Alewife Study Group > W. R. Grace site > community feedback > Mar. 1996, traffic mitigation Search 

Traffic signal timing and traffic mitigation
at Alewife (near W. R. Grace site)

March 1996

To : Liz Epstein, Cambridge Community Development Department

From : Stephen H. Kaiser, traffic analyst

SUBJECT: Traffic Signal Timing and the Potential for Traffic Mitigation at Alewife

Thank you for mailing me copies of the original Rizzo scope letter on the Alewife Center project and the traffic counts made by Rizzo Associates in 1995 and early 1996 at Alewife. Unfortunately, some of the traffic counts are illegible and this is true of both copies I received - - the problem likely is a poor quality original. Under the circumstances, whoever supplied these counts should provide you with legible replacement copies, at no cost to the city. There is no excuse for submitting illegible traffic data.

As I noted in my March 10 letter to Stuart Dash, the Rizzo counts for the City are more extensive than the counts made for Alewife Center. With a 48 hour count, the data shows some remarkable traffic flow results. Near Whittemore Avenue on the parkway, there is a noticeable drop in actual traffic flow during the 5-6 PM peak period. In October 1995, the peak hour flows actually dropped by almost half.

I have been trying to think of all the possible explanations for this strange drop in traffic.

  1. An error in the counts?
  2. Traffic backups from Mass Avenue?
  3. A sudden shift in traffic towards Route 2 and less towards the Parkway. Northbound?
  4. Major congestion at either Route 2 or Whittemore which prevented Parkway traffic from reaching Whittemore?
As best as I can tell, the most likely explanation is #2 above -- that traffic backed up from Mass. Avenue and prevented the regular flow from moving through. There is no way of recreating the actual traffic conditions of October 24 and 25, 1995, but logically we can draw certain conclusions:

  1. We have three locations for data :Alewife Brook Parkway north of Concord, at the shopping Center and near Whittemore. We have both northbound and southbound counts.
  2. Counts near Whittemore were taken on three days, including a January count this year. In all other cases, count data is available only for two days -- October 24 and 25.
  3. As you can see for Graph #1 and #2, the flow drop north of Route 2 near Whittemore is very pronounced between 5 and 6 PM for both October days. Graph #3 shows a slight drop in January 1996 but not as pronounced.
  4. Southbound parkway traffic near Whittemore also dropped significantly, on all three days.
  5. The traffic flow north of Concord Avenue does not have these drops and is much more even.
  6. From field observations, the typical afternoon queue extends back to the Fresh Pond rotary. Also, the Mass Avenue queue begins to extend quite close to Whittemore between 5 and 5:15 PM. At times, the Mass. Avenue queue can extend into the Route 2 intersection and briefly block the intersection. The vehicles most immediately affected by the blockage are in the southbound parkway movement, which is the next phase of the signal.
  7. Other field observations and counts show that at Mass. Avenue, the long signal cycles result in inefficient use of through lanes. The turn slots on the parkway are relatively short and have room only for about 7 vehicles (fewer with buses) Meanwhile the signal has been operating recently with 3 minute cycles, which is more than double what it should be. As a result of the long signal cycles, too many cars overflow the turn slots and block the left hand through lane. Cars in the left lane trapped by the queue may try to switch lanes into the right lane and interrupt that flow as well.
Therefore, long signal cycles and the resulting blocking of lanes seem to be the primary culprit in contributing to drops in peak hour flows on the parkway. I urge that we make more detailed counts and observations and engage Rizzo, Abend and the Cambridge Traffic Department in a discussion of ways to improve signal timing at Mass. Avenue. There may also be some phasing changes which would assist operations as well.

The signal timing at Route 2 and Alewife and at the MBTA station is also not very efficient. Sample counts I have made indicate that signal operation on a 60-second cycle could actually produce a 5% capacity benefit, compared to the current operation on a 120-second cycle. The long queues produced by a 2-minute cycle also extend back to the MBTA station signal and Rindge Avenue.

Overall, I find the long signal cycles producing longer queues, longer delays, and reduced capacity due to the influence of short lane effects. The only benefits of long signal cycles -- reduced start-up losses -- appear to be totally overwhelmed by the magnitude of the short- lane losses at Alewife.

Regardless of the status of a formal MEPA filing for Alewife Center, we can engage is a useful discussion of the traffic limitations and potentials at Alewife. Indeed, the committee report to the City Council in mid-May should probably focus on these matters, as long as the specific development proposal is still in flux. I would note that if we were to consider a site proposal with additional office development, we would need to look at morning peak hour operations as well for any mitigation potential.

The Cambridge traffic counts contain useful information which has not been fully appreciated by City officials. Instead of the obvious initial conclusions that one might draw -- the Alewife area is congested and cannot accept any significant increase in traffic -- it appears that there does exist significant traffic improvement potential without having to go to the extremes of Salvucci's Wall. We should seek to define how to go about achieving these improvements and estimating the magnitude of change likely in traffic capacity.

With regard to a philosophy of traffic capacity enhancements, I concur with the City policy of not seeking to provide increased through capacity for vehicles entering Cambridge. However, in the afternoon, with the vehicles already in the City, there is a convincing logic which says that Route 2 and the Parkway should operate as best as possible to dissipate these vehicles, with a minimum of congestion and pollution. At the same time, residential streets should not be targeted for "mitigation" in the form of increased capacity -- especially the intersection of Rindge Avenue and Sherman Street. Some of the capacity improvements at Mass. Avenue should come in the form of less traffic on Rindge Avenue and more capacity on Mass. Avenue. Currently, even at 6:30 PM, traffic is backed up and not clearing out on Mass. Avenue, waiting to make a left turn onto the Parkway.

As I noted in my earlier comments, the real "heart of Alewife" may be at Mass. Avenue -- not Route 2. The Route 2 intersection may back up 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. We do not know how to deal with queues, short lanes, and merging lanes -- all of which are critical factors at Alewife. The Highway Capacity Manual is batting 0 for 3.

Please send me any further traffic counts you have made in the Alewife area, from Huron to Mass. Avenue. Insist that they be provided to you in legible form, in 15-minute increments.

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