A C560 CREW RPTS A RWY INCURSION AT MKE BECAUSE OF DIFFICULTY SEEING THE RWY 25 HOLD SHORT LINE ON TXWY M.

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I WAS THE PIC ON CESSNA CITATION. WE WERE ON A POSITIONING FLT FROM MKE. CAPT WAS AT THE CTLS IN THE L SEAT AND TAXIING THE ACFT. WE WERE TAXIING TO RWY 25L. AS WE TAXIED EAST TO TXWY M APCHING RWYS 31 AND 25L THE TWR CTLR TOLD US TO STOP. HE THEN TOLD ANOTHER ACFT TO GAR. THE TWR CTLR THEN ANNOUNCED A POSSIBLE PLT DEVIATION RPT. WE WERE THEN CLRED FOR TAKEOFF ON RWY 25L. AFTER TAKEOFF THE CTLR TOLD US TO CALL AT OUR EARLIEST CONVENIENCE. UPON OUR ARR; I CALLED OUR ASSISTANT CHIEF PLT FOR THE CE560XL FLEET AND INFORMED HIM OF THE EVENTS SURROUNDING OUR DEP FROM MKE. HE ASKED ME FOR THE PHONE NUMBER GIVEN US BY THE MKE TWR CTLR AND HE CALLED IT. LATER IN THE DAY I AGAIN CALLED ASSISTANT CHIEF PLT. HE SAID HE CALLED THE NUMBER I HAD GIVEN HIM AND WAS TOLD THERE WAS A PROB ON THAT TXWY. TOO MANY PLTS WERE TAXIING PAST THE RWY'S HOLD SHORT BAR AND THEY INTENDED TO DO SOMETHING ABOUT IT. SINCE WE HAD ALREADY TAXIED ACROSS 3 RWYS (25L-9R; 19R-1L; AND 19L-9R) ALL IN CLOSE PROXIMITY; WE WERE SUPER CAREFUL TO NOT ENCROACH ON RWY 31 & RWY 25L WITHOUT CLRNC. AT THE E END OF TXWY M THERE IS A RATHER LARGE SIGN WITH ARROWS POINTING L TO RWY 31 AND R TO RWY 25L. CAPT WAS FOCUSED ON THAT SIGN WHILE I WATCHED TO BE SURE WE DIDNT TAXI ONTO A RWY BY MISTAKE. SINCE THE RWY'S HOLD SHORT BAR IS A LONG WAY FROM THE RWY; THE SIGNAGE AND MARKINGS NEED TO BE IMPROVED. THE SIGN AT THE E END OF TXWY M; WHILE HELPFUL; WAS A FACTOR IN DISTRACTING US FROM SEEING THE HOLD SHORT BAR IN A TIMELY MANNER. I WOULD SUGGEST A SIGN SAYING 'ALL ACFT STOP HERE' WOULD PREVENT ACFT FROM TAXIING PAST THIS POORLY LOCATED RWY HOLD SHORT BAR. LAX HAS SEVERAL SUCH SIGNS.SUPPLEMENTAL INFO FROM ACN 733879: AT NO TIME DID OUR ACFT CROSS ON TO THE ACTIVE RWY 25; WHILE I SAW THE SIGN INDICATING RWY 25 WAS AHEAD AND TO THE R; THE ONLY HOLD BARS I RECALL SEE WHERE THOSE WE CROSSED EXITING RWY 1R/19L. THE SIGNAGE IN THE AREA IS A DISTRACTION AND CONTRIBUTES TO CONFUSION RATHER THAN CLARITY. BETTER AN ALL ACFT HOLD HERE SIGN IN THE AREA AS I'VE SEEN AT OTHER AIRFIELDS.