CL65 TAXIING FOR DEP AT BWI ENTERS RWY 28 TXWY E WITHOUT ATC CLRNC.

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UPON TAXI OUT OF GATE AT BWI; WE WERE GIVEN INSTRUCTIONS TO TAXI TO RWY 28. COMING OUT OF ALLEY BTWN PIER C AND WESTERN TERMINAL PIER AND W OF ISLAND N OF TXWY A. RATHER THAN MAKE THE TURN ON TO TAXI 'P;' WE CONTINUED ON 'E.' JUST AS THE NOSE CROSSED THE LINE; I REALIZED THE MISTAKE AND STOPPED THE ACFT. GND CTL ALSO NOTIFIED US TO STOP; AND ASKED IF WE COULD STILL MAKE THE L TURN. WE WERE ABLE; AND COMPLIED. THERE WAS AN ACFT IN POS AND HOLDING AT THE END OF RWY 28. CONTRIBUTING FACTORS: 1) THE TRIP WAS A 'STAND-UP' OVERNIGHT WITH LESS THAN 6 HRS AT THE HOTEL. 2) I HAD DONE THIS EXACT TRIP FOR THE LAST 7 WKS; THIS IS THE FIRST TIME WE CAME OUT OF THE ALLEY W OF THE ISLAND BTWN TXWY A AND THE RAMP. WE HAD ALWAYS PUSHED BACK TO DEPART THE RAMP BTWN THE 2 ISLANDS. 3) TXWYS ARE NOT CLRLY MARKED; AND ARPT LAYOUT IS CONFUSING. 2 RWYS ( RWY 10-28 AND RWY 4-22); AND MULTIPLE TXWYS (A; P; P1; E; U; D) ALL INTERSECT IN THAT AREA. ADDITIONALLY; TXWYS CHANGE IDENTITY (P TO P1 TO U); AND WRAP AROUND THE TERMINAL (A; P). I DID NOT SEE ANY TAXI SIGNS MARKING THE RWY OR DIRECTION OF TXWYS. THEY MAY HAVE BEEN COVERED WITH SNOW. BALTIMORE GOT 2+ FT OF SNOW EARLIER IN THE WK. THIS AREA IS A RWY INCURSION HOT SPOT.