Cass Ave., South of 91st st.
Cops like to sit in the entrance to the Argonne Natl. Labs child care center on the East side of Cass Ave, south of 91st street. In case you want to pinpoint it in Google Maps, the exact position is: Latitude = 41.7133, Longitude = -87.9603
I’ve also seen them occasionally in the Argonne Shipping and Receiving entrance, on the west side of the road, just north of the above location, at Latitude = 41.7168, Longitude = -87.9638