RWY TXWY EXCURSION ONTO ADJACENT ISLAND AS FLC MISIDENTS THE TURNOFF AREA FOR TXWY IN A NIGHT OP.

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LANDED RWY 18; TWR SAID TO USE RWY 21 FOR TURNOFF. AFTER TURNING ON 21; WE INTENDED TO TURN ON TXWY 'D.' THERE WAS TFC IN POS FOR TKOF ON RWY 18; TFC ON FINAL FOR RWY 18; AND I BELIEVE TFC FOR RWY 21 TO HOLD SHORT ON LNDG OF RWY 18 (ABOUT 2-3 MI OUT). AFTER TURNING ON RWY 21; WE TURNED ON WHAT WE BOTH THOUGHT WAS 'D;' BUT ACTUALLY THE PAVED ISLAND BTWN 'D' AND 'C' (AND 'J' AND RWY 21). OUR NOSE ENTERED THE 'OFF TXWY (BUT PAVED) AREA' BEFORE WE REALIZED THE ERROR. WE MADE A HARD R 180 DEG TURN BACK TO RWY 21 AND STOPPED; CLR OF RWY 18 AT ALL TIMES. TWR SAID 'STOP' UNTIL THE 18 DEP HAD LIFTED; THEN SAID 'CONTINUE TAXI.' SUBSEQUENT RADIO AND PHONE CONVERSATIONS REVEALED: THERE ARE 3 DIFFERENT 'BRIGHTNESS' FOR ANY GIVEN LIGHT STEP SELECTED (THIS FROM DCA GND CTLR VIA PHONE AND ARPT OPS TRUCK VIA RADIO 10 MIN AFTER INCIDENT; WITH ALL LIGHTS CONFIRMED OPERATING -- NONE INOP); AFTER WE OVERSHOT THE TXWY 'D;' ANOTHER AIRLINE DID THE SAME BUT LEFT HIS TAIL ON RWY 18; REQUIRING A LNDG ACFT. TO GAR; AND IT HAPPENS 2-3 TIMES EVERY NIGHT BY PLTS WITH SUBSTANTIAL KDCA EXPERIENCE; OF BOTH MAJOR AND COMMUTER CARRIERS; AS PER THE GND CTLR. KDCA OPS SAYS IT IS 'ON THE LIST FOR A FIX;' BUT I BELIEVE IT IS AN ITEM REQUIRING IMMEDIATE ATTN IF THAT MANY INSTANCES OCCUR WITH SUCH FREQ. PLEASE USE YOUR GOOD AUTH TO URGE THEM TO REMEDY THIS SIT ASAP. THE SPACING OF THE RWY LIGHTS ON RWY 21 IS SUCH THAT THERE IS A LARGE SPACE AT THE EDGE OF THE ISLAND BTWN C AND D ON RWY 21 AND THAT 'BLACK HOLE' I BELIEVE IS WHAT SUCKERS YOU INTO SUCH A MISTAKE SUPPLEMENTAL INFO FROM ACN 288576: HE ALSO INDICATED THAT THERE ARE 3 DIFFERENT TYPES OF TXWY LIGHTS INSTALLED IN THAT AREA -- SOME BRIGHTER; SOME DIMMER. HE ALSO INDICATED THAT ACFT HAVE TAXIED OFF OF RWY 21 IN THAT AREA IN ATTEMPTING TO UNSUCCESSFULLY ENTER TXWY 'D.' I ALMOST DID TOO! THERE IS A LOT OF CONSTRUCTION AND RESURFACING BEING DONE AT DCA. A LOT OF TEMPORARY LIGHTING FOR TXWYS IS BEING USED. THE LIGHTING AT THE JUNCTION OF RWY 21 AND TXWY 'D' IS EITHER VERY CONFUSING OR INCORRECT. ANOTHER POSSIBILITY ALSO EXISTS; TXWY 'D' MAY ALSO BE INCORRECTLY DEPICTED ON DCA 10-9 CHART.