As final designs for the Central Artery / Tunnel Project were coming together in the late 1980s, engineers were faced with a daunting problem: How to get the new highway across the Charles River without disturbing the existing double-deck I-93 bridge or the MBTA's underwater Orange Line subway tunnel. Charlestown residents had already rejected a complex, 18-lane ramp design known as "Scheme Z" because they believed it would blight their neighborhood.