B06 TRANSITING SBOUND THROUGH PWK CTL ZONE TAKES EVASIVE ACTION TO AVOID DEPARTING C172; EVEN THOUGH BOTH ACFT RPTED THE OTHER IN SIGHT.

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I WAS TRAVELING SBOUND ALONG THE E SIDE OF PWK ARPT FOLLOWING I294 S ON ROUTINE TFC PATROL. A CESSNA 172 WAS DEPARTING RWY 34. WE WERE BOTH IN CONTACT WITH THE CTL TWR; AND WE BOTH HAD VISUAL ON ONE ANOTHER. THE CESSNA INFORMED THE TWR HE WOULD BE TAKING AN EBOUND HDG; AND THE TWR CLRED HIM; BUT TOLD HIM TO MAINTAIN VISUAL CONTACT WITH ME; AND I WITH HIM; WHICH WE BOTH ACKNOWLEDGED. I HAD ALREADY BEEN CLRED TO CONTINUE MY SBOUND HDG SO I MAINTAINED MY FLT PATH. BECAUSE THE CESSNA WAS DEPARTING FROM A HDG OF 340 DEGS; AND BECAUSE OF OUR PROX; I EXPECTED HIM TO CLBOUT AND THEN MAKE HIS EASTWARD TURN BEHIND ME. AS I CONTINUED TO MAINTAIN VISUAL CONTACT; THE AIRPLANE MADE AN UNEXPECTED STEEP R TURN IN FRONT OF ME. I IMMEDIATELY AND AGGRESSIVELY SLOWED DOWN; AND MADE A STEEP R DSNDING TURN TO AVOID HITTING THE CESSNA. MY PAX AND I ESTIMATE THAT WE AVOIDED A MIDAIR COLLISION BY APPROX 100 FT. BECAUSE MY PATH HAD REMAINED UNCHANGED; I EXPECTED THE CESSNA TO MAINTAIN HIS HDG UNTIL I HAD PASSED AND CONTINUE HIS TURNOUT BEHIND ME. HAD THE CESSNA ANNOUNCED HIS INTENTIONS TO TURN IN FRONT OF ME; I WOULD HAVE HAD MORE TIME TO ALTER MY COURSE. ADDITIONALLY; HAD THE TWR GIVEN INSTRUCTIONS FOR ME TO GIVE WAY OR GIVEN THE CESSNA DIRECT INSTRUCTIONS TO TURN BEHIND ME; THIS NEAR ACCIDENT MOST LIKELY WOULD NOT HAVE OCCURRED.