAIN VISUAL SEPARATION FROM ANOTHER ACFT IN A CRITICAL PHASE OF FLT (E.G. SECOND SEGMENT CLB) WHEN IT IS IMPOSSIBLE TO PREDICT THAT ACFT'S CLB PATH. 3) REMEMBERING THAT A PLT'S OPTIONS ARE SEVERELY LIMITED WHEN HIS ACFT IS LOW; SLOW; AND CONFIGURED FOR TKOF OR LNDG.