Low-Cost Improvements to Express-Highway Bottleneck Locations

Project Managers

Seth Asante and Chen-Yuan Wang

Project Principal

Mark Abbott

Data Analysts

Benjamin Erban

Kathy Jacob

Graphics

Ken Dumas

Cover Design

Jane Gillis

Editor

Meghan Connolly

The preparation of this document was supported

by Federal Highway Administration through

MPO 3CPL FFY2019 Contract #105757

Central Transportation Planning Staff

Directed by the Boston Region Metropolitan

Planning Organization. The MPO is composed of

state and regional agencies and authorities, and

local governments.

February 2020

Abstract

The purpose of the Low-Cost Improvements to Express-Highway Bottleneck Locations study is to identify low-cost improvements that will help reduce congestion at freeway bottleneck locations in the Boston Metropolitan Planning Organization (MPO) region. Bottlenecks in the freeway network can occur where geometric elements, such as ramps or lane drops, restrict traffic flow and are a major contributor to recurring congestion. This study was done in cooperation with the Massachusetts Department of Transportation (MassDOT) Highway Division and the Federal Highway Administration Massachusetts Division.

Candidate locations were selected based on input from the MassDOT Highway Division and Congestion Management Process data. The screening process yielded two locations that had the potential to respond to low-cost improvement measures. These locations included:

• Interstate 93 (I-93) northbound between Exit 40 (Route 62) and Exit 41 (Route 125) in Wilmington
• I-93 southbound at the end of the high occupancy zipper lane exit between Exit 7 (Route 3) and Exit 8 (Furnace Brook Parkway) in Braintree and Quincy

Both locations regularly experience poor level of service (LOS) because of one or more freeway bottlenecks during peak travel periods. MPO staff developed low-cost improvement proposals to address each bottleneck. If implemented, the modifications would result in capacity and safety improvements on these two high-volume facilities.  

This report summarizes the analyses and recommendations from the study. The report is divided into multiple chapters, with two chapters covering each study location. Each location chapter summarizes existing conditions, proposes various low-cost measures to address the bottlenecks, and evaluates the efficacy of the proposed alternatives using methodology from the Highway Capacity Manual. The report concludes with a summary of the recommendations, followed by figures that illustrate features of the proposed improvements. As an addendum, the report includes technical appendices that cite the methods used and the data applied.

Table of Contents                                                                                PAGE

Chapter 1—Introduction................................................................................ 9

1.1...... Introduction.................................................................................... 9

1.2...... Background.................................................................................... 9

1.3...... Purpose of Study........................................................................... 10

Chapter 2—Selection of Study Locations...................................................... 13

2.1 ..... Screening Criteria.......................................................................... 14

2.2 ..... Study Locations............................................................................. 14

2.2.1      Location 1: I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125) in Wilmington 15

2.2.2      Location 2: I-93 Southbound at the End of the High Occupancy Vehicle (HOV) Zipper Lane in Quincy and Braintree 15

Chapter 3—Data Collection and Uses............................................................ 17

3.1...... Traffic Volume Data........................................................................ 17

3.2...... Crash Data................................................................................... 17

3.3...... Speed Data.................................................................................. 17

3.4 ..... Level of service Criteria................................................................... 17

Chapter 4—Location 1: I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125) in Wilmington   21

4.1 ..... Existing Freeway Characteristics....................................................... 21

4.1.1      Basic Freeway Section......................................................... 21

4.1.2      Entrance Ramp.................................................................. 21

4.1.3      Exit Ramp......................................................................... 22

4.1.4      I-93/Route 125 Interchange.................................................. 22

4.2...... Problems...................................................................................... 23

4.3...... Causes........................................................................................ 23

4.3.1      High Traffic Volume............................................................. 23

4.3.2      Short Acceleration Lane....................................................... 23

4.3.3      Short Deceleration Lane....................................................... 24

4.3.4      Traffic Congestion from I-93 Northbound Downstream Bottleneck. 24

4.4...... Impacts........................................................................................ 24

4.4.1      Crashes............................................................................ 24

4.4.2      Travel Speed..................................................................... 26

4.4.3      Level of Service.................................................................. 27

4.5...... Improvement Alternatives................................................................ 28

4.5.1      Alternative 1: Create an Auxiliary Lane for Merging and Diverging Traffic    29

4.5.2      Alternative 2: Examine Potential Improvements at the Downstream Lane Drop Location  30

4.6...... Effectiveness and Cost of the Improvements....................................... 30

4.7...... Recommendations......................................................................... 32

Chapter 5—Location 2: I-93 Southbound Segment at the End of the HOV Zipper Lane in Quincy and Braintree      33

5.1...... Existing Freeway Characteristics....................................................... 33

5.1.1      Basic Freeway Section......................................................... 33

5.1.2      Entrance Ramps................................................................. 33

5.1.3      Exit Connector................................................................... 33

5.2...... Problems...................................................................................... 34

5.3...... Causes........................................................................................ 34

5.4...... Previous Configuration.................................................................... 35

5.5 ..... Impacts........................................................................................ 35

5.5.1      Crashes............................................................................ 35

5.5.2      Travel Speed..................................................................... 37

5.5.3      Level of Service.................................................................. 37

5.6...... Improvement Alternatives................................................................ 39

5.6.1      Alternative 1: Lengthen the Distance for the HOV Merge............. 39

5.6.2      Alternative 2: Lengthen Acceleration Distance for Furnace Brook Parkway On-Ramp      39

5.6.3      Alternative 3: Alternatives 1 and 2 Combined............................ 39

5.7...... Effectiveness and Cost of the Improvements....................................... 39

5.7.1      HCS Analysis Results.......................................................... 39

5.7.2      VISSIM Simulation Results................................................... 42

5.7.3      Costs............................................................................... 42

5.8...... Recommendations......................................................................... 43

Chapter 6—Conclusion and Next Steps......................................................... 45

TABLES

Table 1 Inventory of Express-Highway Locations for Screening....................................... 13

Table 2 LOS Criteria for Basic Freeway, Merging/Diverging, and Weaving Segments. 19

Table 3 Crash Summary (2012–16): Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125)      25

Table 4 LOS Analysis–Existing Conditions: Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125)      28

Table 5 2030 LOS Analysis–No-Build and Improvement Alternative 1 for Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125)........................................................................................................... 31

Table 6 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Five-Year Crash Summary (2012–16)     36

Table 7 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Existing Conditions LOS Analysis 38

Table 8 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: 2030 Future LOS Analysis 41

Table 9 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Traffic Simulation Analysis 42

FIGURES

Figure 1 Regional Map of Study Areas.................................................................................. 46

Figure 2 Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Peak Period Traffic Volumes  47

Figure 3  Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Location and Number of Crashes     48

Figure 4 Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Congestion Scan          49

Figure 5 Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Alternative 1—Create Auxiliary Lane 50

Figure 6 Location 2—I-93 Southbound at the End of the HOV Zipper Lane:.................. 51

Figure 7 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Location and Number of Crashes            52

Figure 8 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Five-Year (2012–16) Crashes when the HOV Lane is in Operation (3:00 PM to 7:00 PM)............................................................................. 53

Figure 9 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Congestion Scan    54

Figure 10 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Alternative 1—Lengthen the HOV Lane Merge Distance     55

Figure 11 Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Alternative 2—Lengthen the Acceleration Distance for Furnace Brook Parkway On-Ramp........................................................................................ 56

APPENDICES

Appendix A: Review Comments and Selection Process

Appendix B: ATR and Classification Data

Appendix C: Crash Tables

Appendix D: HCS Printouts

Chapter 1—Introduction

1.1      Introduction

This report summarizes the results of the analyses and improvement alternatives considered in the federal fiscal year 2019 study, Low-Cost Improvements to Express-Highway Bottleneck Locations. The report begins with background information and describes the purpose of the study, followed by the selection of study locations, an assessment of the safety and operational problems, and a discussion of the potential improvement strategies. The final section presents the study recommendations. The report concludes with technical appendices, which cite the study methods, describe how the data and analyses were applied, including detailed reports from the freeway merge and diverge analyses. If implemented, the report’s recommendations would not only result in improvements on the freeway facilities; they would improve traffic safety, make traffic operations more efficient, and reduce congestion at the bottlenecks.

1.2      Background

According to the Federal Highway Administration (FHWA),

Much of the recurring congestion is due to physical bottlenecks—potentially correctible points on the highway system where traffic flow is restricted. While many of the nation’s bottlenecks can only be addressed through costly major construction projects, there is a significant opportunity for the application of operational and low-cost infrastructure solutions to bring about relief at these chokepoints.1

To be consistent with this guidance, the FHWA Massachusetts Division has recommended, as part of its comments on the Unified Planning Work Program process, that the Boston Region Metropolitan Planning Organization (MPO) identify the worst bottlenecks in the region that can be mitigated with low-cost countermeasures and develop recommendations for such countermeasures at these locations.

In general, recurring bottlenecks are influenced by the design or operation present at the point where the bottleneck begins; for example, at merge locations, diverges, lane drops, traffic weaving, and abrupt changes in highway alignment. In previous years, MPO staff analyzed several express-highway bottleneck locations in four consecutive studies, which were well received by the Massachusetts Department of Transportation (MassDOT) and the FHWA.2 ^,3,4,5 Study locations included sections of Interstate 95 (I-95) in Burlington, Lexington, and Weston; sections of Interstate 93 (I-93) in Woburn; sections of Route 3 in Braintree; and sections of Route 24 in Canton and Randolph.

The MassDOT Highway Division has implemented many of the recommendations from those studies, and the FHWA has interviewed MPO staff about these successful implementations, including

• restriping lanes to serve traffic demand better on I-95 northbound at Interchange 24 in Weston;
• restriping lanes to serve traffic demand better on I-95 southbound at Interchange 24 in Weston;
• providing two-lane exits for traffic exiting I-95 northbound to Route 3 northbound and the Middlesex Turnpike at Interchange 32 in Lexington and Burlington; and
• providing two-lane exits for traffic exiting I-95 southbound to Route 3 northbound and the Middlesex Turnpike at Interchange 32 in Burlington.

1.3      Purpose of Study

The purpose of this study is twofold. First, the study aims to identify two bottleneck segments or points where low-cost mitigation improvements seem applicable. Second, the study aims to recommend low-cost mitigation improvements based on analysis of geometric design, traffic volumes and other data, and projected service performance associated with the improvements at each location.

Since 2011, the MPO has conducted four bottleneck studies in the Boston region to identify low-cost methods to reduce congestion, increase safety, and improve traffic operations. In the current study, MPO staff will rely on technical expertise regarding the nature of bottlenecks and will seek input from the MassDOT Highway Division staff, who are familiar with the region’s express-highway system operations, to develop and evaluate a comprehensive list of potential improvements at the bottleneck locations.

Chapter 2—Selection of Study Locations

To select the study locations, MPO staff had to first inventory and screen all candidate locations.⁶ MPO staff developed an initial list of candidate locations in the MPO region based on the following parameters:

• Consultations with the MassDOT Highway Division
• Review of Congestion Management Process (CMP) monitoring data and recent MPO and other planning studies
• Staff knowledge of bottleneck locations in the Boston Region MPO area

Table 1 presents the inventory process, which yielded nine bottleneck locations in the Boston Region MPO area for screening.

Table 1
Inventory of Express-Highway Locations for Screening

Note: Shading indicates locations selected for study

HOV = high occupancy vehicle. I-93 = Interstate 93. I-95 = Interstate 95. MassDOT = Massachusetts Department of Transportation.

Source: Central Transportation Planning Staff

2.1      Screening Criteria

MPO staff used the following three criteria to screen the bottleneck locations:

1. Does the location qualify as a bottleneck?
   A repetitive, long-traffic queue upstream trailing free-flowing traffic downstream usually characterizes the location as a bottleneck. In other words, the location experiences routine and predictable congestion because traffic volume exceeds the available capacity at that location.
2. Is a physical design constraint or operational conflict inherent in the location the cause of the bottleneck?
   Examples of these include the following constraints or conflicts:
1. Lane drop: One or more travel lanes end, requiring traffic to merge
2. Weaving area: Drivers must merge across one or more lanes to access an entry or exit ramp
3. Merge area: On-ramp traffic merges with mainline traffic to enter the freeway
4. Major interchanges: High-volume traffic is directed from one freeway to another
   
3. Can low-cost operational and geometric improvements fix the bottleneck?
   These exclude costly long-term solutions such as expansion or widening of the roadway. Examples of low-cost operational and geometric improvements include the following:
1. Using a short section of shoulder as an additional travel lane or for lengthening an acceleration or deceleration lane
2. Restriping merge and diverge areas to better serve traffic demand
3. Providing all-purpose reversible lanes
4. Changing or adding signs and striping

Locations selected for study must meet these criteria and the number of locations selected for the study is dependent on allocated funding.

2.2      Study Locations

Based on the screening criteria and consultations with the MassDOT Highway Division officials, MPO staff selected location numbers 1 and 2 for study. Figure 1 shows the study locations.7 Although locations 3, 4, 5, 6, 7, and 8 met the screening criteria, MPO staff did not select them for this study due to funding concerns. However, MPO staff would consider these locations in a future bottleneck study. In addition, MassDOT is currently developing a project to address the bottleneck at Location 9. Appendix A contains comments about the study from the MassDOT Highway Division and a memorandum to the MPO that describes the selection process in detail.

2.2.1   Location 1: I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125) in Wilmington

This mile-long segment of highway (with four travel lanes) is a bottleneck because of merging and diverging activities, which causes congestion, especially during the PM peak periods. In the segment, there are two exit ramps and three entry ramps connecting Routes 62 and 125 to I-93. The ramps also carry high volumes of traffic because of office and industrial parks located off Route 125.

At both exits, the northbound ramps have approximately 500 vehicles per hour (vph) and 700 vph exiting I-93 northbound to Route 62 and Route 125 during the AM peak period, respectively, and about 700 vph and 1,000 vph during the PM peak period. During the same periods, the on-ramps from Route 62 and Route 125 to I-93 northbound receive about 500 vph and 300 vph during the AM period, respectively, and about 400 vph and 600 vph during the PM peak period.

This entering and exiting of traffic interacts with approximately 4,000 to 5,500 vph on the mainline during the AM peak period and approximately 7,000 to 7,500 vph during the PM peak period. The merging and diverging maneuvers in the vicinity creates a bottleneck that backs up traffic on the mainline.

2.2.2   Location 2: I-93 Southbound at the End of the High Occupancy Vehicle (HOV) Zipper Lane in Quincy and Braintree

This bottleneck is located on I-93 southbound at the end of the HOV zipper lane, where traffic exits the southbound HOV lane and then merges with the traffic on the mainline. The bottleneck occurs only during PM peak periods when the southbound HOV lane is in operation. Traffic from six lanes is forced onto a four-lane freeway segment. The reduction in the number of lanes, merging, diverging, and weaving of traffic, and the high number of lane-changing maneuvers to disperse traffic to continue on I-93 southbound or head to Route 3 southbound, dramatically reduces capacity in the segment, creating a bottleneck. During PM peak periods, about 5,100 to 5,500 vph pass through the bottleneck—600 vph from the Furnace Brook Parkway on-ramp, 3,500 vph in the four general-purpose lanes, and 1,000 vph in the HOV lane. Given an upstream traffic demand of 8,000 vph, which is far greater than the capacity of the bottleneck, a long traffic queue forms on the mainline, which extends five miles to Columbia Road in Dorchester.

Chapter 3—Data Collection and Uses

3.1      Traffic Volume Data

The MassDOT Highway Division’s Traffic Data Collection Program conducted automatic traffic recorder (ATR) counts for the ramps, freeways, and arterial roadways at the locations selected for study. The ATR counts traffic continuously for at least 48 hours. MPO staff used these counts to determine the average weekday traffic of a highway and operations performance. The traffic volume data are included in Appendix B. In addition, MassDOT collected turning movement counts (TMCs) for the signalized intersections at the ramp-arterial junction on Route 125. MassDOT performed TMCs during the weekday AM peak travel period (6:00 AM to 9:00 AM) and weekday PM peak travel period (3:00 PM to 6:00 PM).

3.2      Crash Data

MPO staff used crash data from January 2012 to December 2016 from the MassDOT’s Registry of Motor Vehicles database to evaluate safety for motorists. Crash data are included in Appendix C.

3.3      Speed Data

The CMP maintains average speed data on express-highway systems in the MPO region with use of the INRIX historical traffic speed data.8 MPO staff used the current speed data from the CMP (spring 2015 and fall 2015) to determine the average weekday travel speeds through the bottlenecks.

3.4      Level of service Criteria

Level of service (LOS) is a quality measure describing operational conditions within a traffic stream, generally in terms of such service measures as speed and travel time, freedom to maneuver, traffic interruptions, and comfort and convenience. Factors influencing LOS are volume, lane width, lateral obstructions, traffic composition, grade, and speed. The Highway Capacity Manual (HCM) methodology demonstrates driving conditions on freeways in terms of LOS ratings from A through F.⁹ The LOS criteria characterize freeway performance measures in terms of density (passenger cars per lane mile, [pc/lane mile]). The LOS criteria has been developed for each freeway component—basic freeway, ramp merge/diverge, and weaving segments. The locations and traffic flow characteristics at each of the components are described below.

• Basic freeway segments are outside of the influence area of ramps or weaving areas of the freeway. The flow in such segments occurs more smoothly than segments with merging, diverging, or weaving. The exact point at which basic freeway segments begin or end depends on local conditions, particularly the LOS operating at the time. If traffic flow is light, the influence may be negligible, whereas under congested conditions, queues may be extensive.
• Freeway merging segments are ramp junction areas where two separate traffic streams (mainline and on-ramp) join to form one stream on the mainline. The influence areas of merging segments depend on local traffic conditions.
• Freeway diverging segments are ramp junction areas where one traffic stream diverge to form two separate streams (mainline and off-ramp). The influence areas of merging segments depend on local traffic conditions.
• Weaving segments are areas of the freeway where two or more vehicle flows must cross paths along a length of the freeway in order to continue. They are usually formed when merge areas are closely followed by diverge areas.
• A major merge occurs when two multilane freeway segments combine to form a single freeway segment with three or more lanes. Likewise, a major diverge occurs when a freeway segment with three or more lanes splits into two multilane basic freeway segments. While these locations can create turbulence in the traffic flow, they are less restrictive than freeway ramps because speed differences are smaller and lane changes are often unnecessary.

Table 2 shows the LOS criteria for basic freeway, merge/diverge, and weaving segments.

Table 2
LOS Criteria for Basic Freeway, Merging/Diverging, and Weaving Segments

LOS = level of service. pc/lane mile = passenger cars per lane mile. V/C = volume-to-capacity ratio.

Source: Highway Capacity Manual 2010.

LOS A represents the best operating conditions (unrestricted operations) while LOS F represents the worst operating conditions. LOS A through LOS D represent acceptable operating conditions. LOS E represents operating conditions at capacity. LOS F represents failing conditions (demand exceeds capacity).

The traffic operations analyses conducted by MPO staff were consistent with HCM methodologies. Using the data collected, MPO staff then built traffic analysis networks for the AM and PM peak hours using the 2010 Highway Capacity Software (HCS) to assess the capacity and quality of traffic flow at the two bottleneck areas.

Chapter 4—Location 1: I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125) in Wilmington

Study location 1 is a stretch of I-93 northbound in Wilmington. Figure 1 shows the location of the bottleneck within the MPO region. The northbound off- and on-ramps connect to and from Route 62 (Salem Street) and Route 125 (Ballardvale Street), and a service plaza. The bottleneck conditions form primarily during the PM peak period, when high volumes of rush hour traffic heads northbound on I-93. This interchange and the roadways are under the jurisdiction of the MassDOT Highway Division, located in District 4.

4.1      Existing Freeway Characteristics

Operations at this bottleneck are associated with the following freeway components.

4.1.1   Basic Freeway Section

The basic freeway section of I-93 northbound has four 12-foot travel lanes, a 12-foot right shoulder, and a 6-foot left shoulder. This section carries approximately 4,000 to 5,500 vph during the AM peak period and 7,000 to 7,500 vph during the PM peak period.10 The posted speed limit is 65 miles per hour (mph). Freeway exit signs are posted at one-mile and one-half-mile intervals to guide drivers to Routes 62 and 125.

4.1.2   Entrance Ramp

The entrance ramp from Route 62 to I-93 northbound is a one-lane, one-way roadway. It carries about 250 to 450 vph during the AM peak period and 250 to 400 vph during the PM peak period. The length of the acceleration lane for traffic entering the section from Route 62 westbound is approximately 300 feet long, and the design speed of the entrance ramp is presumed to be 25 mph (no posted speed limits observed).11 Based on highway design and entrance ramp curve design speeds, the length of the acceleration lane does not meet MassDOT standards. The MassDOT Highway Division’s current Project Development and Design Guide specifies a minimum acceleration lane of 1,220 feet for a freeway facility with a design speed of 65 mph, an entrance ramp curve design speed of 25 mph, and a grade of two percent or less.12

4.1.3   Exit Ramp

The exit ramp from I-93 northbound to Route 125 is a one-way, one-lane roadway that gradually widens and splits into two approaches to connect Route 125 eastbound and westbound at the end of the ramp. The Route 125 eastbound (right-turn) approach has two lanes and the Route 125 westbound (left-turn) approach has only one lane, and the intersection is signalized. The ramp carries about 500 to 850 vph during the AM peak period and about 750 to 1,050 vph during the PM peak hour.

The length of the deceleration lane is approximately 400 feet long and the posted speed limit on the exit ramp is 30 mph. Based on highway design and exit ramp curve design speeds, the length of the deceleration lane is insufficient to meet the MassDOT design standards. The MassDOT Project Development and Design Guide specifies a minimum deceleration length of 440 feet for a freeway facility with a design speed of 65 mph, an exit ramp curve design speed of 35 mph, and a grade of two percent or less. The deceleration lane is the parallel type (at least half the length of the deceleration lane is parallel with the mainline) as recommended by the MassDOT design guide.

4.1.4   I-93/Route 125 Interchange

The exit ramp at Route 125 is signalized as part of a coordinated system that includes the traffic signals at I-93 southbound exit ramp and at Ballardvale Street. It is essential to ensure that traffic operations at the interchange would not cause traffic backup on the off-ramp into the I-93 northbound mainline, especially in the PM peak period when the ramp carries a high volume of traffic.

MPO staff conducted intersection capacity analyses and traffic simulations for the three intersections on Route 125 by using the Synchro traffic analysis and simulation program.13 The PM peak hour analyses indicate that both intersections at the interchange operate at a desirable LOS A, and the intersection of Route 125 at Ballardvale Street operates at an acceptable LOS D. At the I-93 northbound off-ramp and Route 125 intersection, the off-ramp is evaluated to operate at LOS C, with an average queue length of about 100 feet and an estimated 95th percentile queue length of about 150 feet14. Traffic simulation runs showed no traffic queues from the ramp backing up into the I-93 northbound travel lanes.

4.2      Problems

The existing bottleneck creates an intense interruption of traffic flow primarily during PM peak travel periods, experienced by virtually all drivers in this section. Meanwhile, traffic on I-93 northbound is already congested during the same travel periods, due to a lane-drop bottleneck on the I-93 mainline about 1.2 miles downstream from this location.15 Travel speeds on the freeway mainline in this section usually reduce to under 55 mph during the PM peak period. In addition, staff identified a number of crashes on I-93 northbound from Exit 40 to Exit 41, especially in the diverge area of the exit ramp to Route 125.

4.3      Causes

The following factors contribute to traffic congestion in this I-93 northbound section:

• A high volume of traffic during the PM peak hours
• A short acceleration lane at the ramp from Route 62
• A short deceleration lane at the ramp to Route 125
• Traffic congestion from I-93 northbound downstream bottleneck

4.3.1   High Traffic Volume

Figure 2 shows the traffic flows during the AM and PM peak periods. In the AM peak period (6:00–10:00), the I-93 mainline and the entrance and exit ramps generally carry moderate traffic volumes and do not cause serious traffic congestion. However, in the PM peak period (3:00–7:00), the I-93 mainline and the exit ramp carry high traffic volumes, causing traffic congestion at this bottleneck location.

4.3.2   Short Acceleration Lane

The short acceleration lane for the traffic entering I-93 northbound from Route 62 forces drivers to merge quickly and does not give them the distance needed to reach safe freeway speeds. Meanwhile, the merging maneuver is difficult during the PM peak period due to the congested conditions on the freeway mainline. The merging operation slows down I-93 mainline traffic, affecting traffic flow upstream from the merge location.

4.3.3   Short Deceleration Lane

Although the acceleration lane for the exit ramp to Route 125 is just short of MassDOT design standards, it carries a high volume of traffic during the PM peak hours. Under the congested conditions, drivers usually experience delays in reaching the exit ramp and occasionally some drivers would use the breakdown lane to access the ramp.  

4.3.4   Traffic Congestion from I-93 Northbound Downstream Bottleneck

I-93 northbound reduces from four to three travel lanes about 1.2 miles downstream from this section. During the PM peak travel periods, traffic queues from the downstream bottleneck location frequently spill back into this section, increasing difficulties for the merging and diverging operations.

4.4      Impacts

4.4.1   Crashes

Table 3 presents a summary of the crashes in this segment. There were 102 crashes in this area based on MassDOT crash data from 2012–16. Figure 3 shows the location of these crashes. The majority of crashes (69 crashes to be exact) occurred near the off-ramp at Exit 41.

Table 3
Crash Summary (2012–16): Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125)

Note: The weekday evening peak period is 3:00 PM to 7:00 PM from Monday through Friday.
Source: Central Transportation Planning Staff.

A summary of the crashes in this segment are as follows:

• 30 percent of the crashes resulted in injury
• 56 percent of the crashes were rear-end collisions
• 25 percent of the crashes were single vehicle collisions
• 37 percent of the crashes occurred during the PM peak travel periods
• 77 percent of the crashes occurred under daylight conditions
• 87 percent of the crashes occurred under dry roadway conditions

4.4.2   Travel Speed

Figure 4 is a congestion scan that covers the I-93 northbound stretch about three miles upstream and downstream of this study location. It shows the average travel speeds on I-93 northbound from the Concord Road interchange (Exit 39) to the Dascomb Road interchange (Exit 42). The 2015 spring and fall midweek Tuesday to Thursday travel time data (provided by INRIX) were used for this analysis.¹⁶

Travel speeds at the study location, in most cases, reduce to under 55 mph from 2:45 PM to 3:15 PM and from 4:30 PM to 6:00 PM. Travel speeds during the AM peak are less affected, and remain greater than 55 mph. The congestion scan shows that the downstream lane drop location (approximately at the 36.5-mile marker) is actually a more severe bottleneck than this study location. Travel speeds generally reduce to under 45 mph from 2:30 PM to 3:30 PM and from 4:30 PM to 6:00 PM. Field observations indicate that I-93 northbound traffic queues frequently extend from this location to the vicinity of the Route 62 interchange. The congestion scan also shows an interesting dual-peak phenomenon (2:30–3:30 PM and 4:30–6:00 PM) at the lane-drop bottleneck location. It is a combined effect of the lane drop bottleneck and the travel demand management strategy (use of the shoulder as a travel lane during the PM peak period), currently applied to the I-93 sections downstream use of the breakdown lane.

During the weekday PM period from 3:00 to 7:00, travel in the I-93 northbound breakdown lane is permitted beginning approximately a quarter mile north of the lane drop all the way to the north side of Merrimack River before Exit 46. The first wave of congestion begins around 2:30 PM when I-93 traffic gradually increases but travel in the breakdown lane is prohibited. After 3:00 PM, when travel in the breakdown lane is permitted, traffic congestion gradually dissipates. The severe congestion begins after 4:00 PM, and peaks around 5:30 PM. The congestion gradually dissipates after 6:00 PM, when the I-93 northbound sections carry most commuter traffic heading home in the north.

4.4.3   Level of Service

MPO staff conducted traffic operations analyses consistent with HCM methodologies. Using data from MassDOT, MPO staff built traffic analysis networks for the AM and PM peak hours with the HCS suite to assess the capacity and quality of traffic flow at the bottleneck area.¹⁷ Full HCS reports are included in Appendix D.

Table 4 presents the results of the LOS analyses for existing conditions at Location 1. It contains the HCM merging and diverging analyses. The section between the merge area and diverge area was not analyzed as a basic freeway section because it is shorter than 1,000 feet and it is entirely within the merge and diverge influence areas.18   

Table 4
LOS Analysis–Existing Conditions: Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125)

^a Refers to ramp influence area speed for merge/diverge areas.

^b Refers to the freeway section’s volume-to-capacity ratio.

^c LOS A through LOS D represent acceptable operating conditions; LOS E represents operating conditions at capacity; and LOS F represents failing conditions (demand exceeds capacity).

^d In HCM merge and diverge analyses, acceleration and deceleration lanes are measured from the tip of the painted gore to the end of the taper. This may differ from the AASHTO length. AASHTO, or The American Association of State Highway Transportation Officials, is a nonprofit, nonpartisan association representing highway and transportation departments in the 50 states, the District of Columbia, and Puerto Rico. It represents all transportation modes, including air, highways, public transportation, active transportation, rail, and water. Its primary goal is to foster the development, operation, and maintenance of an integrated national transportation system.

HCM = Highway Capacity Manual. LOS = level of service. mph = miles per hour. pc/lane mile = passenger cars per lane mile. V/C = volume-to-capacity. vph = vehicles per hour.

Table 4 illustrates that both areas operate at LOS C during the AM peak hour, while the merge area operates at LOS D and the diverge area operates at LOS E during the PM peak hour. The PM peak-hour analysis shows that the diverge area has a high density of vehicles, causing delay for the traffic exiting to Route 125. Traffic congestion on I-93 northbound mainline frequently builds up from the downstream lane-drop location to the vicinity of this location, adding difficulties to the diverge operation. In addition, the estimated freeway speeds in the PM peak hour could be somewhat higher than the observed speeds (see Figure 4), as the downstream bottleneck congestion is not applicable to the HCM analyses. 

4.5      Improvement Alternatives

The analyses identified that the on-ramp acceleration length and the off-ramp deceleration length do not meet MassDOT standards. A simple solution could be to extend their lengths; however, it would not be suitable at this location. As the two ramps are located in close proximity (about 1,600 feet), further extending the acceleration or deceleration length and reducing the space in between would potentially increase crash risk.19^,20 The analyses also identified that one of the key problems at this location is the I-93 mainline congestion, causing difficulties for the merge and diverge maneuvers. However, this congestion is mainly due to the downstream lane drop bottleneck.

Under these conditions, there are not many applicable low-cost improvement alternatives. However, MPO staff developed two improvement alternatives to address safety and operational issues at this location. These improvements include:

• Alternative 1: Create an auxiliary lane for merging and diverging traffic
• Alternative 2: Examine potential improvements at the downstream lane drop location

4.5.1   Alternative 1: Create an Auxiliary Lane for Merging and Diverging Traffic

An auxiliary laneis defined as the portion of the roadway adjoining the traveled freeway for speed change, merging, diverging, weaving, and other purposes supplementary to through-traffic movement. Alternative 1 would create an auxiliary lane between the on-ramp at Exit 40 and the off-ramp at Exit 41. This lane would extend the distance available for merging or diverging traffic maneuvers and would provide sufficient distance to accommodate speed changes and vehicle weaving. The auxiliary lane would also upgrade the acceleration lane to meet MassDOT standards.

Figure 5 shows the improvements recommended in Alternative 1, including:

• Restriping I-93 northbound between Exit 40 and Exit 41 (about one-third of a mile) to accommodate a fifth 12-foot auxiliary lane on the right. This would bring the total lane width to 60 feet along this distance. Using the existing paved area on both the left and right shoulders would provide the required additional width. The highway alignment would also need to be shifted to the left by approximately two feet to accomplish this. It would reduce the left shoulder to approximately four feet and the right shoulder to a minimum of two feet.
• Relocating existing guide signs or installing new guide signs and pavement markings to direct drivers to merge onto the mainline or to use Exit 41.
• Modifying pavement markings to delineate the auxiliary lane from the mainline travel lanes.

Although improvements could be made, Alternative 1 does present some design difficulties. First, the less-than-minimal right shoulder areas would require a design exception report. When conditions warrant, a design exception may be granted for a project design that proposes one or more controlling substandard design elements if it can be documented that a lesser design is the best practical alternative. Second, while the paved shoulder along this stretch of highway is wide enough to accommodate an extra travel lane, it is not wide enough to fit an additional emergency pullover or stopping area. Any such area would require additional paving and grading work due to a moderate slope to the right of the roadway.

4.5.2   Alternative 2: Examine Potential Improvements at the Downstream Lane Drop Location

This study identified that traffic congestion at this bottleneck location is mainly caused by a downstream bottleneck where the I-93 northbound mainline reduces from four to three lanes. Traffic operations at this location would be improved if the congested conditions from the downstream bottleneck can be substantially decreased. One possible solution is to open up the breakdown lane for travel earlier downstream of the bottleneck. Based on observations throughout the region, it is acknowledged that the peak congestion periods are expanding, so opening the breakdown lane may help to alleviate this bottleneck. However, this solution is beyond the scope of the current study.

4.6      Effectiveness and Cost of the Improvements

Table 5 presents the 2030 future LOS analyses compiled using HCS software, and compares the results from the no-build scenario and Alternative 1, where modifications would affect system operations. Full HCS reports are included in Appendix D. All scenarios use a uniform four percent growth for 2030 traffic volumes estimated based on the MPO regional travel demand model for this study area. Alternative 2 is a proposed future study and therefore, not included in Table 5. Approximations made as part of the HCM analysis are provided when applicable.

Table 5
2030 LOS Analysis–No-Build and Improvement Alternative 1 for Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125)

^a Refers to ramp influence area speed for merge/diverge areas and average of weaving and non-weaving speeds for the weaving segment.

^b Refers to the freeway section’s volume-to-capacity ratio.

^c LOS A through LOS D represent acceptable operating conditions; LOS E represents operating conditions at capacity; and LOS F represents failing conditions (demand exceeds capacity).

HCM = Highway Capacity Manual. LOS = level of service. mph = miles per hour. pc/lane mile = passenger cars per lane mile. V/C = volume-to-capacity. vph = vehicles per hour.

A crash modification factor (CMF) is an estimate of the change in crashes expected after the implementation of a countermeasure. Using CMFs from the HSM and the CMF Clearinghouse show that adding continuous auxiliary lanes for weaving between the entrance ramp and the exit ramp would reduce crashes by 20 to 25 percent.21^,22 In addition, the LOS for Exit 41 would improve from D to C during the AM peak and from E to D during the PM peak by lengthening the acceleration lane. However, the merge from Route 62 would remain at LOS C in the AM peak and LOS D in the PM peak.

Implementing the changes in Alternative 1 would require pavement restriping to shift the northbound highway alignment to the left by approximately two feet, beginning at Exit 40 and extending to Exit 41. No right-of-way acquisition, pavement widening, or alignment changes should be required, other than providing an emergency pullover, if needed. Alternative 1 is estimated to cost between $50,000 and $75,000 to construct and would require realignment of the lanes, pavement restriping, relocating existing guide signs or installing new guide signs, and adding pavement markings.   

4.7      Recommendations

MPO staff recommends Alternative 1 because of the beneficial effects on safety and operational efficiency. Alternative 1 provides more space for entering and exiting traffic to move to and from the mainline travel lanes and would potentially reduce crashes in this I-93 northbound section. In addition, staff recommends studying possible alternatives, similar to those presented in Alternative 2, as a proposed future study for the I-93 northbound lane drop bottleneck about 1.2 miles downstream from this location. Traffic operations and crash risk at this location would be greatly improved if the congested conditions from the downstream bottleneck could be substantially decreased.

Chapter 5—Location 2: I-93 Southbound Segment at the End of the HOV Zipper Lane in Quincy and Braintree

The affected segment, approximately 0.8 miles long, extends from the beginning of Furnace Brook Parkway on-ramp at Exit 7 to the Route 3 diverge at Exit 8. The interchange and freeway, which are located in District 6, are under the jurisdiction of the MassDOT Highway Division. Figure 1 shows the location of the bottleneck within the MPO region. It is located on the I-93 southbound barrel at the end of the HOV zipper lane. The bottleneck occurs during PM peak southbound HOV lane operations, which begins at 3:00 PM and ends at 7:00 PM. This bottleneck is where traffic merges and weaves in order to continue onto I-93 southbound or Route 3 southbound. It is also where the on-ramp from Furnace Brook Parkway merges onto I-93 southbound. Just 0.4 miles north of the end of the HOV zipper lane exit, traffic heading to Route 3 southbound diverges off I-93.

5.1      Existing Freeway Characteristics

5.1.1   Basic Freeway Section

The basic freeway section is about 1,000 feet long when the HOV lane is in operation. It has four 12-foot travel lanes, and an 11- to 12-foot right shoulder. There is no left shoulder at this section because of HOV lane operations during peak travel periods and storage of the zipper barriers during off-peak travel periods.

5.1.2   Entrance Ramps

There are two ramp merge areas relevant to this bottleneck location: the on-ramp from Furnace Brook Parkway and the HOV lane exit. Furnace Brook Parkway is a one-lane, on-ramp with an 800-foot acceleration lane.23 Traffic in the HOV lane exits and merges with the mainline traffic about 200 feet north of Furnace Brook Parkway. The exit and merge area of the HOV is about 1,000 feet long.

5.1.3   Exit Connector

The Exit 7 connector to Route 3 southbound is a two-lane major diverge with design speed similar to that of the freeway. It is important to note that downstream bottlenecks on Route 3 southbound during the PM peak travel period create a traffic queue that extends into the subject bottleneck and exacerbates traffic conditions.

5.2      Problems

The existing bottleneck creates a recurring long traffic queue on the mainline, which extends five miles northerly to Columbia Road in Dorchester. It reduces traffic flow to stop-and-go conditions with average travel speeds of less than 25 mph, even though the posted speed limit is 55 mph. In addition, the congested conditions lead to many crashes, pollution, and high person-hours of delay.

5.3      Causes

Physical design constraints and operational conflicts inherent in the location create the bottleneck, including the following:

• Lane drop: One or more travel lanes end, requiring traffic to merge onto the mainline.
• Weaving areas: Drivers must change lanes or cross each other’s path along a length of the freeway in order to continue on I-93 southbound or Route 3 southbound.
• Merge areas: Furnace Brook Parkway on-ramp traffic merges with mainline traffic to enter the freeway and then, shortly ahead, traffic in the HOV lane exits and merges with the mainline traffic.  
• Major diverge: High-volume traffic from the freeway diverges to Route 3 southbound at Exit 8.

At the bottleneck, traffic from six lanes (four on the mainline and one each on the HOV lane and the on-ramp from Furnace Brook Parkway) are forced onto four travel lanes in a short segment about 1,500–2,000 feet long. In addition, a high number of lane-changing maneuvers (merging, weaving, and diverging) take place within the segment to disperse traffic to continue on I-93 southbound or head to Route 3 southbound. These factors along with the close proximities of the merge and diverge areas dramatically reduce capacity in the segment.

Figure 6 shows the PM peak-period traffic volumes. During this period, when the HOV lane is in operation, the entry ramp from Furnace Brook Parkway serves about 600 vph while the HOV lane serves an average of 1,200 vph. The mainline serves about 3,500 vph. Traffic data are included in Appendix B.

In all, the bottleneck serves between 5,200 vph and 5,400 vph while the traffic demand is around 8,000 vph during PM peak periods. This suggests that traffic demand at the bottleneck greatly exceeds the capacity and results in a long traffic queue trailing the bottleneck for over five miles to Columbia Road in Dorchester. Interestingly, between 2:00 PM and 3:00 PM when the HOV is not in operation, the four I-93 southbound lanes carry around 6,000 vph, which is even greater than the volume when the HOV lane is operation.24

5.4      Previous Configuration

Over the years, there have been several lane reconfigurations at the bottleneck to address safety operations and safety issues. The current configuration started in 2014. Before that, the mainline had the same four lanes; however, about 1,000 feet to the diverge to Route 3 southbound, the lane next to the rightmost lane widened to about 22 feet, which was used as a shared lane for traffic heading to either Route 3 southbound or continuing on I-93 southbound. In effect, three lanes headed to Route 3 southbound and two lanes went to I-93 southbound.

Assessment of the current configuration indicates that it reduces the lane changing maneuvers at the bottleneck and streamlines traffic heading to Route 3 southbound in anticipation of downstream bottlenecks. While the current configuration has safety benefits, a tradeoff to this may be reduced traffic flow. 

5.5      Impacts

5.5.1   Crashes

Table 6 presents a summary of the crashes at the bottleneck. There were 132 crashes in this section between 2012 and 2016 (Appendix C). Figure 7 shows the location of these crashes. The majority, 108 of the crashes, occurred near the diverge area at Exit 7. Interestingly, the number of crashes between 2012 and 2016 (132 crashes) represent a 10 percent reduction to the five-year total between 2005 and 2009 (146 crashes). This suggests that the change in lane configuration around 2013 significantly influenced crash frequency. Figure 8 shows the location of the 31 crashes that occurred in the same period when the HOV lane is in operation, which represents 23 percent of the crashes.

Table 6
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Five-Year Crash Summary (2012–16)

Note: Weekday evening peak period is 3:00 PM to 7:00 PM Monday through Friday.
Source: Central Transportation Planning Staff.

A summary of the crashes in this segment are as follows:

• Twenty-eight percent of the crashes resulted in injury
• The largest share of crashes (54 percent) were rear-end crashes
• Many of the rear-end and sideswipe crashes were caused by drivers changing lanes, merging or diverging
• Forty-seven crashes (36 percent) occurred outside of daylight conditions, including the one unreported crash
• Eighty-three percent of the crashes occurred on dry roadway conditions
• Sixty-four percent of the crashes occurred outside daylight conditions

5.5.2   Travel Speed

Figure 9 is a congestion scan that shows the average travel speeds on I-93 southbound at the bottleneck. Based on these data, the bottleneck reduces travel speeds to less than 25 mph between the hours of 3:00 PM and 7:00 PM. Vehicle speeds this far below free-flow speed correlate with LOS F conditions on the freeway. In addition, speeds at this bottleneck can fall to less than 25 mph and stay that low for two to three hours. The congestion scan also shows severe slowdowns (queue) trailing the bottleneck. The gradual relief after the bottleneck suggests that the HOV lane merge and Furnace Brook Parkway on-ramp merge are not the only causes of congestion, and that more issues are present further downstream on I-93 southbound and Route 3 southbound.

5.5.3   Level of Service

Traffic operations at the bottleneck are complex as merging, diverging, and weaving maneuvers all take place within the segment at the same time. MPO staff conducted traffic operations analyses consistent with HCM methodologies. Using the MassDOT data, MPO staff built traffic analysis networks for the PM peak hours with the HCS suite to assess the capacity and quality of traffic flow at the bottleneck area.²⁵ The analyses included

• merge analysis of the HOV lane;
• merge analyses of the on-ramp from Furnace Brook Parkway;
• weave analysis of the traffic from HOV lane to I-93 southbound; and
• weave analysis of the traffic from the Furnace Brook Parkway on-ramp to Route 3 southbound.

The HCM methodology has some limitations concerning weaving analysis, as it does not address the following conditions, which exist at the bottleneck, such as:

• Special lanes, for example, HOV lanes within weaving segments
• Specific operating conditions when oversaturated conditions exist
• Effects of downstream or upstream congestion
• Multiple weaving segments

Some simplifications were necessary to align the HCS model with observed conditions, such as the breaking up of multiple weave segments into merge and simple weave segments for analysis.

Table 7 presents the results of the LOS analyses for existing conditions. The results indicate that traffic in the HOV lane and Furnace Brook Parkway on-ramp operate at LOS D when they merge with the mainline traffic. In addition, the analyses indicate that traffic operating conditions at the weaving segment (where traffic weaves from the HOV lane, Furnace Brook Parkway on-ramp, and the freeway onto Route 3 southbound and I-93 southbound) is LOS F. Full HCS reports are included in Appendix D.

Based on these analyses, the primary cause of the bottleneck is intense weaving due to a lack of lane balance rather than merging or diverging conditions. The volume-to-capacity (v/c) ratios for the weaving segment was 1.4, meaning that the traffic demand is higher than maximum throughput of the weave segment with the given features. HCS suite does not provide density and speed data for scenarios that result in LOS F or have a v/c greater than 1.0.

Table 7
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Existing Conditions LOS Analysis

^a HCM does not provide density and speed data for scenarios that result in LOS F.

^b Refers to the freeway section’s v/c ratio.

^c LOS A through LOS D represent acceptable operating conditions; LOS E represents operating conditions at capacity; and LOS F represents failing conditions (demand exceeds capacity).

HCM = Highway Capacity Manual. HOV = high-occupancy vehicles. LOS = level of service. mph = miles per hour. pc/lane mile = passenger cars per lane mile. v/c = volume-to-capacity. vph = vehicles per hour.

Source: Central Transportation Planning Staff.

5.6      Improvement Alternatives

MPO staff developed the following improvements to address safety and operational issues at the bottleneck. These improvements include:

• Alternative 1: Lengthen the distance of the HOV lane merge
• Alternative 2: Lengthen the acceleration lane distance for Furnace Brook Parkway on-ramp
• Alternative 3: Alternatives 1 and 2 combined

All three alternatives have the objective of lengthening the merging and weaving segments to give drivers more space to carry out their intended maneuvers.

5.6.1   Alternative 1: Lengthen the Distance for the HOV Merge

Alternative 1 would move the HOV exit about 600 feet further north to lengthen the distance in which HOV traffic merges with the mainline traffic. The extension is expected to provide HOV lane drivers with ample distance for merging and weaving safely and comfortably to continue on I-93 southbound and Route 3 southbound. Figure 10 shows the improvements recommended in Alternative 1.

5.6.2   Alternative 2: Lengthen Acceleration Distance for Furnace Brook Parkway On-Ramp

Figure 11 shows the improvements recommended in Alternative 2. Alternative 2 would extend the acceleration lane distance for the Furnace Brook Parkway on-ramp about 600 feet using the existing right shoulder. An 11- to 12-foot right shoulder exists along the entire length of the bottleneck.

5.6.3   Alternative 3: Alternatives 1 and 2 Combined

Alternative 3 combines the improvement recommendations of both Alternatives 1 and 2 for added benefits.

5.7      Effectiveness and Cost of the Improvements

The improvement alternatives were analyzed using a total growth factor of five percent over the existing traffic demand to project short-term traffic volumes in 2030. Because the traffic operations at the bottleneck are at capacity for the entire four-hour PM peak period, it is likely that any growth in traffic demand would worsen the queue trailing the bottleneck.

5.7.1   HCS Analysis Results

Table 8 presents the results of the 2030 LOS analyses compiled using the HCS suite. The HCS analyses results show marginal benefits with the recommended improvements but does not appear to reduce the impacts of intense weaving at the bottleneck, resulting in a LOS F for all weaving analyses.

Table 8
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: 2030 Future LOS Analysis

^a HCM does not provide density and speed data for scenarios that result in LOS F.

^b Refers to the freeway section’s volume-to-capacity ratio.

^c LOS A through LOS D represent acceptable operating conditions; LOS E represents operating conditions at capacity; and LOS F represents failing conditions (demand exceeds capacity).

HCM = Highway Capacity Manual. HOV = heavy-occupancy vehicle. LOS = level of service. mph = miles per hour. pc/lane mile = passenger cars per lane mile. v/c = volume-to-capacity. vph = vehicles per hour.

Source: Central Transportation Planning Staff.

5.7.2   VISSIM Simulation Results

In addition to the HCS suite, MPO staff used VISSIM traffic simulation software to analyze future traffic operations at the bottleneck.26 Due to the complex nature of the bottleneck and limitations of the HCS analyses, VISSIM was used to account for all the maneuvers at the bottleneck simultaneously. Table 9 presents the results of the VISSIM analyses for the existing conditions and improvement alternatives. The primary performance measure in the simulation analysis was the total volume of traffic simulated through the bottleneck—the higher the total volume, the more effective the alternative. The results show that separately, Alternatives 1 and 2 would have minimal effect on the bottleneck and the trailing queue. VISSIM simulations show that while Alternative 3 may improve traffic operations at the bottleneck to some extent, it would not be able to reduce the trailing traffic queue significantly.

Table 9
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Traffic Simulation Analysis

HOV = high-occupancy vehicle. mph = miles per hour. vph = vehicles per hour.

Source: Central Transportation Planning Staff.

5.7.3   Costs

Alternative 1 is estimated to cost between $300,000 and $500,000 to construct. This estimate includes the relocation of existing median barriers, preparing the median area for staging HOV operations, restriping travel lanes, and the relocation and installation of signs and pavement markings.

Alternative 2 is estimated to cost between $100,000 and 300,000 to construct, and would require restriping travel lanes, possibly repaving, relocating rumble strips, creating an emergency pullover lane, and drainage systems.

Alternative 3 is estimated to cost about $1.0 million to construct, and includes all of the same items listed for Alternatives 1 and 2.

5.8      Recommendations

MPO staff recommends Alternative 1, as it produces safety and operational benefits.

The HOV lane is a contra-flow reversible lane meaning that a lane is borrowed from the off-peak direction to serve the peak direction. The HOV lane heads northbound in the morning peak period using a lane borrowed from the southbound direction, and in the afternoon peak period, that operation is reversed. Both the morning and afternoon HOV operations share a common space for staging and operations. Because of the shared space, the space requirements for the proposed improvements would need further evaluation regarding HOV staging and operations for the morning northbound HOV operations. MPO staff advises further consultation with personnel involved with HOV lane operations and maintenance.

Alternative 2 would have operational improvements during the PM peak period; however, it would also eliminate the existing shoulder for disabled vehicles. Another concern with Alternative 2 is that drivers would choose to enter the longer acceleration lane to "bypass" slow traffic in the general travel lanes. For these concerns, MPO staff cannot recommend Alternatives 2 and 3 without further assessment of safety and operational effects.

Chapter 6—Conclusion and Next Steps

MPO staff, working in conjunction with the MassDOT Highway staff, identified, developed, and evaluated improvements for two bottleneck locations in the MPO region. The study provides the MassDOT Highway Division with an opportunity to assess the most critical needs at the two bottleneck locations and to start planning design and engineering efforts. If implemented, these low-cost, short-term improvements would increase traffic safety, make traffic operations more efficient, and reduce congestion at the bottlenecks. The study aligns with the MPO’s goals of managing capacity and improving mobility, and increasing safety on the region’s highway system.

Figure 1
Regional Map of Study Areas

Figure 2
Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Peak Period Traffic Volumes

Figure 3
 Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Location and Number of Crashes

Figure 4
Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Congestion Scan

Figure 5
Location 1—I-93 Northbound between Exit 40 (Route 62) and Exit 41 (Route 125): Alternative 1—Create Auxiliary Lane

Figure 6
Location 2—I-93 Southbound at the End of the HOV Zipper Lane:

Peak Period Traffic Volumes

Figure 7
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Location and Number of Crashes

Figure 8
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Five-Year (2012–16) Crashes when the HOV Lane is in Operation (3:00 PM to 7:00 PM)

Figure 9
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Congestion Scan

Figure 10
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Alternative 1—Lengthen the HOV Lane Merge Distance

Figure 11
Location 2—I-93 Southbound at the End of the HOV Zipper Lane: Alternative 2—Lengthen the Acceleration Distance for Furnace Brook Parkway On-Ramp

 APPENDICES