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SUBJECT: Comments on the 1995 and 1996 Traffic Counts at Alewife by
Thank you for the traffic counts made by Rizzo Associates in 1995 and early 1996 at Alewife. These counts are more extensive than the one- day, two-hour counts made for Alewife Center, since the Rizzo counts covered a 24-hour period over two days.
One possible explanation for this sudden drop in traffic flow is that at Mass. Avenue the left turn slots on the parkway become overloaded, blocking the left through lane. The congestion spills over and affects both northbound lanes on the parkway. In addition, pedestrians may actuate the pushbutton phase more frequently in the 5-6 peak hour.
While making my own counts, I have observed that the left lane on the parkway was becoming blocked by backups from the turn slot, especially towards the end of the green phase. It is a classic short lane effect, caused in part by cycle times which are too long and insufficient green time for the left turn traffic.
The result of the reduced capacity at Mass. Avenue is traffic backups into the Route 2 intersection and blocking of this intersection and Rindge Avenue as well. This blocking action restricts all moves, including the southbound parkway, and such flow reductions are shown in Rizzo's southbound counts as well. These blocking effects do not take place the same way every day, and some days are better or worse than others.
The Abend counts at the same location were taken two weeks before in October and do not show any significant flow reduction between 5 and 6 PM. The trigger to the massive flow reduction of late October 1995 may be either a change in traffic volumes, congestion in the turn slot, changed signal timing at Mass. Avenue (including longer cycles), or defective signal operation. Both my own and Rizzo's counts of January 1993 show that flow reductions still occur in the peak hour, although not as abrupt as were recorded on October 24 and 25.
This inefficiency in traffic movement is both a disappointment and an opportunity. A solution which would maintain intersection efficiency should be counted as traffic mitigation. I suggest that we make a full inventory at Alewife of inefficient traffic operations so that a full listing of possible mitigation measures can be determined. Logically, the magnitude of this gain in efficiency can be counted towards overall mitigation allowances covering traffic growth at Alewife, including no build and Alewife Center growth.
While we are seeking efficient flow on major roadways, we should not apply similar "mitigation" objectives to local roadways such as Rindge Avenue and Sherman Street. Improved flow and lower delays on these streets will likely attract more traffic diverting from the Parkway. A good general policy is for Parkway traffic to stay on the Parkway.
These observations lead me to believe that at the moment the real "heart of Alewife" may be at Mass. Avenue and not Route 2. The Route 2 intersection may backup into Rindge Avenue, but Mass. Avenue backs up into Route 2 and makes Rindge significantly worse. Currently, we do not have a traffic analysis model which reflects these practical limitations at Mass. Avenue, Route 2 or Rindge Avenue. The Highway Capacity Manual is batting 0 for 3.
I urge the City to continue its counting program to understand the variations in traffic flow at Alewife. I am hopeful that with good weather citizens armed with laptop computers will be able to make lane- by-lane traffic flow measurements which will be able to show both road capacities and congestion events, allowing us to calibrate the intersections for all their merging, short lanes and queuing effects which are not covered by the traffic manual.
Contact the Alewife Study Group, North Cambridge Massachusetts, by email at email@example.com