sentences:  For roughly a half-mile (0.8 km), I-81 heads east–west, running alongside I-690 and connecting to the highway by way of a series of closely spaced ramps.Like I-81, the incomplete interchange is entirely elevated, passing over parts of several downtown blocks.Past I-690, I-81 transitions from an elevated road to a sub-surface highway as it cuts across the north half of downtown and proceeds northwest past an old industrial area that was once Syracuse's Inner Harbor.