6:1 Shortcut at 7:00 am
Tales of the Old Central Artery
Here's a story of a "shortcut" on the old Central Artery that gained time at a success rate of about one time per six attempts. That's sort of like playing inverted Russian Roulette, with five bullets in the revolver and one empty chamber (artery traffic was extremely frustrating to say the least, hence the analogy).
It's 7:00 a.m. in the late 1980s. Commuters would drive through the Sumner Tunnel from East Boston to get downtown. At the North End portal, 80% of the traffic would take the sharp right at Cross Street to get to the southbound artery on-ramp (a gridlocked u-turn, which was avoided by using this shortcut).
To access Cross Street, it was a good idea to stay on the right in the Sumner Tunnel, but many drivers would stay left and then cut right at the North End portal (this blocked access to the northbound on-ramp on the left, backed traffic into the tunnel, and annoyed drivers that had stayed right but then got cut-off).
A couple of prerequisites had to occur for the 6:1 shortcut to work. If you were forced into the left lane in the tunnel (or chose it), about 20 or so vehicles in a row had to take the northbound on-ramp, opening up a huge swath of space. You'd speed up to a brisk 20 miles per hour and then peek up at the northbound artery once outside the tunnel; if trucks were passing by quickly, then traffic wasn't backed up in that direction.
The shortcut was to zip up the northbound on-ramp on the left, and then get off at Causeway Street. At Causeway Street, you'd take a u-turn back under the artery and then head southbound. This was a tricky intersection; first a stop sign, then a left turn across a divided thoroughfare, then take a left back across and get on the artery south.
This alternate route could save 10 minutes or more. If Causeway Street had heavy traffic—four lanes of traffic with two in each direction—you'd get stuck on the off-ramp queued up behind the stop sign and lose time.
The Big Dig System, aka The Sponge, has paid for itself with increased productivity and reduced air pollution; all $14 billion dollars' worth.