ACR PLT DEPARTING ORD WAS CONCERNED WHEN A CLRNC FOR TKOF WAS GIVEN BEFORE THE PRECEDING ACFT HAD LIFTED OFF.

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ON TXWY W APCHING RWY 4L AT ORD; TWR CLRED AN ACR Y ON TXWY E TO TAKE OFF ON RWY 4L AHEAD OF US. WE WERE THEN INSTRUCTED TO 'KEEP IT MOVING AND STAY AS CLOSED AS YOU CAN BEHIND ACR Y; YOU ARE GOING RIGHT OUT.' UNSURE OF WHETHER HE WANTED US TO CROSS THE HOLD LINE; WE WAITED FOR A 'TAXI INTO POS AND HOLD;' WHICH CAME WITH ACR Y NOT HAVING RELEASED BRAKES YET. WHILE STILL MOVING UP ON THE TXWY; WE RECEIVED A 'CLRED FOR TKOF; TURN 090 DEGS' CLRNC. WE DECLINED THE CLRNC STATING 'WE ARE NOT GOING TO RELEASE BRAKES UNTIL ACR Y LIFTS OFF.' ACR Y ROLLED ANOTHER 2000-3000 FT BEFORE HIS NOSE GEAR LIFTED OFF. ALL THE WHILE TWR WAS PRESSURING US TO ROLL DUE TO LNDG TFC ON RWY 9L. I ESTABLISHED VISUAL CONTACT WITH THE TFC APCHING ON RWY 9L. I DECIDED IT WAS LESS HAZARDOUS TO ROLL THROUGH THE INTXN OF RWYS 4L AND 9L THAN TO STAY SO NEAR THE CROSS RWY IN MY PRESENT POS. WE RELEASED BRAKES AND TOOK OFF NORMALLY AS SOON AS ACR Y AHEAD OF US LIFTED OFF. UPON FURTHER REFLECTION; 2 TROUBLING SAFETY ISSUES STAND OUT. 1) AN ORD TWR CTLR ISSUED AN UNSAFE TKOF CLRNC WITH THE PRECEDING ACFT STILL WELL BELOW V1 AND ONLY 1/2 WAY THROUGH HIS TKOF ROLL. 2) CLRNC TO TAKE OFF WAS GIVEN WITH AN ACFT DANGEROUSLY CLOSE TO LNDG ON A CROSS RWY. HAD WE DELAYED OUR TKOF JUST 10-15 SECONDS OR HAD WE PERFORMED A LOW SPD ABORT; THE TFC ABOUT TO LAND WOULD HAVE NEEDED TO PERFORM A GAR AT VERY LOW ALT TO PREVENT A CATASTROPHIC COLLISION.