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Use of Freeway Shoulders for Travel — Guide for Planning, Evaluating, and Designing Part-Time Shoulder Use as a Traffic Management Strategy

Chapter 5. Environmental Analysis

Although the National Environmental Policy Act (NEPA) process is described earlier, this chapter highlights how to consider and conduct an analysis for the three most likely/typical/common environmental issues associated with part-time shoulder use:

  • Air quality
  • Greenhouse gas emissions
  • Noise

This chapter describes how an agency could or should go about assessing their impact on the feasibility of the project during the planning and NEPA stages and, later, addressing them during the environmental clearance stage. This chapter describes how and when these environmental analyses are conducted and discusses the potential effects of part-time shoulder use on air quality, emissions, and noise. This chapter identifies standard guides on conducting these analyses, and points out special considerations when evaluating the use of an existing paved shoulder (possibly with minor improvements) to allow use by cars and/or buses during peak hours on a regular basis.

Air Quality Analysis

Given the variety of characteristics of shoulder use projects, it is difficult to generalize the effect of shoulder use on air quality. Shoulder use may reduce congestion, which is generally beneficial to air quality. Shoulder use also has the potential to increase volume, which generally worsens air quality. There may also be no net effect on traffic characteristics that would affect air quality pollution concentrations.

Nonattainment and Maintenance Areas

Air quality analysis of federal transportation projects is required by the Environmental Protection Agency (EPA) in areas that do not meet, or previously did not meet, federal air quality standards, identified as “non-attainment” and “maintenance” areas, respectively. The definition of a federal project is broad and includes “any highway or transit project which is proposed to received funding assistance or approval through the Federal Aid Highway program or Federal mass transit program, or required FHWA or FTA approval for some aspect of the project, such as connection to an Interstate highway or derivation from applicable design standards on the Interstate system” is subject to transportation conformity. This definition, particularly the “derivation from applicable design standards,” will encompass most part-time shoulder use projects. Projects in non-attainment and maintenance areas must be incorporated into the region’s Transportation Improvement Plan (TIP) and, per the Clean Air Act, must demonstrate their consistency with the regional conformity determination and address potential localized emissions impacts. Conformity requirements cover four pollutants for which federal standards are set—ozone (O3),
nitrogen dioxide (NO2), carbon monoxide (CO), and particulate matter (PM2.5 and PM10).

Areas that do not meet or previously did not meet federal air quality standards are identified as “non-attainment” and “maintenance” areas, respectively. Air quality analysis of federal transportation projects is required by the Environmental Protection Agency (EPA) in these designated areas for the transportation-related pollutants—ozone, nitrogen dioxide, carbon monoxide, and particulate matter per the transportation conformity rule. Shoulder-use projects are typically federal projects because they require design exceptions. Projects in metropolitan non-attainment and maintenance areas must be incorporated into regional emissions analyses associated with the region’s Transportation Improvement Plan (TIP) and long-range plan (MTP). Transportation conformity rules also require the analysis of potential localized emissions impacts where applicable. Non-attainment and maintenance areas have standing transportation conformity procedures in place that address how projects are handled to assess for conformity status (exempt/non-exempt) and whether a project-level analysis is required. These procedures may assist in assessing individual part-time shoulder lane projects.

More information on conformity can be found on these websites:

FHWA Conformity Website:

EPA Conformity Website:

Other Areas

Outside of nonattainment and maintenance areas, air quality analyses may be conducted as part of a NEPA analysis. Agencies may consider the following questions when deciding whether or not to conduct air quality analysis in these areas:

  • Is there concern about the project within the community?
  • Is the shoulder only open to buses, or will a greater percentage of the fleet have access?
  • If the shoulder is only open to buses, is service being added or will bus headways remain the same?
  • For PM impacts, are diesel vehicles being moved closer to sensitive roadside receptors?
  • Is the project in a dusty area where dust will be stirred up when the shoulder opens each day?

The degree of analysis conducted should be proportional to the project scope. Qualitative analysis without the use of modeling software is likely acceptable for low impact projects and more-complex analysis may provide useful information for projects with a higher potential for impact.

Analysis Tools and Techniques

For quantitative air quality analysis, conformity guidance should be used. Tools for quantitative analysis include the latest MOVES (Motor Vehicle Emissions Simulator) and EMFAC models. MOVES is used in all states, except California, which uses EMFAC. The models require inputs such as volumes and speeds, as shown in Table 7.

Table 7. Air Quality tools and data needs.
Tool Inputs


(Motor Vehicle Emissions Simulator)

Vehicle Operating Mode: vehicle speed, vehicle acceleration, road grade, rolling resistance, vehicle mass

Service Hours Operating: actual time a vehicle spends within certain operating modes (captures emissions from idling)


VMT-based emission model: daily VMT by vehicle speed (at 5 mph increments) and vehicle class

More information on these tools can be found on these websites:

U.S. Environmental Protection Agency, Motor Vehicle Emissions Simulator (MOVES):

California Air Resources Board, Emission Factor Model (EMFAC):

When analyzing part-time shoulder use, practitioners may consider two unique aspects that differ from a conventional widening project:

  • During the hours the shoulder is closed to traffic, dust may accumulate on it, and this dust may be stirred up when the shoulder reopens each day.
  • Use of the right shoulder will place emission sources (vehicles) closer to sensitive receptors, if any are present.

If a quantitative analysis is conducted for transportation conformity purposes, practitioners should follow EPA guidance for conducting hot-spot analysis for particulate matter and carbon monoxide, where applicable.

Greenhouse Gas Emissions Analysis

In some states—currently California, Massachusetts, New York, and Washington—analysis of greenhouse gas (GhG) emissions is required for some transportation projects. Part-time shoulder use projects in these states may require GhG analysis, depending on the circumstances.

Resources, such as the FHWA’s Handbook for Estimating Transportation Greenhouse Gases for Integration into the Planning Process, provide information on how to analyze on-road greenhouse gas emissions at the state and regional level. The majority of greenhouse gas (GhG) emissions from transportation are carbon dioxide (CO2) emissions from the combustion of petroleum-based products. Methods to reduce GhG emissions include improving traffic operating conditions, such as avoiding rapid acceleration and braking, reducing idling, and reducing travel demand. MOVES and EMFAC (described in the previous section) are used for GhG emissions analysis.

Similar to air quality, it is difficult to generalize the effect of part-time shoulder use on GhG emissions. A study of the M42 part-time shoulder uses pilot project in the UK captured this.(25) In general, regardless of part-time shoulder use, emissions on the M42 fall per mile as average speed increases from congested conditions to 40-50 mph where the fuel efficiency of engines is greatest and then rises as the average speed increases towards 70 mph and fuel efficiency falls. On a per-vehicle basis, the GhG emission benefit of part-time shoulder use was greatest in lower speed ranges when it reduced start-stop traffic. However, the effect is likely to be outweighed by additional emissions from higher speeds above the 40-50 mph range and changes in peak period traffic volume. The study concluded by stating “we expect that shoulder [use] will lead to an increase in traffic emissions compared to a ‘do nothing’ scenario. However, the impact of [part-time shoulder use] on traffic emissions is expected to be lower than the impact of road widening.”

Noise Analysis

The level of highway noise primarily depends upon traffic volume, traffic speed, truck volume, and to a lesser extent it depends on other factors such as topography and pavement type.

The Federal noise regulation in 23 CFR 772 constitutes the Federal noise standard. For the purposes of meeting the requirements in 23 CFR 772, a noise analysis is required for all Federal or Federal-aid projects that are defined as Type I, per the regulation. Noise measurements are conducted to determine existing noise levels, and future levels are predicted using the FHWA Traffic Noise Model (TNM). A noise impact occurs when the predicted noise level approaches or exceeds the Noise Abatement Criteria (NAC) in 23 CFR 772, Table 1, or represents a substantial increase over existing noise levels. If noise impacts are determined, then noise abatement must be considered. If noise abatement is found to be feasible and reasonable, per 23 CFR 772 and that state’s noise policy, then the noise abatement measure must be constructed. For the purposes of NEPA, a noise analysis may also compare the project noise level of a no- build or no action condition to the existing noise levels.

There are eight parts to the Type I definition, but there are only three that may encompass a part- time shoulder use project, depending on the type of shoulder and any restrictions and/or requirement for its uses. Those three parts include:

  • “The physical alteration of an existing highway where there is…substantial horizontal alteration. A project that halves the distance between the traffic noise source and the closest receptor between the existing condition and the future build condition.”
  • “The addition of a through-traffic lane(s). This includes the addition of a through-traffic lane that functions as an high-occupancy vehicle (HOV) lane, High-Occupancy Toll (HOT) lane, bus lane, or truck climbing lane.”
  • “Restriping the existing pavement for the purpose of adding a through-traffic lane or an auxiliary lane.”

Similar to air quality analysis, design exceptions required for part-time shoulder use projects will make these projects federal and, thus, subject to 23 CFR 772.

The level of noise analysis necessary will depend upon the type of part-time shoulder use. For bus-on-shoulder (BOS), noise analysis may be qualitative because the number of additional vehicles and changes in speed are small or nonexistent. For static and dynamic part-time shoulder use, noise analysis will typically be conducted in a manner similar to a conventional widening project. For the noise analysis, the location of the part-time shoulder use would affect the proximity to sensitive receptors. Left side part-time shoulder use is less likely to have noise impacts compared to right side part-time shoulder use, which places the traffic closer to sensitive receptors. Noise analysis and determination of noise mitigation needs is focused on peak noise conditions. This may or may not correspond to peak volume conditions (when shoulder lanes are typically open), and analysis will determine if part-time shoulder use affect peak noise or not. Predicted noise levels are determined by using the FHWA Traffic Noise Model.(4) If part-time shoulder use does increase peak noise, and there are impacts associated with it, then noise abatement must be considered and implemented if found to be feasible and reasonable.

Existing part-time shoulder use demonstrates the differences in noise analysis and mitigation needs. For example, part-time shoulder use on US 2 in Everett, Washington, was implemented on a bridge over wetlands. There were no sensitive receptors (i.e. land uses where noise would cause impacts) in the project area, so no noise analysis was conducted. Washington State is currently planning part-time shoulder use on another freeway—I-90 east of Seattle—and noise analysis was conducted because there were sensitive receptors. The analysis indicated the need for noise walls, which will be constructed as part of the part-time shoulder use project.(33)

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