UG: Charles River Basin Infrastructure Synchronization Project

FINAL REPORT

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Appendix A: Study Limits and Schedule Appendix B: Study Results Appendix C: Combination Analysis Results with Mitigation

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Volume Il

Appendix D: Combination Analysis Results without Mitigation Appendix E: List of Provided Documentation

Appendix F: Existing Traffic Volumes

Appendix G: Structure Staging Concepts

Appendix H: Traffic Reassignment Routes

Appendix |: Level of Service Analysis Results (Individual Projects) Appendix J: Schedule Detail

Appendix K: Condition Summary Reports

Appendix L: Anticipated Permitting Status

Appendix M: Select Link Analysis

Appendix N: Photographs

Appendix 0: SYNCHRO Analysis Files

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The transportation infrastructure within the Charles River Basin is

entering a critical period in which many structures are simultaneously

nearing the end of their expected useful service lives. The potential traffic congestion triggered by the myriad of lane restrictions and detours necessary to implement the appropriate rehabilitation programs threatens to disrupt the economy and vitality of Boston and surrounding communities. To address the situation, the Executive Office of Transportation and Public Works retained BETA Group, Inc. to study options for synchronizing the rehabilitation of twelve of these critical transportation assets over the next decade-and-a-half in a manner that would address public safety while minimizing impacts to traffic operations.

This Final Report presents a_ planning-level schedule for sequencing the rehabilitation of these twelve critical transportation assets. The sequencing of these projects, and the underlying analysis performed, strived to minimize traffic delays and congestion while maintaining public safety and convenience throughout the entire duration of the scheduled rehabilitation projects. The overall purpose of this Study is to utilize the schedule developed as an aid in scheduling design consultant procurement; determining required annual _ capital improvement funding; and monitoring and updating the schedule based upon the actual status, process, and duration of each _ individual

rehabilitation project. In this way, changes in the overall schedule can be

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reviewed to determine any resulting traffic implications such that they may be mitigated prior to them becoming and an issue.

The results of this Study find that the synchronization schedule

presented herein (Appendix A Figure A3) represents a logical sequence for the rehabilitation of these twelve structures while mitigating, to the extent possible, the anticipated traffic congestion. The resulting concurrent rehabilitation projects coupled with possible signal retiming and phasing found two time periods that exhibit the highest levels of unacceptable intersection level of service (LOS). These periods include a 30 month period commencing in 2012 in which two intersections will experience a level of service (LOS) F in the morning peak hour and two intersections will operate at an LOS E and one at LOS F during the afternoon peak hour. The second period is for a 12 month period commencing in 2015 where one intersection will operate at an LOS E and three at LOS F during the morning peak hour and two intersections will operate at an LOS E during the afternoon peak hour. Together these occurrences include seven intersections within the basin, of which four are located on Memorial Drive, two on Soldiers Field Road, and one on Massachusetts Avenue. From a global point of view, and an appreciation for the amount and magnitude of the construction underway during these periods, these results are remarkable. It is not expected that an alternate schedule depicting different or modified concurrent rehabilitation projects would result in improved intersection operations.

Also included in this Final Report are the methodology and

processes used as a basis for the Study. Should the next phase of the

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Study be implemented by EOT, some of the results obtained may be revised based upon different methodologies utilized to determine traffic re-assignment routes. In addition, as the results of the Study are presented to the stakeholders and public, additional information may be uncovered or presented. This information as well may have an influence

over the results of this Study.

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In March 2008 BETA Group, Inc was retained by the Executive Office of

Transportation and Public Works (EOT) to develop a comprehensive

construction synchronization plan (the Study) for critical components of the transportation infrastructure in the Charles River Basin (the Basin). The objectives of the Study are to assess the current condition of the identified infrastructure assets, prioritize them by their impact to public safety and convenience, and develop a reasonable order and timing of the projects to minimize traffic and pedestrian disruptions. Only by looking at the entire transportation network in the Basin can the Study achieve an overall logical and workable solution.

The transportation network within the Charles River Basin encompasses numerous vehicular and pedestrian bridges and tunnel structures owned and maintained by the Department of Conservation and Recreation (DCR). Many of these structures are simultaneously nearing the end of their expected useful service lives and are in need of major rehabilitation efforts. Due to the advanced state of deterioration and concern to public safety, some of the structures are undergoing interim repairs in order to extend their useful service lives until a full rehabilitation program can be undertaken. Over the next decade-and-a- half, the transportation network within the Basin will be disrupted by these interim and full scale rehabilitation projects. With close to 450,000 vehicles using these structures daily it is imperative that a global plan for the Basin be developed to establish a logical sequence for the

rehabilitation of these structures. This plan needs to consider public

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safety, maintenance of traffic flow, and the management and mitigation of construction impacts in order to maintain the economy and vitality of

Boston and surrounding communities.

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The Study encompasses the geographical area (see Appendix A Figure

A1) along the Charles River bounded to the west by the Eliot Bridge, to

the east by the Craigie Bridges (Route 28), to the south by the Massachusetts Turnpike (I-90), and to the north by Washington Street in

Cambridge.

Infrastructure Assets

Within the study area, the following infrastructure assets have been at | FF identified by EOT and DCR as critical transportation structures to be included in the Study:

1. Boston University (BU) Bridge 2. Bowker Interchange - Ramp H (NB off-ramp to Storrow Dr. WB) 3. Bowker Interchange - Phase Il a. Main Line over Beacon Street and Commonwealth Avenue b. Main Line over 190, RR and Ipswich Street . Off-Ramp to Commonwealth Avenue over Muddy River . Southbound On-Ramp from Storrow Drive EB . Southbound On-Ramp from Storrow Drive WB Northbound Off-Ramp to Storrow Drive WB . Northbound Off-Ramp to Storrow Drive EB

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. Storrow Drive EB over Bowker Interchange On- and Off-Ramps Cambridge Street (River Street)

Craigie Dam Bridge

Craigie Drawbridge

Eliot Bridge

North Harvard Street (Larz Anderson Bridge)

oC anon.

Longfellow Bridge 10. Memorial Drive over Brookline Street (Reid Overpass) 11. Storrow Drive Tunnel Eastbound Underpass

12. Storrow Drive Tunnel Berkley Street Underpass

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13. Western Avenue Bridge

14. Memorial Drive - Phase II For the purposes of this Study, the structures that constitute the

Bowker Interchange have been group together as one project identified

as the Bowker Interchange - Phase Il. The structure designated Ramp H, the Northbound Off-Ramp to Storrow Drive WB, is identified as a separate project because its rehabilitation has commenced with a_ recently- awarded construction contract.

Memorial Drive - Phase Il is a proposed DCR roadway rehabilitation project extending along Memorial Drive between Massachusetts Avenue and Memorial Drive over the CSX Bridge. Although this proposed project does not encompass any bridge or tunnel structures, it will have a significant impact to traffic, and has therefore been included in the Study.

The list of structures included in Study is not an all inclusive list of the transportation structures in the Charles River Basin only those

structures deemed critical by EOT and DCR have been included.

Intersections

Within the Study area are numerous signalized and un-signalized intersections. For the purposes of this Study, forty-two intersections were identified as key locations (see Appendix A Table A-1 and Figure A2) from which to gauge the traffic flow and traffic conditions within the Study area. The majority of the identified locations are signalized intersections, while a small number are controlled via regulatory signing (i.e. stop, yield,

or merge signs).

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The Study has been divided into two phases in order to accommodate

funding and schedule commitments. Phase 1 encompasses the Initial

Study phase while Phase 2 encompasses the Final Study phase. This Report presents the findings of Phase 1 which used the following methods:

e Review documents and information provided

e Develop traffic systems and models

Screen and prioritize identified structures by relevant Study

parameters

Perform traffic management and level of service analysis for each

individual project

Determine occurrences of concurrent project constructions

Superimpose traffic re-assignments for the identified project combinations

e Perform level of service analysis for the project combinations

e Review impacts to traffic operations as a result of project

combinations and determine mitigation strategies

Apply mitigation and re-run level of service analysis for the project

combinations

The goal of the prioritization was to maximize the construction effectiveness while minimizing local and regional traffic impacts. The resulting synchronization Schedule (see Appendix A Figure A3) is based upon the expected durations of the permitting, design, and construction phases of each rehabilitation project. Potential delays due to outside

forces, challenges, or funding were not accounted for in the Study.

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Following is a discussion of the methods used in the Study to

generate the synchronization Schedule.

Data Review

In order to gain an understanding of the existing conditions of the structures within the Basin, the Study included a review of available documentation provided by EOT, DCR, the Massachusetts Bay Transportation Authority (MBTA), and other outside sources. This documentation included:

e Design and as-built plans for each identified structure

e DCR Parkway Design Standards and Guidelines

e Bridge and tunnel inspection and load rating reports

e Progress prints of rehabilitation and repair plans currently in design, including staging and traffic management concepts

e Traffic studies and reports A complete list of the documentation provided for the Study has

been included in Appendix E.

Field Reconnaissance

The BETA Team performed cursory visual field observations of each identified infrastructure site to review the general use and overall condition of the structure, visually confirm some of the information contained in the latest Inspection Reports, observe potential impacts to traffic patterns, identify potential staging areas, and review the overall

constructability of the potential rehabilitation programs.

Initial Ranking

After the review of the available information and completion of the field reconnaissance, an initial ranking of the identified structures based upon

the selected evaluation criteria was performed. The ranking was based, in

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general, on the reported severity of each structure’s condition; traffic volume; detour length; functional classification; current design status; and a qualitative review of ability, or inability, to be constructed concurrently with other structures. A matrix of the evaluation criteria is

presented in Appendix G Table G-1.

The initial process resulted in the following order of construction

for the identified structures.

Bowker Interchange - Ramp H (NB Off-Ramp to Storrow Dr. WB) Craigie Dam Bridge and Craigie Drawbridge

Boston University (BU) Bridge

Longfellow Bridge

Cambridge Street (River Street)

Bowker Interchange - Phase Il

Western Avenue

Storrow Drive Tunnels

Memorial Drive over Brookline Street (Reid Overpass)

Eliot Bridge

. North Harvard Street (Larz Anderson) 12.

Memorial Drive Phase II

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Although the Initial Prioritization and Decision Matrix presented in Table G-1 presents a summary of the evaluation criteria, not all of the

criteria can be easily represented in such a tabular format. The additional

criteria and other information used in addition to those found in Table G-1 are detailed below:

The rehabilitation contract for Bowker Interchange - Ramp H had been advertised for construction at the time the rankings were generated. Because no other project in the Basin was as advanced in the design process this structure was ranked first.

An assumption was made that Route 28 (Craigie Dam Bridge and Craigie Drawbridge) would serve as a logical alternate route to mitigate traffic delays during the construction of the Longfellow Bridge. At the time that the rankings were generated, the designs of the Craigie structures’ rehabilitations were at the pre-25% stage, with a scheduled completion date of December 2008. In addition, DCR was in discussions with the US Coast Guard to obtain winter shutdowns (non-operational periods) of the drawbridge from November 1, 2009 to March 15, 2010 and again from November 1, 2010 to Marchi5, 2011 to accommodate the rehabilitation. Therefore, these structures were ranked second, with the additional assumption that they would be administered as one construction contract.

The design of the BU Bridge rehabilitation was at a 90% level of completion at the time the rankings were generated, with minimal permitting remaining. Because of perceived limited overlapping traffic

impacts with the Bowker Interchange - RampH and Craigie Bridge

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projects, the BU Bridge was ranked third. This ranking would allow the bridge’s rehabilitation to be completed in advance of the need to use it as

a detour route for other nearby projects.

The Longfellow Bridge rehabilitation was at the pre-25% design level at the time the rankings were generated; and there was a clear understanding of the required construction staging, extent of rehabilitation efforts and anticipated construction duration. Interim repairs are currently underway to strengthen the structural elements under the roadways. It was assumed that the full rehabilitation of the Longfellow Bridge and the Storrow Drive Tunnels cannot occur simultaneously. In addition, the planned interim repairs to the Storrow Drive Tunnels were designed with the assumption that the full rehabilitation program would commence within five years after the completion of the interim repairs. Therefore, the Longfellow Bridge was ranked fourth, with the goal of completing its rehabilitation before the commencing the full rehabilitation of the Storrow Drive Tunnels.

Cambridge Street (River Street), the Bowker Interchange - Phase Il, and Western Avenue were ranked fifth, sixth, and seventh because of their overall structural condition in general, and because of the visual distress to the concrete elements and steel cross beams of the Bowker Interchange in particular. The rehabilitations of these structures were sequenced after the completion of the BU Bridge rehabilitation in order to avoid perceived overlapping traffic impacts with that project.

The Storrow Drive Tunnels (Eastbound Underpass and the

Berkley Street Underpass) were ranked eighth. The interim repairs to

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these structures currently underway are anticipated to extend the tunnels’ service lives for an additional five years, at which point a full

rehabilitation program must be undertaken. These two structures will be

administered as one construction contract and_ rehabilitated simultaneously and as a result have been shown as one structure throughout this Report. The full rehabilitation of the tunnel structures should not commence until after the completion of the Longfellow Bridge rehabilitation and the majority of the Bowker Interchange - Phase Il rehabilitation. It is anticipated that a minor amount of overlapping construction may occur between the Storrow Drive Tunnels and the Bowker Interchange - Phase II project.

Memorial Drive over Brookline Street (Reid Overpass) was ranked ninth because of the relatively good condition of its main structural elements in comparison to the other structures included in the Study. In addition, the rehabilitation projects for the BU Bridge and Cambridge Street (River Street) Bridges should ideally be completed prior to undertaking the Reid Overpass project.

The Eliot Bridge is in relatively good structural condition. The rehabilitation program is anticipated to be mostly cosmetic in nature. This structure was therefore ranked tenth.

The Larz Anderson Bridge was ranked eleventh due to its overall condition relative to the other structures included in the Study.

Memorial Drive - Phase Il was ranked twelfth because of the

non-critical nature of the roadway work envisioned, and the ability to

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delay the start of this project until the completion of all other identified

projects affecting traffic on Memorial Drive.

Structure Rehabilitation Plans / Staging

Of the twelve identified structures, five have rehabilitation projects

actively in design. In-progress rehabilitation plans and construction staging schemes for these structures were therefore available for use in the Study. For the remainder of the identified structures, a probable approach for rehabilitation was developed in order to derive summary level construction staging. The developed staging concepts were utilized as a basis for determining anticipated traffic restrictions for each individual structure. These summary level staging concepts have been

included in Appendix G as Figures G1 - G12.

Summary Condition Reports

Summary Condition Reports (Appendix K) for each identified structure were prepared based upon data obtained from existing inspection reports, load rating reports, plans, and other provided documentation. These reports provide summary level information regarding: the bridge owner and lead agency responsible for its rehabilitation; overall structural configuration; fracture critical elements; information on the structure’s usage; numerical structural condition ratings; qualitative descriptions of main structural elements; utilities carried; probable rehabilitation plan; rehabilitation design status; a listing of the documentation reviewed; and

a list of assumptions used in the Study.

Environmental Permitting

In support of the development of the synchronization Study, an

assessment was performed in regards to the anticipated permitting

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process required for each identified rehabilitation project. This assessment included a review of the probable rehabilitation programs

and resulting construction tasks in order to determine which regulatory

agencies would be involved and the resulting level of permitting review required. From this, an anticipated duration and timing for the permitting process was developed for each identified structure.

Because the environmental permitting process is an essential component to the success of rehabilitating the structures within the Basin, a Permit Briefing Meeting was held with EOT, DCR, MEPA, DCR, and DEP to discuss the potential environmental permits needed for the structures within the Study. Appendix L contains a complete breakdown of anticipated environmental permits, notifications, and reviews that may be required for each structure, as well as anticipated timeframes for the reviews and approvals. The following assumptions were used in the permitting assessment process:

e All structure rehabilitations will be federally-funded.

e All structures will include work or staging within a 100 foot wetland buffer and/or a 25 foot riverfront area, and will therefore fall under the jurisdiction of the Wetlands Protection Act (310 CMR 10.00). All projects will therefore require filings with the

local Conservation Commissions (Boston, Cambridge, or both).

In order to ensure that the permitting schedule does not impede the construction schedule, early coordination with federal, state, and local agencies will be a design requirement for all projects.

Early coordination should begin at the initiation of project design.

Most bridge work will require the use of barges in the Charles

River for staging and access.

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e All structures have been licensed by Chapter 91 or are exempt

from licensing through legislative acts.

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This section presents the findings of the traffic analyses conducted to

evaluate existing conditions, and the phase of construction for each syn-

chronization project which is anticipated to have the most severe traffic impact.

Traffic Volume Data Collection

The Study utilized data gathered from a combination of historical and primary data collection sources. The primary sources consisted of peak hour intersection Turning Movement Counts (TMC) collected as part of the Storrow Drive Tunnel Reconstruction Project and daily roadway vol- umes collected by the Massachusetts Highway Department and provided by the Central Transportation Planning Staff (CTPS). A list of other data sources reviewed for the Study is included in Appendix E.

The peak hour traffic volumes on all structures crossing the Charles River between the Eliot Bridge and the Charlestown Bridge are included in Appendix F Figure F1. Peak hour turning movement volumes at the forty-two analyzed intersections are presented in Figures F2 - F5 in

Appendix F.

Analysis Methods

Turning movement volumes, field sketches of intersection geometry, and observations of prevailing traffic conditions at all study area intersections were combined to develop existing traffic conditions. The SYNCHRO traffic analysis software (Version 6 Build 614) was used to develop morning

peak hour and afternoon peak hour models. SYNCHRO implements the

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ECT

analysis procedures for signalized and unsignalized intersections con- tained in the 2000 Highway Capacity Manual.

Traffic operations are defined by level-of-service (LOS), which is a

qualitative measure that associates LOS with vehicle delays. The criteria for signalized intersections are different than for unsignalized intersec- tions because drivers expect different performance levels from each type of intersection. The relationship between LOS and delay is summarized in

the following table:

Signalized Intersection Unsignalized Intersection LOS Criteria Criteria Average Total Delay Average Total Delay (Seconds per Vehicle) (Seconds per Vehicle) A < 10.0 < 10.0 B 10.1 to 20.0 10.1 to 15.0 C 20.1 to 35.0 15.1 to 25.0 D 35.1 to 55.0 25.1 to 35.0 E 55.1 to 80.0 35.1 to 50.0 F > 80 > 50

Source: Highway Capacity Manual, Transportation Research Board; Washington, DC; 2000

Traffic volumes are expected to fluctuate as construction activi- ties within the Charles River Basin commence, and are expected to in- crease commensurately along a few key segments of various roadways. These increased volumes often translate to a corresponding increase in vehicle delay and degradation of peak hour operations at the intersec- tions along the roadway. Conversely, reduced traffic volumes are ex- pected along other key segments. These reduced traffic volumes often translate to a corresponding reduction in vehicle delay and improvement

in peak hour operations at the intersections along each affected roadway.

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The study area intersections were initially analyzed for existing weekday morning and weekday afternoon peak hour conditions. The most

severe construction stage for each synchronization project was selected

for analysis. These analyses have assumed that no other synchronization projects will be occurring simultaneously. This information was utilized when looking at the resulting level of service under each combination in order to asses whether any degradation in level of service was due to an individual project, or as result of a combination of projects.

For consistency, all signalized intersection analyses were per- formed using existing traffic signal phasing and timing and have not been optimized to accommodate anticipated shifts in traffic patterns. The re- sults for signalized intersection analyses are summarized in Tables |-1 and I-2 in Appendix | while the results for unsignalized intersections are summarized in Tables I-3 and I-4. This information is presented in a geo-

graphic format in Figure I1 in Appendix I.

Existing Traffic Conditions Morning Peak Hour Analysis

Nearly all intersections included in the study area operate at LOS D or better during the peak hours under existing conditions. There are six sig- nalized intersections which operate at LOS E or F during the morning peak hour. The following two signalized intersections operate at LOS E during the morning peak hour:

e River Street @ Memorial Drive,

e Columbia Street & Main Street @ Massachusetts Avenue, The following four signalized intersections currently operate at

LOS F during the morning peak hour:

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e Memorial Drive @ JFK Street, e O'Brien Highway @ Leverett Circle, e Embankment Road eastbound @ Leverett Circle,

e Land Boulevard @ Monsignor O’Brien Highway

Afternoon Peak Hour Analysis

During the afternoon peak hour, twelve signalized intersections operate at LOS E or F. The following five signalized intersections operate at LOS E during the afternoon peak hour:

e Western Avenue @ Memorial Drive,

e Boston University Bridge @ Commonwealth Avenue, e Beacon Street @ Berkeley Street,

e Beacon Street @ Arlington Street,

e O’Brien Highway @ Leverett Circle,

The following seven signalized intersections operate at LOS F dur- ing the afternoon peak hour:

e Memorial Drive @ JFK Street,

e River Street @ Memorial Drive,

e Boylston Street @ Massachusetts Avenue,

e Beacon Street @ Charles Street,

e Embankment Road eastbound @ Leverett Circle,

e Land Boulevard @ Monsignor O’Brien Highway,

e Causeway Street @ Commercial Street @ North Washington Street

The stop sign controlled northbound right turn from Memorial Drive onto the Longfellow Bridge operates at LOS E during the morning peak hour and LOS F during the afternoon peak hour. The southbound right turn from Memorial Drive to the Longfellow Bridge operates at LOS D and LOS C during the morning and afternoon peak hours, respectively.

The southbound through movement at the unsignalized intersection of

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the Longfellow Bridge and the Storrow Drive westbound Off-Ramp oper- ates at LOS D during the morning peak hour and LOS C during the after-

noon peak hour. The adjacent right turn operates at LOS C and LOS B in

the morning and afternoon peak hours, respectively. The northbound right turns from Rutherford Avenue and Main Street onto Alford Street operate at LOS F during the morning and afternoon peak hours.

The higher number of intersections operating at LOS E or F during the afternoon peak hour is typical throughout the Boston area. This is in part due to elevated traffic volumes and in some cases elevated pedes-

trian volumes as well.

Construction Impacts

The following section will discuss the methodology and assumptions used to create construction impacts and analyze traffic operations for each synchronization project.

Traffic Reassignment Process

A traffic reassignment process was conducted for each project to account for traffic diversions due to construction. The process began by evaluat- ing the volume of traffic currently passing through each project area dur- ing the morning and afternoon peak hours.

The proposed construction staging was then reviewed for each project. Details on the number and width of the travel lanes and any pro- posed intersection modifications to be implemented during each stage of construction were reviewed. The single construction stage anticipated to have the most severe traffic impact was selected for analysis. A construc-

tion period traffic link capacity was generated for each study project, and

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the number of trips which would be likely to divert due to congestion was estimated.

BETA also obtained a set of “select-link” analyses for each study

project from CTPS to aid in determining trip origins and destinations. These select-links provide trip origin and destination data for trips pass- ing through a pre-specified roadway link in the regional transportation system. The CTPS model currently contains 2,727 analysis zones which represent the 101 cities and towns served by the Boston MPO. The 2,727 analysis zones were aggregated by CTPS into a more manageable 39 zones for use by BETA in this analysis (See Appendix M).

CTPS conducted a model run for each project, selecting a single “select link” in one travel direction. During each model run, origin and destination data were collected for each trip passing through the link in the specified direction during both the morning (6AM - 9AM) and the af- ternoon (3PM - 7PM) peak periods. The direction specified for each “se- lect link” was inbound. It was assumed that the inbound direction would be reversed for outbound traffic. The results of the select-link analyses are located in Appendix M.

The select-link data and the volume of traffic expected to divert from a project area were used to generate reassignment routes. The re- assignment routes generated for each study project are provided as Fig- ures H1 through H12 in Appendix H.

Other assumptions include:

e Existing intersection level of service at intersections adjacent to a

link were considered in the reassignment process.

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Reassignments were made primarily to collector and arterial roadways. Local / residential routes were not chosen for reas- signments.

e In some cases, regional diversions were made to circumferential

and radial highways including and within the I-95 belt.

Secondary diversions (reactions to the initial reassignment proc-

ess) have not been accounted for at this stage.

Shifts in transportation mode have not been accounted for at this

stage.

Traffic Capacity Assumptions

The following section will describe the assumptions made related to traf- fic capacity provided during the most serve traffic condition during each of the study projects.

Bowker Interchange (Ramp H): During the most severe traffic im- pact stage of construction the Bowker Line A to Storrow Drive WB will be closed. Line A traffic destined for Storrow Drive westbound would exit to the Charlesgate East surface road, continue along Charlesgate East to the intersection with Beacon Street, and take the Line A on-ramp to ac- cess Storrow Drive westbound. No physical improvements to capacity have been recommended by the traffic management plan for the inter- sections along the proposed detour route.

Craigie Dam Bridge and Drawbridge: The most severe traffic im- pact stage of construction would provide a single travel lane in each di- rection in the area of the Craigie Drawbridge. Since the two structures are closely spaced, traffic capacity restrictions imposed at one structure will effectively be imposed on the other. It was assumed that two-thirds of the

inbound traffic and one-half of the outbound traffic would be diverted.

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Reducing the inbound roadway to one lane significantly reduces the queuing space available on the approach to the traffic signals located at

Leverett Circle.

Boston University (BU) Bridge: The traffic management plan pro- poses to provide one travel in each direction throughout the entire dura- tion of construction. It was assumed that 50% of the traffic currently us- ing the bridge would find an alternate route. This equates to a per lane flow rate of approximately 1,000 vehicles per hour per lane. This value would be low for an isolated lane, but in this case the traffic signals lo- cated at either end of the bridge control the volume of traffic which can flow over the bridge.

Longfellow Bridge: The construction stage anticipated to have the most adverse effect on traffic would provide two travel lanes in the in- bound travel direction and one travel lane in the outbound direction. The two inbound lanes would not be side-by-side, but would be placed on ei- ther side of the MBTA red line tracks located in the center of the bridge. One inbound lane would remain on the inbound side of the bridge. The second lane would be diverted in Cambridge to a contra flow lane on the outbound side of the bridge, and it would terminate at a new signalized intersection in Boston. The new signalized intersection is currently an un- signalized intersection which serves outbound traffic to the bridge and existing traffic from Storrow Drive westbound. Inbound traffic traveling in the contra flow lane would be permitted to access the Storrow Drive eastbound on-ramp and Charles Street north of Charles Circle. It was as-

sumed that approximately half of the traffic which currently travels in-

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bound on the Longfellow Bridge and is destined for Storrow Drive east- bound or Charles Street north of Charles Circle would use the Contra Flow

lane. All other movements would be served by the single lane on the in-

bound side of the bridge.

Cambridge Street (River Street): Throughout the construction pe- riod it is proposed that the existing three lane section be reduced to two lanes. As with other structures in the Basin, traffic flow on this structure is regulated by traffic signals on each end. The ability of the traffic signals to serve vehicles is reduced by the reduction in the number of lanes on the bridge. It was assumed that approximately one-third of the traffic cur- rently using the bridge will choose to find an alternative route to reach their destination.

Bowker Interchange - Phase Il: The stage of construction which is assumed to create the most severe traffic impacts is Phase 7, where the Line B Off-ramp and the Storrow Drive eastbound mainline are under construction simultaneously. During this stage the lane width of the Line B Off-ramp would be narrowed, and the Storrow Drive eastbound mainline would be reduced to one travel lane, reducing the traffic capac- ity of the mainline lane by approximately 50%.

Western Avenue: Throughout the construction period it is pro- posed that the existing three lane section be reduced to two lanes. As with other structures in the Basin, traffic flow on this structure is regu- lated by traffic signals on each end. The ability of the traffic signals to serve vehicles is reduced by the reduction in the number of lanes on the

bridge. It was assumed that approximately one-third of the traffic cur-

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rently using the bridge will choose to find an alternative route to reach their destination.

Storrow Drive Tunnels: Extensive traffic modeling was conducted

by CTPS for the Storrow Drive Tunnels Reconstruction project. The infor- mation derived for the “Option A” construction period was used by this Study to represent the most serve traffic impact period. During this phase of construction two lanes of traffic would be maintained during peak hours. The eastbound Off-Ramp to Clarendon Street and the on-ramps from Berkeley Street would be closed to accommodate a temporary road for eastbound traffic which would pass through the Berkeley Street tun- nel. An eastbound off-ramp to Dartmouth Street would be opened when the Clarendon Street off-ramp is closed. The westbound off-ramp to Ar- lington Street would also remain open during peak hours.

Memorial Drive over Brookline Street (Reid Overpass): Mainte- nance of eastbound traffic on the overpass is proposed for the duration of construction. When construction activities are taking place on the eastbound lanes, eastbound traffic would be shifted to the westbound lanes. Westbound traffic would be diverted to the Brookline Street sur- face rotary. Temporary traffic signals are proposed at two locations on the north side of the rotary to accommodate the increased traffic demand. When work on the eastbound roadway of the overpass is complete, east- bound traffic will resume travel in the eastbound lanes and the west- bound lanes will be closed. It is assumed that no eastbound vehicles on

Memorial Drive will choose to divert during construction activities. It is

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assumed that 10% of Memorial Drive westbound traffic will choose to divert from the Brookline Street surface rotary to an alternate route.

Eliot Bridge: Two 11-foot wide traffic lanes are proposed for the

duration of construction activity. This matches the number of lanes cur- rently provided, but the lanes would be slightly narrower than the existing 12-foot lanes. It was assumed that 10% of traffic currently using the bridge would choose to use an alternate route to avoid the construction activities.

North Harvard Street (Larz Anderson Bridge): Throughout the construction period it is proposed that the existing four lane section be reduced to two lanes (one lane in each direction). As with other structures in the Basin, traffic flow on this structure is regulated by traffic signals on each end. The ability of the traffic signals to serve vehicles is reduced by the reduction in the number of lanes on the bridge. It was assumed that approximately one-half of the traffic currently using the bridge in each direction will choose to find an alternative route to reach their destina- tion.

Memorial Drive (Phase II): It was assumed that construction ac- tivities associated with this project would be primarily confined to reduc- tion of one lane at any time, which would maintain three open lanes at any one given time. Since traffic volumes through this area are directional in nature, a reversible lane traffic management strategy would be effec- tive. This strategy would maintain two lanes eastbound and one lane westbound during the