As predicted, traffic on Tuesday was bad. Real bad. Of course, it was not possible to predict a rolled-over tractor trailer at the most inopportune portion of the Massachusetts Turnpike, near Charlton. Due to the rollover, heavy traffic lasted from 1 pm until after midnight, instead of the expected 2 pm until 10 pm timeframe.

What Was the Traffic Impact?

We visualized speeds (in mph) on the Massachusetts Turnpike; each segment (Boston at the top, Sturbridge at the bottom) is colored from red (slow) to green (fast):

The numbers in the image above correspond to the following:

  1. The aforementioned tractor trailer overturns around 2 pm, shutting down 2 of the 3 lanes; with heavy volumes, the ensuing 3-to-1 merge immediately brings traffic to a halt.
  2. The jam backs up at a rate of about 10 miles per hour, and upstream sections of roadway east of Auburn quickly devolve into gridlock around 3 pm.
  3. At the same time, the segment of I-90 east of Route 128 slows down due to normal rush hour traffic, plus holiday travelers.
  4. By 4 pm, even the I-90 segments between 128 and 495 are slowing down.
  5. The tractor trailer is cleared around 4 pm, and the faster speeds cascade through the roadway. On a low-traffic day, speeds would quickly return to normal, but with the high holiday and rush hour volumes, traffic never returns to free-flow.
  6. Around 6 pm, the entire Turnpike from Boston to Sturbridge is a traffic jam.
  7. Around 7 pm, the eastern portion of I-90 returns to normal free-flow as rush hour ends, but the western portion continues to be congested.
  8. By about 9 pm, I-90 east of I-495 finally clears out, and once a segment speeds up, it does so quickly.
  9. The section between I-495 and Auburn is mostly clear by 11 pm (still quite late) but 20 mph traffic continues on the road out to Sturbidge until after midnight.

Why is the Massachusetts Turnpike so prone to heavy traffic?

The Sturbridge tolls are not the cause; even with heavy westbound I-90 traffic, the ramps leading to the tolls rarely have heavy traffic. Instead, congestion on the Massachusetts Turnpike near Charlton and Sturbridge is caused by the following:

  1. Vehicles traveling west on I-90 must sort themselves (change into the proper lane) in order to continue west on I-90 or to take the exit ramp to I-84
  2. Vehicles leaving the Charlton Service Plaza must merge with traffic already attempting to sort itself (#1, above); this service plaza also sometimes has a full parking lot, causing vehicles to backup onto I-90W.
  3. Heavy feeder traffic from I-290, I-395 and I-495

Next year? Take the train.