B757 CREW ENCOUNTERED CONFUSING HOLD SHORT MARKINGS FOR RWY 31C AND RWY 31R IN THE TXWY ECHO AREA AT MDW.

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WE WERE CLRED TO TAXI TO RWY 31C VIA TXWYS PAPA AND GOLF AT MIDWAY ARPT. ON TXWY GOLF BEHIND A B727 WE REQUESTED A L TURN ONTO TXWY KILO TO BE IN POS TO COMPLY WITH DEP CLRNC TIME OF XA:59 MINS PAST THE HOUR. GND CTL DENIED OUR REQUEST AND AT THE SAME TIME THE B727 TAXIED FORWARD AND MADE A 90 DEGREE R TURN ONTO TXWY ECHO 3 TO HOLD SHORT OF RWY 31C. NOW IT WAS POSSIBLE; WE THOUGHT; TO APCH THE HOLD SHORT LINE FOR RWY 31C BY TAXIING BEHIND THE B727 AND MAKING A R TURN ONTO TXWY ECHO 2. THIS IS A COMMON PROC AT MIDWAY ARPT. THE CAPT TAXIED BEHIND THE B727 AND IT APPEARED TO ME THAT WE WOULD CROSS INTO THE RWY 31R PROTECTED AREA WITH OUR L WINGTIP. SINCE I HAD SEEN THIS PROC BEFORE AT MIDWAY; AND IT WAS LATER CONFIRMED BY THE TWR THAT THIS PROC OCCURS; I DID NOT SPEAK UP. AS THE CAPT MADE THE R TURN ONTO 'ECHO 2' IT BECAME EVIDENT THAT HE COULD NOT TAXI FORWARD AND MAINTAIN A SAFE DISTANCE FROM THE B727 ON 'ECHO 3;' AS A RESULT HE STOPPED THE ACFT. AT THAT TIME I SPOKE UP AND STATED THAT I FELT WE WERE POSSIBLY ACROSS THE RWY 31R HOLD SHORT LINE. THE CAPT SAID HE DID NOT THINK WE WERE; SHORTLY THEREAFTER; THE TWR NOTIFIED US THAT WE MADE A L TURN AND CROSSED INTO RWY 31R. THE CAPT HAD VEERED TO THE L TO CLEAR THE B727 AS WE TAXIED BEHIND IT. THE TWR SAW THIS AS A L TURN. THE CAPT CONFUSED THIS WITH THE L TURN THAT WAS DENIED US BY GND CTL. THE CAPT HAD COMPLIED WITH GND CTL BY NOT TURNING L ONTO 'KILO' AS A RESULT THERE WAS CONFUSION OVER THE 2 L TURNS. A TELEPHONE CALL TO THE TWR RESOLVED THE SIT. THE CAUSE OF THIS PROB IS 2-FOLD: NO 1: LACK OF SPACE AT MIDWAY ARPT. THE CLEARWAY FOR RWY 31R CAUSES A TRAPEZOIDAL HOLD SHORT AREA FOR BOTH RWY 31R AND RWY 31C WITH THE NARROW END OF THE TRAPEZOID AT THE TRESHOLD OF RWY 31C. RWY 31C IS THE MAIN RWY FOR MIDWAY AND ALL MAJOR AIRLINES APCH RWY 31C FROM EITHER 'GOLF' OR 'ECHO'. THERE ARE 3 PARALLEL TXWY CENTERLINES IN THIS HOLD SHORT AREA NOTED AS 'ECHO 1;2 AND 3.' TFC IS ROUTED ON ALL 3 TXWYS TO ACCOMMODATE AS MANY ACFT AS POSSIBLE. AS A RESULT; IT IS COMMON; AGAIN ACKNOWLEDGED BY ATC; FOR ACFT TO PASS BEHIND OTHER ACFT ON 'ECHO 3' TO HOLD SHORT ON ECHO 2 WHICH MAY TEMPORARILY CAUSE A RWY 31R INCURSION INTO THE CLEARWAY AT LEAST 100 FT PRIOR TO THE RWY 31R THRESHOLD. THERE ARE NO NOTES OR INSTRUCTIONS REGARDING WHICH ACFT CAN BE ACCOMODATED SIDE BY SIDE ON ECHO 1;2 AND 3 OR WHICH ACFT WILL FIT ON THESE TXWYS BTWN RWY 31C AND RWY 31R HOLD SHORT LINES. ALL ACFT HAVE BEEN IN ALL POS. NO 2: THE CREW'S ANTICIPATION OF A DEP CLRNC TIME. IT WAS 'WHEELS UP' TIME AS WE TAXIED BEHIND THE B727 TRYING TO BE IN POS AT THE PROPER TIME AND NOT LOSE OUR SLOT. COMBINED WITH THE OBSERVED PRACTICE OF TAXIING BEHIND 'ECHO 3' ACFT AND NOT REALIZING THE CLRNC LIMITATIONS FOR ACFT SIDE BY SIDE ON 'ECHO 2 AND 3'; THIS COULD HAVE BECOME A SAFETY ISSUE FOR ACFT APCHING RWY 31R. THE SOLUTION INCLUDES: 1) A MORE ASSERTIVE STATEMENT BY THE FO AS SOON AS IT APPEARED THAT THERE MAY HAVE BEEN A RWY 31R CONFLICT COMBINED WITH A RADIO CALL TO ATC TO COORDINATE ACTION. 2) MORE IMPORTANTLY; NOTES ON THE MIDWAY ARPT DIAGRAM THAT SPELLS OUT WHICH ACFT CAN BE ACCOMMODATED AT WHAT TIMES. FOR EXAMPLE; DURING IFR OPS WITH RWY 31R NOT IN USE; IT IS ACCEPTABLE FOR ACFT TO AWAIT RWY 31C DEPS WHILE INTRUDING INTO RWY 31R WITHOUT SPECIFIC ATC AUTHORIZATION. 3) WHEN IN DOUBT AS TO THE REQUIRED CLRNC FROM ANY OBJECT; STOP AND VERIFY THE APPROPRIATE ACTION AS NECESSARY WITH THE APPROPRIATE FAC.