Traffic consultants presented updated modeling at the Sept. 29 meeting showing that travel times through Needham Center would increase during the worst peak-hour conditions if all vehicles remained on Great Plain Avenue — and that a substantial share of the AM peak volume appears to be long-distance through‑traffic.

What was presented: the study used automatic traffic recorder counts from Sept. 9, calibrated intersection turning‑movement counts, and a stochastic micro‑simulation engine (replica/VISSIM-style modeling). The consultants compared four‑lane (retain lanes), hybrid (three‑lane), and two‑lane road‑diet alternatives and kept pedestrian-exclusive phasing at the two signalized intersections.

Key technical points and why they matter

- Peak periods and measurement: consultants focused on three critical windows — AM peak (07:45–08:45), school dismissal (14:45–15:45) and PM peak (16:00–17:00) — because those periods produce the greatest delay and therefore the greatest incentive for drivers to divert.

- Delay and queuing: the consultants showed that, with every vehicle forced to remain on the network, the two‑lane alternative produced the highest travel times and longest queues during the AM peak. As an example presented in the analysis, an existing eastbound travel time cited as roughly 2.9 minutes in the study comparison rose to about 8 minutes in the two‑lane scenario if no vehicles diverted. The consulting team emphasized that those results are a “worst‑case” comparison that assumes no drivers change routes or timing; they said measured field diversion would reduce the modeled delay.

- Cut‑through composition: consultants reported that about 55% of the trips cutting through the center in the AM are commuter-related, and that 28% of cut-through trips are longer than roughly 32 miles; if the distance threshold is lowered to 16 miles, roughly 60% of cut‑through traffic are over that length. The consultant said these origin‑destination patterns were derived from cell‑phone data and indicate many through‑drivers are making long trips and not stopping in Needham Center.

- Adaptive signal coordination: town staff noted an in‑progress state-funded “bottleneck” signal coordination project. A staff member said the bid did not include the most advanced adaptive detection and that a change order with the state will be required to install more sophisticated adaptive hardware; the town expects to implement the upgraded signals after negotiating the change order and still hopes to put initial signal changes in place before the end of the year.

- Diversion and local impacts: consultants said a meaningful portion of delay under constrained lane options is likely to produce diverted traffic. They also warned that diversion effects depend on origins/destinations; long‑distance commuters would likely choose highways, while some local trips could shift to nearby streets. The team said they can add additional analysis showing where diverted traffic would appear on the local street network if requested.

Quotes from presenters

- “This shows a variation of traffic that travels along Great Plain Ave over a 24‑hour period… the AM peak is especially high,” the consultant said when introducing the ATR data.

- Regarding cut‑through origins: “55% of the trips are commuter trips, and 28% of the trips are longer than 32 miles,” the consultant said, adding that much of the cut‑through traffic originates well outside town.

Next steps and context

Consultants and staff said future modeling rounds will incorporate anticipated diversion and the state signal upgrade. They also suggested additional checks of the dataset (for example, Pacer AI or other cell‑phone datasets) to estimate how many through‑drivers ever stop in town — information local merchants requested to assess whether diverted traffic represents potential lost customers.

Ending: The modeling results presented were scenario comparisons intended to show trade-offs; the consultants emphasized that including estimated diversion substantially changes projected delays and that the project team will provide more targeted diverted‑traffic mapping on request.