Scheme Y Interchange
Tales of the Old Central Artery
In the early 1990s, Scheme Z was a complicated stacked clover-leaf interchange that was proposed for the Central Artery/Tunnel project. Scheme Z would have connected I-93 and U.S.-1 with the then un-built Big Dig Tunnel system.
What was originally constructed on the old Central Artery had evolved into what can be described as "Scheme Y," or Scheme Yikes! The apex of the old Central Artery was at High Bridge, a double-deck steel truss bridge over the Charles River. A high-volume half clover-leaf interchange existed at High Bridge.
Driving north on the old artery, three lanes would greatly narrow at North Station. Once on High Bridge, you could either bear left and head north on I-93, or bear right and go north on U.S.-1. If you were merging from Storrow Drive from the left, and needed to get on Route 1 on the right, you'd have to cut-off Central Artery traffic and cross over to the right. If you were in the right lane on the Central Artery, and had to get on I-93 north on the left, you'd have to cut-off Storrow Drive traffic and cross over to the left to get on 93.
The flow of vehicles on the lower deck of High Bridge was the exact opposite. I-93 traffic heading south would have to bear left to get on the artery, while U.S.-1 traffic heading over to Storrow Drive would have to bear right.
At the height of evening rush hour, Scheme Y also created a tremendous bottle neck. Cars heading north at the old lane-drop before North Station would often not merge, and four lanes of traffic would exist instead of three. Storrow Drive on-ramp traffic would also often not merge, and two lanes of traffic instead of one lane would converge onto High Bridge. Thus, it was physically possible at times to have five lanes of traffic on the south side of High Bridge instead of three.
Scheme Yikes was partially replaced in the early 1990s with a tunnel at City Square in Charlestown. Heading north, instead of bearing right to get on Route 1, a left-hand loop replaced the old ramp on the right. This created a queuing area for cars from the central artery to move over to the left lane, while at the same time allowed traffic from Storrow Drive to stay to the left even if they were heading to Route 1.
The Big Dig System, aka The Sponge, has paid for itself with increased productivity and reduced air pollution; all $14 billion dollars' worth.