Complex sentence: The Post Office is a landmark which is sometimes used in referring to the end of I-290 in downtown Chicago; for example, a traffic reporter might say "... forty minutes from Mannheim to the Post Office...".
Split into simpler sentences: The Post Office was a landmark that was sometimes used in referring to the end of I-290 in downtown Chicago. For example, a traffic reporter might say "... forty minutes from Mannheim to the Post Office...".