Low-Cost Improvements to Express-Highway Bottleneck Locations

Abstract

Executive Summary

Chapter 1: Introduction

Chapter 2: Selection of Study Locations

Chapter 3: Data Collection and Analysis Methods

Chapter 4: Operations and Safety Analysis

Chapter 5: Proposed Low-Cost Improvements

Chapter 6: Conclusion and Next Steps

Appendices

Project Manager

Chen-Yuan Wang

Project Principal

Mark Abbott

Data Analysts

Julie Dombroski

Kathy Jacob

Graphics

Kim DeLauri

Cover Design

Kim DeLauri

The preparation of this document was supported

by the Federal Highway Administration through

MPO Planning Contract #112310.

Central Transportation Planning Staff is

directed by the Boston Region Metropolitan

Planning Organization (MPO). The MPO is composed of

state and regional agencies and authorities, and

local governments.

May 2022

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Abstract

The section of Interstate 95 (I-95) between Exits 57 and 61 that runs through Reading, Wakefield, and Lynnfield, is one of the most frequently traveled corridors in the state. Motorists on this section of I-95 regularly experience delays resulting from highway bottlenecks, particularly during peak travel periods in the morning and afternoon. The federal fiscal year 2021 Low-Cost Improvements to Express-Highway Bottleneck Locations study examined these conditions to identify the cause of the traffic delays and develop low-cost solutions to improve traffic operations and safety in this area. Recommendations are given for reconfiguring acceleration and deceleration lanes and adding an auxiliary lane to alleviate congestion. The results of this study provide the MassDOT Highway Division with information for beginning to design and engineer improvements to these bottleneck locations.

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Executive Summary

The purpose of the Low-Cost Improvements to Express-Highway Bottleneck Locations study was to identify low-cost improvements that will help reduce congestion at freeway bottleneck locations in the Boston Metropolitan Planning Organization (MPO) region. The study was accomplished through cooperation with the Massachusetts Department of Transportation (MassDOT) Highway Division.

Bottlenecks on the freeway network can occur where geometric elements, such as ramps or lane drops, restrict traffic flow and become a major contributor to recurring congestion. Candidate locations in this study were identified based on input from the MassDOT Highway Division and review of the MPO’s Congestion Management Process data. The screening process yielded two locations on Interstate 95 (I-95) that had the potential for low-cost improvements: 1) I-95 southbound between Exit 58 (Route 129) and Exit 57 (North Avenue) in Wakefield and Reading, and 2) I-95 northbound at Exit 61 (Walnut Street) in Lynnfield.

After further review of the areas adjacent to the two locations and discussions with MassDOT Highway Division District 4 staff, MPO staff selected the northbound and southbound sections of I-95 between Exit 57 and Exit 61 for a comprehensive and systematic analysis to identify bottleneck locations for low-cost improvements. Travelers on this section of I-95 regularly experience poor levels of service and endure additional delays while entering or exiting the freeway during the weekday AM and PM peak travel periods.

MPO staff performed a series of operational and safety analyses, including traffic-volume estimations and projections, and analyses of highway capacity, travel speeds, and crash data. Staff then developed low-cost improvement proposals for the identified bottleneck locations.

The locations and proposed improvements are as follows:

• I-95 northbound between Exit 57 and Exit 58: Expand the existing paved shoulder and add an auxiliary lane between the exit ramp and the entrance ramp.
• I-95 northbound merge segment at Exit 58: Extend the acceleration length.
• I-95 northbound diverge segment at Exit 60: Extend the deceleration length.
• I-95 southbound between Exit 58 and Exit 57: Expand the existing paved shoulder and add an auxiliary lane between the exit ramp and the entrance ramp.
• I­-95 southbound merge segments at Exit 59 and Exit 61: Extend the acceleration length.
• I­-95 southbound diverge segments at Exit 58 and Exit 59: Extend the deceleration length.

The study aligns with the MPO’s goals of managing capacity, improving mobility, and improving safety on the region’s highway system. The proposed low-cost improvements would improve the freeway facilities, make traffic operations more efficient, and reduce congestion at the identified bottleneck locations. The improvements also would reduce crashes and improve safety for the freeway users.

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Chapter 1—Introduction

1.1      Background

According to the Federal Highway Administration (FHWA), “Much of 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  

The cause and duration of highway bottlenecks are various. In general, recurring bottlenecks, the subject of this study, are influenced by the design or operation present at the point where the bottleneck begins. These points may be where the highway design is less than optimal; where traffic merges, diverges, or weaves; where there are abrupt changes in highway alignment, lane drops, or narrowing of lanes; at low-clearance structures; or where traffic is disrupted intentionally for management purposes.

Since 2011, MPO staff analyzed several express-highway bottleneck locations in five previous studies; these studies were well received by the Massachusetts Department of Transportation (MassDOT) and the FHWA.2  Previous 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; sections of Route 24 in Canton and Randolph; sections of I-93 northbound in Wilmington; and sections of I-93 southbound in Quincy and Braintree.

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.

Cost estimates for low-cost bottleneck improvements that have been implemented by the MassDOT Highway Division, or currently are in design status, range between $10,000 and $1 million.

1.2      Purpose of Study

The purpose of this study was twofold. First, the study aimed to identify bottleneck locations where low-cost mitigation improvements are potentially applicable. Second, the study aimed 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.

MPO staff conducted a series of highway operations and crash data analyses to identify low-cost methods to reduce congestion, increase safety, and improve traffic operations. Technical expertise was provided by 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.

1.3      Report Organization

The following chapters of this report describe the selection of study locations, an assessment of the safety and operational problems, and potential improvement strategies. The final section presents the proposed low-cost improvements for several locations and recommendations. This report also includes a series of technical appendices, which cite the study methods, highway capacity analysis reports, and intersection capacity analysis reports for the signalized interchanges in the study area.

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Chapter 2—Selection of Study Locations

2.1      Inventory of Candidate Locations

MPO staff developed an initial list of candidate locations in the Boston region based on the following:

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

The inventory process also included a review of the locations collected in the previous studies that have potential for low-cost improvements. The process yielded over 20 bottleneck locations in the Boston region for further screening.

2.2      Screening Candidate Locations

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 constraints or conflicts include the following:

• Lane drop: One or more travel lanes end, requiring traffic to merge
• Weaving area: Drivers must merge across one or more lanes to access an entry or exit ramp
• Merge area: On-ramp traffic merges with mainline traffic to enter the freeway
• Major interchanges: High-volume traffic is directed from one freeway to another
  

3. Can low-cost operational and geometric improvements fix the bottleneck?

These improvements exclude costly long-term solutions such as expansion or widening of the roadway. Examples of low-cost operational and geometric improvements include the following:

• Using a short section of shoulder as an additional travel lane or for lengthening an acceleration or deceleration lane
• Restriping merge and diverge areas to better serve traffic demand
• Providing all-purpose reversible lanes
• Changing or adding signs and striping

Based on the suggestions from MassDOT Highway Division officials and the screening criteria, MPO staff identified nine express-highway bottleneck locations that have the potential to be corrected with low-cost mitigation strategies (see Table 1). Among them, Locations 1 and 2 are considered as having high potential because there is sufficient highway layout and few constraints in the adjacent areas. The other locations were not selected in this round because of funding resources. Those locations can be considered in future bottleneck studies.

Table 1
Express-Highway Locations with Potential for Low-Cost Improvements

Notes: 1) Shading indicates locations identified as having high potential for low-cost improvements in this study.
2) All the exit numbers are based on the recently (2020–21) updated mile-based numbers.

HOV = High occupancy vehicle. MassDOT = Massachusetts Department of Transportation.

Source: Central Transportation Planning Staff.

2.3      Selected Bottleneck Locations for Study: I-95 Northbound and Southbound between Exit 57 (North Avenue, Wakefield) and Exit 61 (Walnut Street, Lynnfield)

After the screening process, staff further reviewed the two locations with high potential for low-cost improvements and the adjacent highway segments and found that both the northbound and southbound sections of I-95 between Exit 57 and Exit 61 were suitable for study.

Figure 1 shows the selected sections of I-95 in the study area. These sections contain five interchanges:

• Exit 57: North Avenue in Wakefield
• Exit 58: Route 129 in Wakefield and Reading
• Exit 59: Vernon Street in Wakefield and Main Street in Lynnfield
• Exit 60: Salem Street and Pleasure Island Road in Wakefield
• Exit 61: Walnut Street in Lynnfield

The distances for traffic to merge and diverge on I-95 northbound and southbound at these interchanges are generally short, some less than 500 feet. However, most of these segments appear to have sufficient highway layout with no significant terrestrial constraints. Therefore, there is space to extend the distances where traffic may merge and diverge.

Both directions of I-95 between Exit 57 and Exit 61 are usually congested in the peak periods. In general, the traffic in the peak direction (southbound in the AM peak period and northbound in the PM peak period) is much more congested than the off-peak direction. In addition to the mainline congestion, localized bottlenecks develop at the areas where traffic merges and diverges when high entry and exit volumes are present in the peak hours.

Review of the crash data also indicated that there were significant numbers of crashes on I-95 at the locations of merging and diverging traffic. This study provided a systematic review of this frequently congested section of I-95 to identify short-term improvement strategies and support long-term improvement planning.

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Chapter 3—Data Collection and Analysis Methods

3.1      Traffic Volume Data

The MassDOT Transportation Data Management System provides traffic volume data on major express highways, arterials, and major collectors throughout the state.3 The database contains four permanent count stations in the study area:

• I-95 between Exit 57 and Exit 58 (Location ID: 4137)
• I-95 between Exit 58 and Exit 59 (Location ID: 4423)
• I-95 between Exit 59 and Exit 60 (Location ID: 4121)
• I-95 between Exit 60 and Exit 61 (Location ID: 5099)

In addition, it contains ramp counts collected in October 2020 at all the interchanges in the study area. MPO staff used these counts and available historical data to estimate traffic volumes on the I-95 northbound and southbound mainline and ramps. Staff also adjusted some of the counts collected during the COVID-19 pandemic period.4

3.2      Crash Data

MPO staff used crash data from January 2015 to December 2017 from the MassDOT Registry of Motor Vehicles database to analyze crash severity, collision type, and roadway conditions and to evaluate safety for motorists.

3.3      Speed Data

The CMP maintains average speed data on express-highway systems in the Boston region, incorporating the INRIX historical traffic speed data.5 MPO staff used the current speed data from the CMP (spring 2019 and fall 2019) to determine the average weekday travel speeds through the bottlenecks.

3.4      Level of Service Criteria

Level of service (LOS) is a quality measure of effectiveness 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 volumes, 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.6

The LOS criteria have 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 merge 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 diverge segments are ramp junction areas where one traffic stream diverges to form two separate streams (mainline and off-ramp). The influence areas of merge 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. 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 LOS criteria characterize freeway performance measures in terms of density (passenger cars per lane mile).

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.

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 and applied the Highway Capacity Software (HCS) to assess the capacity and quality of traffic flow at the potential bottleneck locations.7

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Chapter 4—Operations and Safety Analysis

4.1      Existing Freeway Characteristics

As explained in Section 3.4, in general, three types of segments are found on express highways (freeways):

• Freeway merge and diverge segments: Segments in which two or more traffic streams combine to form a single traffic stream (merge) or a single traffic stream divides to form two or more separate traffic streams (diverge).
• Freeway weaving segments: Segments in which two or more traffic streams traveling in the same direction cross paths along a significant length of freeway without traffic control devices. In most cases, weaving segments are formed when a diverge segment is closely followed by a merge segment.
• Basic freeway segments: All segments that are not merge, diverge, or weaving segments.

The study locations in the I-95 northbound and southbound sections between Exit 57 and Exit 61 are associated with the freeway merge segment, freeway diverge segment, and basic freeway segment operations. Figure 2 shows the three freeway segment types at the locations analyzed in this study. The operational components of the three types of freeway segments are further discussed in the next section.

In addition, freeway operations could be affected by operations at interchanges where the freeway entrance and exit ramps meet the arterial and major roadways, especially when the interchange traffic is busy and causes traffic queues extending into the freeway. Three of the five interchanges (Exits 57, 60, and 61) are under traffic signal controls and their operations in the peak hours could potentially interfere with I-95 traffic operations. To examine their potential impacts to I-95 operations, this study reviewed traffic operations at the three interchanges based on peak-hour turning movement counts collected in the summer of 2021.8

Another major factor that affects I-95 operations in the study area is the traffic conditions at the interchange of I-93 and I-95 (Exit 28 on I-93 and Exit 55 on I-95). The interchange carries significant traffic volumes from both interstate highways and is one of the most congested locations in the state. During peak periods, the congestion has a significant impact on I-95 and causes significant delays for users of both highways. In the study area, the interchange congestion mainly affects the traffic operations on I-95 between Exit 57 and Exit 58, especially in the southbound direction during the AM peak period and in the northbound direction during the PM peak period.

The interchange of I-93 and I-95 is a major bottleneck in the region and its congestion is a much larger issue that can only be addressed through long-term transportation planning. However, the low-cost improvements proposed in this study would meaningfully improve the freeway merge and diverge operations and enhance the safety for users in the freeway sections north of the major interchange.

4.2      Traffic Conditions and Operations

A number of factors potentially contributed to traffic congestion at various locations in the I-95 northbound and southbound study sections:

• High volume of traffic during the peak hours
• Short deceleration lanes affecting freeway diverging operation
• Short acceleration lanes affecting freeway merging operation
• Basic freeway segments operating at capacity during the peak hours
• Congestion at the interchange of I-93 and I-95 affecting the freeway traffic flow in the study sections
• Potential traffic queues from the interchange traffic operations

These factors are further analyzed in the following sections.

4.2.1   Traffic Volumes

The section of I-95 between Exits 57 and 61 is one of the most frequently traveled corridors in the state. This section of highway carried approximately 140,000 to 150,000 vehicles per average weekday in recent years, from 2015 to 2019, before the COVID-19 pandemic.9 Daily traffic on I-95 in the study area reduced significantly during this pandemic period, varying from approximately 10 to 25 percent in general to more than 50 percent in the peak pandemic period (spring 2020).10

MPO staff therefore chose 2019 as the base year for the analysis of the existing traffic conditions and used the available traffic data from 2015 to 2019 to estimate the traffic volumes on an average weekday during active traveling months in the spring and fall. Figure 3 shows the estimated daily traffic volumes on the freeway and entrance and exit ramps in both the northbound and southbound directions.

From the collected data, staff found that weekday peak periods in this stretch of I-95 had been expanding to at least four hours: 6:00–10:00 AM and 2:30–6:30 PM. To perform a highway capacity analysis for assessing the existing traffic conditions, staff further identified AM and PM peak hours and estimated peak-hour traffic volumes. Figures 4 and 5 show the traffic flows in the AM peak hour (6:00–7:00 AM in the southbound direction and 7:00–8:00 AM in the northbound direction) and PM peak hour (5:00–6:00 PM in both directions) separately.

During the peak hours, the I-95 mainline and the entrance and exit ramps generally carry high traffic volumes, especially at Exits 57 and 58. In general, all the study locations carry high traffic volumes on the I-95 mainline in the peak direction (northbound in the AM and southbound in the PM peak hour). Field observations indicated that the merging and diverging activities are frequently impeded by the high mainline traffic volumes during the peak hours.

4.2.2   Diverging Operations

There are a number of variables that influence the freeway diverging and merging operations at the ramp-freeway junctions. Attributes influencing the merging and diverging operations include the length and type (taper or parallel) of acceleration or deceleration lanes, sight distances, speed, the distribution of vehicles across lanes, and free flow speeds of upstream freeway traffic.

Tables 3 and 4 show key attributes observed in the diverge areas on I-95 northbound and southbound by the study interchange locations and the freeway-interchange ramp types. Appendix A contains the classification of the interchange ramp types based on A Policy on Geometric Design of Highways and Streets (as known as the Green Book) by the American Association of State Highway and Transportation Officials (AASHTO).

The length of the deceleration lane is one of the key attributes of particular importance to the diverging operations. Based on the design guidance for minimum deceleration lengths in MassDOT’s Project Development and Design Guide, deceleration lanes at the study locations generally require a minimal length of 440 or 470 feet.11 Appendix B exhibits the design standards and the applicable minimal deceleration lengths for this study.

Table 3
Diverging Operation Attributes: I-95 Northbound Locations

^a See Appendix A.

^b The length of the deceleration lane is measured from the starting point of the taper to the tip of the painted gore, as defined by the Highway Capacity Manual for capacity analysis purposes. This length may differ from the length defined by AASHTO, which is for geometric design purposes. In this study, these deceleration lengths were estimated approximately from the digital orthophotos of US Geological Survey Massachusetts 2019 and Google Earth 2021.

AASHTO = American Association of State Highway and Transportation Officials. mph = miles per hour.

Source: Central Transportation Planning Staff.

Table 4
Diverging Operation Attributes: I-95 Southbound Locations

^a See Appendix A.

^b The length of the deceleration lane is measured from the starting point of the taper to the tip of the painted gore, as defined by the Highway Capacity Manual for capacity analysis purposes. This length may differ from the length defined by AASHTO, which is for geometric design purposes. In this study, these deceleration lengths were estimated approximately from the digital orthophotos of US Geological Survey Massachusetts 2019 and Google Earth 2021.

AASHTO = American Association of State Highway and Transportation Officials. mph = miles per hour.

Source: Central Transportation Planning Staff.

Among the I-95 northbound study locations, the deceleration lane at Exit 60 has a deceleration length of approximately 200 feet, which is much shorter than the minimal design standards of 470 feet. This location (under an advisory speed of 25 miles per hour) is of particular concern as the exit ramp is short in length with a sharp compound curvature and ends at a traffic signal. During peak hours, vehicles on the ramp were observed to queue back onto the freeway section from time to time, especially in the PM peak period.

Among the I-95 southbound study locations, the deceleration lane at Exit 59 has a deceleration length of approximately 275 feet, which is shorter than the minimal design standards. A quick review of the area’s digital orthophotos indicates that there is potential to extend the deceleration lane in the diverge area.

4.2.3   Merge Operations

In order to allow vehicles on ramps to accelerate for merging, acceleration lanes generally require a longer distance than deceleration lanes. Based on the design guidance in MassDOT’s Project Development and Design Guide for minimum acceleration lengths, the assumptions for initial on-ramp speed before merging is 30 miles per hour (mph) for slip ramps and 25 mph for loop ramps. The acceleration lanes in at the study locations generally require a minimal acceleration length of 1,000 and 1,120 feet to meet design standards. Appendix C exhibits the design standards and the applicable minimal deceleration lengths for this study.

Tables 5 and 6 show key operation attributes in the merge areas on I-95 northbound and southbound at the study interchange locations. The study locations generally do not meet the minimum acceleration length design standards. Among them, the merge operations at Exits 59 and 61 on I-95 southbound are of particular concern. The acceleration lengths at both locations are much shorter than the minimal design standards. Meanwhile, drivers at both on-ramps have a limited sight distance when merging due to the loop ramp geometrics.

It is challenging to meet the minimal acceleration length standards for express highways in urban areas where the adjacent areas are densely developed. Highway capacity analyses for the merge segments in the study area indicated that opportunities exist at some of the merging locations that could improve the merging capacity and operations, although design standards still would not be met.

Table 5
Merging Operation Attributes: I-95 Northbound Locations

^a See Appendix A.

^b The initial merging speed is assumed to be comparable to the advisory speed of the exit ramp at the interchange.

^c The length of the acceleration lane is measured from the tip of the painted gore to the end point of the taper, as defined by the Highway Capacity Manual for capacity analysis purposes. This length may differ from the length defined by AASHTO, which is for geometric design purposes. In this study, these acceleration lengths were estimated approximately from the digital orthophotos of US Geological Survey Massachusetts 2019 and Google Earth 2021.

AASHTO = American Association of State Highway and Transportation Officials. mph = miles per hour.

Source: Central Transportation Planning Staff.

Table 6
Merging Operation Attributes: I-95 Southbound Locations

^a See Appendix A.

^b The initial merging speed is assumed to be comparable to the advisory speed of the exit ramp at the interchange.

^c The length of the acceleration lane is measured from the tip of the painted gore to the end point of the taper, as defined by the Highway Capacity Manual for capacity analysis purposes. This length may differ from the length defined by AASHTO, which is for geometric design purposes. In this study, these acceleration lengths were estimated approximately from the digital orthophotos of US Geological Survey Massachusetts 2019 and Google Earth 2021.

AASHTO = American Association of State Highway and Transportation Officials. mph = miles per hour.

Source: Central Transportation Planning Staff.

4.2.4   Basic Freeway Segment Operations

As shown in Figure 2, in the operational analysis the sections between the exit ramps and the entrance ramps at the interchanges were assumed to be operating as basic freeway segments. The only other locations identified for such analysis were the northbound and southbound sections of I-95 between Exit 59 and Exit 60. According to the Highway Capacity Manual, the merge segments have an influence area 1,500 feet downstream of the merge point and the diverge segments have the same influence upstream of the diverge point. In this study, therefore, the basic-freeway-segment analysis was applied only to the sections where the merge and diverge segments’ influence areas do not overlap.

Attributes influencing basic-freeway-segment operation include the number of lanes, lane widths, lateral clearances, terrain and grades, degree of curvature, and total ramp density.12 The basic freeway segments on I-95 northbound and southbound in the study area generally contain three 12-foot travel lanes, a 10-foot or more right shoulder, and a six-foot left shoulder. The terrain where the segments are located is level, with a grade generally less than two percent.

4.2.5   Highway Capacity Analyses

Based on the freeway attributes and the estimated base-year (2019) peak-hour traffic volumes, staff conducted highway capacity analyses for the merge, diverge, and freeway basic segments in the study area. Figures 6 and 7 summarize the AM and PM analyses by the segment locations on I-95 northbound and southbound. Appendices D and E contain full HCS reports of the AM and PM peak-hour capacity analyses for the various study locations.

The analyses indicated that most of the diverge segments operated at acceptable LOS D, except the diverge segment on I-95 southbound at Exit 57. The segment is estimated to operate at LOS E and near its capacity.

For the merge segments, several locations were assessed to operate at LOS E and near their capacities. These include the segment at Exit 57 on I-95 northbound during both the AM and PM peak hours, Exit 58 on I-95 southbound during both the AM and PM peak hours, Exit 59 on I-95 southbound during the AM peak hour, and Exit 61 on I-95 southbound during the AM peak hour.

All the basic freeway segments were assessed to operate at acceptable LOS D, with the segments at Exits 57, 58, and 59 generally having a high volume to capacity ratio near 0.90.

Note that HCS is not a traffic simulation software and the above capacity analyses do not include the congestion effects from the major bottleneck at the interchange of I-93 and I-95. The analyses, however, provide a baseline of the exiting traffic conditions for analyzing and assessing the future year no-build and proposed scenarios.

4.2.6   Freeway Travel Speeds

Figures 8 and 9 depict congestion scans that cover I-95 in the study area in the northbound and southbound direction separately. They show the average travel speeds on I-95 northbound from Exit 57 to Exit 61 and the average speeds on I-95 southbound from Exit 61 to Exit 57. The 2019 spring and fall midweek (Tuesday to Thursday) travel-time data (provided by INRIX) were used for this analysis.

As shown in Figure 8, travel speeds at the study locations on I-95 northbound generally slow to less than 55 mph from 2:30 PM to 6:30 PM, with the impact to travel speeds and the length of the congested period gradually decreasing from Exit 57 to Exit 61. The congestion scan clearly indicates the significance of the congestion effect from the major bottleneck upstream at the interchange of I-93 and I-95. The severe congestion at the interchange often causes stop-and-go traffic conditions on I-95 northbound during the PM peak hours when travel demand is high. Meanwhile, the exit and entrance ramps on I-95 northbound also carry high traffic volumes during the PM peak hours and the diverging and merging operations further impede the recovery of normal freeway travel speeds.

As shown in Figure 9, travel speeds at the study locations on I-95 southbound generally slow to less than 55 mph from 6:00 AM to 10:00 AM, with the impact to travel speeds and the length of congested period gradually increasing from Exit 61 to Exit 57. The congestion scan indicated the severe congestion at the major bottleneck downstream significantly reduces the travel speeds on I-95 southbound, especially between Exist 57 and Exit 59. Meanwhile, the exit and entrance ramps on I-95 southbound also carry high traffic volumes during the AM peak hours and the diverging and merging operations further reduce travel speeds in the mainline sections.

4.2.7   Interchange Traffic Operations

The study area contains five interchanges of different types, including diamond interchanges, a traffic rotary, a combined diamond, and one quadrant. Table 7 summarizes the configurations of these interchanges, the functional classification of the surrounding minor roadways, and the types of traffic controls present.

Table 7
Interchange Configurations and Traffic Operations

NB = northbound. SB = southbound.

Source: Central Transportation Planning Staff.

Based on field observations, staff considered that the three signalized interchanges (Exits 57, 60, and 61) were more likely to cause traffic queues backing up onto the freeway. To examine the potential impacts of the traffic operations, staff requested MassDOT’s assistance in collecting AM and PM peak-period turning movement counts at the three interchanges (collected on August 4, 2021) and conducted intersection capacity analyses for the three interchanges by using the Synchro traffic analysis and simulation program.13

Appendix F contains the capacity analyses at the signalized locations in both the AM and PM peak hours for the three interchanges. In general, all the signalized locations were assessed to operate at an acceptable LOS, except the Exit 60 ramp of I-95 northbound at Salem Street in Wakefield. The signalized location was assessed to operate at LOS F, with an average delay of nearly two minutes per vehicle and a queue length of approximately 300 feet during peak traffic periods.

Note that the analyses were based on the counts collected during the period that traffic increased significantly from the spring in 2020 when the COVID-19 pandemic was prevalent. However, the traffic had not reached the pre-pandemic level, according to MassDOT’s COVID-19 traffic monitoring reports. The traffic operations at these interchanges should be reassessed when the traffic returns to the normal conditions.

4.3      Crash Data Analysis

MPO staff collected the recent three-year (2015–17) crash reports from the MassDOT Crash Data Portal for the selected I-95 northbound and southbound sections and conducted crash data analysis.14 In total, 463 crashes were reported in the three-year period at different locations in the study area, consisting of 206 crashes on I-95 northbound and 257 crashes on I-95 southbound. Tables 8 and 9 summarize key attributes of the total crashes in the I-95 northbound and southbound study sections separately.

Nearly 30 percent of the total crashes on the northbound side and approximately 25 percent on the southbound side caused personal injuries, including a fatal crash on the northbound side in 2015. Nearly 60 percent of the total crashes on the northbound side and nearly 70 percent on the southbound side were reported as rear-end collisions. Single vehicle (out-of-control) and sideswipe collisions (same direction) were also prevailing on the express highway in both directions.

The data indicate that 45 percent or more of the total crashes occurred during the peak traffic periods when I-95 is congested. Meanwhile, the data show that there were more crashes on the southbound side than the northbound, which may be attributed to the congestion on I-95 as the southbound is generally more congested than the northbound during the peak times of the day (see Figures 8 and 9).

Table 8
Summary of Crash Data 2015–17:
I-95 Northbound from Exit 57 to Exit 61

* Peak periods are defined as 6:00–10:00 AM and 2:30–6:30 PM.

Source: Massachusetts Department of Transportation Crash Data Portal.

Table 9
Summary of Crash Data 2015–17:
I-95 Southbound from Exit 61 to Exit 57

* Peak periods are defined as 6:00–10:00 AM and 2:30–6:30 PM.

 Source: Massachusetts Department of Transportation Crash Data Portal.

Staff further examined the crashes by their locations in the freeway corridor. Figures 10 and 11 show the approximate crash cluster sizes and locations of the crashes in the merge, diverge, and the in-between areas at Exits 57 to 61 on I-95 northbound and southbound separately.15 Appendices G and H summarize key attributes of the crashes at each of the interchanges on I-95 northbound and southbound separately.

On the northbound side (Figure 10), Exits 57 and 58 had a number of crashes and an injury crash rate higher than the other locations. Meanwhile, Exit 60 had a relatively high number of crashes and a high injury crash rate. In general, most of the crashes occurred in the merge and diverge areas and only a few or none occurred in the in-between areas.

On the southbound side (Figure 11), all the interchanges generally had a high number of crashes but with injury crash rates somewhat lower than those on the northbound side. This may be attributed to the slower travel speeds and more congested conditions on the southbound side. Meanwhile, none or only a few crashes occurred in the in-between areas at all the interchanges and the basic freeway segment between Exit 60 and Exit 59.

Note that the data indicated that more crashes occurred in the diverge areas than the merge areas at most of the interchanges in the study area; this finding could be the result of geocoding defaults due to insufficient information of the exact crash locations.16 However, even with a smaller sample size, the data indicate that crashes in the merge areas generally had a higher injury crash rate than those in the diverge areas at almost all the interchanges.

4.4      Future-Year Highway Capacity Analyses

Staff chose 2030 as the future year and projected traffic volumes for the various freeway analysis segments by using growth factors estimated from the Boston Region MPO’s regional transportation planning model. The models projected that traffic in the study area would increase by seven percent (about 0.6 percent annually) on the northbound side and six percent (about 0.55 percent annually) on the southbound side in the AM peak hour, and by six percent on the northbound and seven percent on the southbound in the PM peak hour from 2019 to 2030. Figures 12 and 13 show the projected AM and PM peak hour traffic volumes at the different locations in the study area on an average weekday in 2030.

Staff then conducted highway capacity analyses based on the projected volumes and the identified freeway segment types at each study location. Figures 14 and 15 summarize the AM and PM peak hour highway capacity analyses by segment locations in the I-95 northbound and southbound directions. Appendices I and J contain full HCS reports of the 2030 AM and PM peak-hour capacity analyses for the various study locations.

The AM peak hour analyses (Figure 14) indicate that several locations on I-95 southbound would deteriorate from LOS D to LOS E with their adjacent freeway sections near capacity under the projected 2030 traffic conditions. Noticeably, the diverge segment at Exit 57 (Segment 3), the diverge segment at Exit 58 (Segment 6), and the merge segment at Exit 59 (Segment 7) would operate at LOS E with the adjacent freeway sections at capacity. Moreover, the merge segments at Exit 57 (Segment 1) and at Exit 58 (Segment 4) would fail at LOS F with the volume-to-capacity ratio of the adjacent freeway sections equal to 1.01.

The PM peak hour analyses (Figure 15) indicate that many merge and diverge segments on both I-95 northbound and southbound would deteriorate from LOS D to LOS E. Noticeably, in the peak direction (northbound) the merge segment at Exit 57 (Segment 3) and the diverge and merge segments at Exit 58 (Segments 4 and 6) would operate at LOS E with the adjacent freeway sections reaching capacity limits.

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Chapter 5—Proposed Low-Cost Improvements

The analyses described in the previous chapter identified locations in the study area with operational and safety concerns. Most of them are in the merge and diverge segments where the lengths of the acceleration and deceleration areas are insufficient. Localized bottlenecks can develop at these locations when the freeway mainline sections are congested during the peak hours.

For these locations, extending the acceleration or deceleration length is one effective low-cost strategy if the adjacent terrain is suitable and the right-of-way of the freeway is sufficient. Meanwhile, for locations where the entrance and exit ramps at interchanges are closely spaced, operational efficiency may be improved by creating a continuous auxiliary lane between the entrance and exit ramps.17

The sections of I-95 studied are generally located in level terrain with sufficient right-of-way, especially on the southbound side. Based on the projected 2030 traffic volumes, MPO staff conducted a series of highway capacity analyses to assess the potential application of these improvement strategies at the various study locations. Staff identified five locations on I-95 southbound and three on I-95 northbound that have potential for such low-cost improvements.

5.1      Proposed Improvements on I-95 Southbound

The locations and proposed low-cost improvements at the five locations on I-95 southbound are as follows:

1. Exit 58 entrance ramp to Exit 57 exit ramp: Expand the existing paved shoulder and add an auxiliary lane between the two ramps.
2. Exit 58 diverge segment: Extend the deceleration length to approximately 750 feet long.
3. Exit 59 merge segment: Extend the acceleration length to approximately 800 feet long.
4. Exit 59 diverge segment: Extend the deceleration length to approximately 450 feet long.
5. Exit 61 merge segment: Extend the acceleration length to approximately 800 feet long.

Figures 16 and 17 summarize the AM and PM peak hour highway capacity analyses at the locations proposed for improvements on I-95 southbound and northbound. For reference, they also include the capacity analyses under the 2030 no-build scenario at locations with no proposed improvements. Appendices K and L contain full HCS reports of 2030 AM and PM peak hour capacity analyses for the locations proposed for improvements. Figures 18 and 19 show the approximate locations and conceptual diagrams of the proposed low-cost improvements in the study area.

At Location 1, the section between the Exit 58 entrance ramp and Exit 57 exit ramp (Figure 18), the two ramps are located in close proximity (approximately 1,600 feet apart).18 Further extending the acceleration or deceleration length and reducing the space in between would potentially increase the risk of crashes.19 HCS tests of different deceleration and acceleration lengths also indicated that extensive lengths would require improvements to the diverging operation at Exit 58 and the merging operation at Exit 57, which are not possible in this relatively short section.

Thus, the proposed recommendation from this study is to expand the existing paved shoulder and create a 12-foot auxiliary lane with a 10-foot shoulder between the two ramps. A review of the digital orthophotos and highway layouts of the area indicated that there should be sufficient space for expansion. At the design stage of an improvement project, when there is more precise geometry information, a 12-foot shoulder should be considered if the available space can accommodate it.20

With the proposed auxiliary lane, the section between the two ramps would become a highway weaving segment that could facilitate the merging operation for drivers entering from Exit 58 and the positioning and diverging operation for drivers moving toward Exit 57. The capacity analyses for the proposed weaving segment (Location 1 on I-95 southbound in Figures 16 and 17) indicated that traffic operations during both the AM and PM peak hours at this location would significantly improve to the desirable LOS D from the undesirable LOS F or at-capacity LOS E under the merging and diverging operations in the no-build scenario (Segments 3 and 4 on I-95 northbound in Figures 14 and 15).

In addition, the proposed improvement would reduce the number of crashes and improve safety for freeway users at this location. A crash modification factor (CMF) is an estimate of the change in the number crashes expected after the implementation of a countermeasure. Applying CMFs from the Highway Safety Manual (HSM) and the CMF Clearinghouse show that adding continuous auxiliary lanes for weaving between the entrance ramp and the exit ramp would generally reduce crashes by 20 to 25 percent.21

For other locations on I-95 southbound (Locations 2, 3, 4, and 5), the recommendations from this study are to extend the deceleration or acceleration lengths for the respective diverging and merge segments. The recommended lengths would depend on the adjacent terrain and development conditions.

The highway capacity analyses with the proposed improvements (Figures 16 and 17) indicated that traffic operations at these locations would improve to the desirable LOS D from the near- or at-capacity LOS E under the 2030 no-build scenario.

More importantly, the proposed improvements would reduce the number of crashes and improve safety for freeway users at these locations, especially at Location 3 (Exit 59) and Location 5 (Exit 61) where drivers enter the freeway via a loop on-ramp with a limited sight distance and a relatively short acceleration lane. Applying CMFs for lengthening an acceleration lane showed that the proposed acceleration lane extensions would reduce crashes at the merge areas by 10 to 20 percent.

Implementing the proposed improvement at Location 1 (Exit 58–Exit 57) would require widening, resurfacing, and restriping the shoulder and its adjacent areas between the entrance ramp at Exit 58 and the exit ramp at Exit 57. No right-of-way acquisition or major alignment modifications would be required other than realignment of the adjacent travel lane, adding pavement markings, and relocating existing guide signs or installing new guide signs. The proposed improvement is estimated to cost between $500,000 and $750,000 to construct.22   

Implementing the proposed improvements at Locations 2, 3, 4, and 5 would require mainly restriping the areas near the exit and/or entrance ramps at Exits 58, 59 and 61, with minor widening of the adjacent shoulders if necessary. No right-of-way acquisition or alignment modifications would be required other than adding pavement markings and relocating existing guide signs or installing new guide signs. The proposed improvements are estimated to cost between $50,000 and $100,000 per location to construct, depending on the length of the acceleration or deceleration lane extensions and the conditions of the adjacent areas.

5.2      Proposed Improvements on I-95 Northbound

Unlike the southbound side, the areas adjacent to I-95 northbound generally have more constraints. These constraints include slopes and wooded areas, adjacent residential and commercial developments, and a series of sound barriers running along the shoulder area between Exit 59 and Exit 60. As such, only three locations were identified as suitable for low-cost improvements. The locations and proposed low-cost improvements are as follows:

1. Exit 57 entrance ramp to Exit 58 exit ramp: Expand the existing paved shoulder and add an auxiliary lane between the two ramps.
2. Exit 58 merge segment: Extend acceleration length to approximately 1,050 feet long.
3. Exit 60 diverge segment: Extend deceleration length to approximately 450 feet long.

The conditions at Location 1 are similar to its counterpart on the southbound side. The Exit 57 entrance ramp and Exit 58 exit ramp are located in close proximity (approximately 1,750 feet apart). Further extending the acceleration and deceleration lengths at the entrance and exit ramps is not adequate and would potentially increase the risk of crashes.

Thus, the proposed recommendation of this study is to expand the existing paved shoulder and create a 12-foot auxiliary lane with a six-foot shoulder between the two ramps (see Figure 18). The capacity analyses for the proposed weaving segment (Location 1 on I-95 northbound in Figures 16 and 17) indicated that traffic operations at this location would significantly improve during both the AM and PM peak hours to the desirable LOS D from the undesirable, near- or at-capacity LOS E under the merging and diverging operations in the no-build scenario (Segments 3 and 4 on I-95 northbound in Figures 14 and 15).

In addition, the proposed improvement would reduce the number of crashes and improve safety for freeway users at this location. Applying 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 generally reduce crashes by 20 to 25 percent.

A review the digital orthophotos indicated that the areas next to the shoulder are slightly sloping toward the south. To widen the shoulder area for adding the auxiliary lane, minor widening of the adjacent areas may be required.  At minimum, a six-foot wide shoulder is currently recommended. When more information is available at the project design stage, a 10-to-12-foot shoulder should be considered.

For Location 2 (Exit 58) and Location 3 (Exit 60) on I-95 northbound, the recommendation from this study is to extend the acceleration length for the merge terminal at Exit 58 and the deceleration length in for the diverge terminal at Exit 60.

The highway capacity analyses with the proposed improvements (Figures 16 and 17) indicated that traffic operations at the two locations would improve to the desirable LOS D from the near- or at-capacity LOS E under the 2030 no-build scenario. In addition, the proposed improvement would reduce the number of crashes and improve safety for freeway users at these locations. Applying CMFs for lengthening an acceleration lane showed that the proposed acceleration lane extensions would reduce crashes at each of the two locations by 10 to 20 percent.

Implementing the proposed improvement at Location 1 would require widening, resurfacing, and restriping the shoulder and its adjacent areas between the entrance ramp at Exit 57 and the exit ramp at Exit 58. No right-of-way acquisition or major alignment modifications would be required other than realignment of the adjacent travel lane, addition of pavement markings, relocation of existing guide signs or installation of new guide signs, and potential minor modifications of the slope areas next to the shoulder. The proposed improvement is estimated to cost between $750,000 and $1,000,000 to construct.   

Implementing the proposed improvements at Locations 2 and 3 would require mainly restriping the areas near the entrance ramp at Exit 58 and the exit ramp at Exit 60, with minor widening of the adjacent shoulders if necessary. No right-of-way acquisition or major alignment modifications would be required other than adding pavement markings and relocating existing guide signs or installing new guide signs. The proposed improvement at Location 2 is estimated to cost between $75,000 and $125,000 to construct. The proposed improvement at Location 3 is estimated to cost between $50,000 and $100,000 to construct due to minor modifications of the adjacent slope area may be necessary.

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Chapter 6—Conclusion and Next Steps

In this study, the MPO staff performed a series of operational and safety analyses for the bottleneck locations on the northbound and southbound sections of I-95 between Exit 57 and Exit 61. Low-cost improvements were then recommended for several locations on I-95 northbound and southbound:

• I-95 northbound between Exit 57 and Exit 58: Expand the existing paved shoulder and add an auxiliary lane between the exit ramp and the entrance ramp.
• I-95 northbound merge segment at Exit 58: Extend the acceleration length.
• I-95 northbound diverge segment at Exit 60: Extend the deceleration length.
• I-95 southbound between Exit 58 and Exit 57: Expand the existing paved shoulder and add an auxiliary lane between the exit ramp and the entrance ramp.
• I­-95 southbound merge segments at Exit 59 and Exit 61: Extend the acceleration length.
• I­-95 southbound diverge segments at Exit 58 and Exit 59: Extend the deceleration length.

This study provides the MassDOT Highway Division with an opportunity to assess the most critical needs in the study area and to start planning design and engineering efforts. The sections between Exits 57 and 58, particularly on I-95 southbound, were considered priority locations. The implementation of improvements to priority locations would depend on available resources and further evaluations at the project design stage.

The MPO staff conducted this study in conjunction with MassDOT staff as part of the ongoing work to identify, develop, and evaluate improvements to bottleneck locations in the Boston region. The study aligns with the MPO’s goals of managing capacity and improving mobility and safety on the region’s highway system. If implemented, these low-cost improvements would improve the freeway facilities, make traffic operations more efficient, reduce congestion at the identified bottleneck locations, reduce crashes, and improve safety for the freeway users.

Figure 1
Study Area Map

Figure 2
Freeway Segments under Analysis

Figure 3
Estimated Daily Traffic Volumes: 2019 Average Weekday

Figure 4
Estimated AM Peak-Hour Traffic Volumes: 2019 Average Weekday

Figure 5
Estimated PM Peak-Hour Traffic Volumes: 2019 Average Weekday

Figure 6
AM Peak-Hour Highway Capacity Analyses: 2019 Average Weekday

Figure 7
PM Peak-Hour Highway Capacity Analyses: 2019 Average Weekday

Figure 8
Freeway Congestion Scan: I-95 Northbound

Figure 9
Freeway Congestion Scan: I-95 Southbound

Figure 10
 Crashes on I-95 Northbound from Exit 57 to Exit 61: MassDOT Crash Data 2015–17

Figure 11
 Crashes on I-95 Southbound from Exit 61 to Exit 57: MassDOT Crash Data 2015–17

Figure 12
Projected AM Peak-Hour Traffic Volumes: 2030 Average Weekday

Figure 13
Projected PM Peak-Hour Traffic Volumes: 2030 Average Weekday

Figure 14
AM Peak-Hour Highway Capacity Analyses: 2030 No-Build Scenario

Figure 15
PM Peak-Hour Highway Capacity Analyses: 2030 No-Build Scenario

Figure 16
AM Peak-Hour Highway Capacity Analyses: 2030 Proposed Improvements Scenario

Figure 17
PM Peak-Hour Highway Capacity Analyses: 2030 Proposed Improvements Scenario

Figure 18
Approximate Locations and Conceptual Diagrams: Proposed Improvements at Exits 57, 58, and 59

Figure 19
Approximate Locations and Conceptual Diagram: Proposed Improvements at Exits 60 and 61

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