Narrative:

During takeoff roll [on runway 21R]; it appeared another aircraft was crossing our runway after we were given takeoff clearance. At about 80-100 knots; the captain elected to reject the takeoff. I was the PF. I made a radio call to the tower to let them know we rejected. Tower asked why and I replied due to an aircraft crossing our runway. Tower replied the aircraft was on a taxiway and that it was a common occurrence. I did not ask whether it was common for aircraft to reject or just think there was an aircraft crossing. We returned to the gate for fuel and brake cooling. Rejected takeoff was otherwise uneventful.

Google
 

Original NASA ASRS Text

Title: B737 flight crew reported they thought an aircraft was crossing at the departure end during takeoff from DTW so the Captain elected to reject the takeoff when in actuality the other aircraft was on a perpendicular taxiway.

Narrative: During takeoff roll [on Runway 21R]; it appeared another aircraft was crossing our runway after we were given takeoff clearance. At about 80-100 knots; the Captain elected to reject the takeoff. I was the PF. I made a radio call to the Tower to let them know we rejected. Tower asked why and I replied due to an aircraft crossing our runway. Tower replied the aircraft was on a taxiway and that it was a common occurrence. I did not ask whether it was common for aircraft to reject or just think there was an aircraft crossing. We returned to the gate for fuel and brake cooling. RTO was otherwise uneventful.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.