Boston Transportation Department Traffic Analysis

BTD’s December 6, 2012 presentation includes some important information related to traffic volumes and how they will be handled under the Surface Option and the Underpass Option at Austin Street.  We copied a few of the pages of the presentation and explain their significance below.

BTD took traffic counts throughout the project area in 2008 before the economic downturn and before several major construction projects began.   In response to community comments, they took additional counts in 2011 in the Austin Street underpass.  What they found was that the numbers did not change much over the three years.

Traffic Volumes During the Morning Rush Hour

Blue numbers are car counts at the surface and red numbers are car counts in the underpass.  

The number of cars turning off of Rutherford Ave inbound onto the Gilmore Bridge (829 vehicles) and the number of cars heading toward downtown (1,515 + 70 vehicles) are significantly higher than any other traffic movement.  However, in a later slide (page 13), BTD notes that a single traffic lane can accommodate 1,000 vehicles per hour.  This means that even though the number of cars heading toward downtown seems high, it represents less than 2 lanes worth of traffic.  

Traffic Volumes During the Evening Rush Hour 

There is a strong demand coming from the Gilmore Bridge and turning left onto Rutherford Ave toward Sullivan Square (745 vehicles) or right on Rutherford Ave toward downtown or the Tobin Bridge (554 vehicles).  Interestingly, there are actually more cars inbound toward downtown in the afternoon than in the morning.  Since one lane can accommodate 1,000 vehicles per hour, the inbound traffic heading into the intersection (1,600+122 vehicles) only requires 2 lanes.  Below the intersection, it may be important to keep a separate lane for the Tobin Bridge traffic.  This can be looked at in the next phase of design.

Traffic Volumes Applied to the Underpass Option    

Here BTD models traffic volumes in the Underpass Option using the morning and afternoon commute numbers, with afternoon numbers in parentheses.  Note that there is one arrow per travel direction, regardless of the number of lanes planned.  For example, coming off of the Gilmore Bridge, there will be 2 lanes turning left, but only one blue arrow is shown.  

A few points of interest:

• The 353 vehicles traveling from the Gilmore Bridge directly into Charlestown in the afternoon will need to use the same lane as people turning left onto Rutherford Ave (up to 782 vehicles). 
• The 43 vehicles turning left into the neighborhood from Rutherford Ave inbound in the morning will be in the same lane as the 74 people going straight.  These may be impacted by vehicles turning right onto the Gilmore Bridge in the morning.  [Question remains whether the 90 degree right turn shown next to the Community College can really accommodate the 870 vehicles turning during the morning commute (see where it says “870(492)”.)]
• Traffic demand outbound requires less than 1 lane (454 vehicles in the morning when one lane can handle 1,000 vehicles) but BTD needs to build 2 lanes in case there is an accident or breakdown in the underpass.    
  
• BTD does not address what will happen in the middle of the intersection – a place where vehicles back up right now.  BTD presents this as a simple 4-way intersection, but it is actually 2 signal lights that are supposed to work together.  Consider what happens when someone goes through a yellow light and gets stuck in the middle.

Traffic Volumes Applied to the Surface Option    

Here BTD models traffic volumes in the Surface Option using the morning and afternoon commute numbers, with the afternoon numbers in parentheses.  Note that there is one arrow per travel direction, regardless of the number of lanes planned.  For example, coming off of the Gilmore Bridge, there will be 2 lanes turning left, but only one blue arrow is shown. Also, there are two right lanes onto Gilmore Bridge and only one blue arrow is shown.

A few points of interest:

• The 353 vehicles traveling from the Gilmore Bridge directly into Charlestown in the afternoon will have their own lane.
  
• Two surface lanes are sufficient to accommodate inbound traffic in the morning (1,591 vehicles) and the afternoon (1,808 vehicles) since they will be able to handle 2,000 vehicles per hour combined.
  
• A “free” right separates traffic heading onto the Gilmore Bridge from the traffic heading straight through the intersection inbound.
• There is no space for vehicles to block the middle of the intersection.  They will need to clear the center after each light change.
  

A few points on how the traffic signals will work:    

• Basically, all will be activated by demand (bullet 1).  This means that if there are no vehicles trying to exit the side streets like Essex or Baldwin Streets, the lights on Rutherford Ave will not automatically turn red.  Vehicles in the night or early morning hours will likely have a uninterrupted flow through the area.
• The BTD Management Center will be able to observe if any traffic congestion occurs (bullet 2) and will be able to change the light timing remotely (bullet 4).
• The lights will be timed to allow vehicles to flow smoothly on Rutherford Ave, i.e., vehicles going the correct speed limit will be able to flow through a series of green lights (bullet 3).

Here BTD compares how much time will be given to vehicles exiting Austin Street within each sequence of lights.  Under both options, the light timing will be changed to a 2 minute sequence (120 seconds).  Austin Street will be given 24-26 seconds under the Surface Option and 30-32 seconds under the Underpass Option – a difference of only 6 seconds between the two options.

What the slide doesn’t say is that in 30 seconds of green, approximately 18 vehicles can move through the intersection per lane.  This means that with the 24 seconds provided in the Surface Option (PM peak), 28-30 vehicles will be able to get through the light each time it turns green - assuming that the left turn lane and the right turn-through lane (i.e., 2 lanes total) get the green at the same time.

Given that only 365 vehicles leave Austin Street during the peak afternoon commute, it is unlikely there will ever be 28-30 vehicles cued up at any one time at that location…