Rural Conformity:

A Survey of Practice

Requested by:

American Association of State Highway 

and Transportation Officials (AASHTO)

Standing Committee on Planning

Prepared by:

ICF Consulting

9300 Lee Highway

Fairfax, Virginia 22031

In association with:

Sarah J. Siwek & Associates

The information contained in this report was prepared as part of NCHRP Project 08-36, Task 28, 

National Cooperative Highway Research Program, Transportation Research Board. 
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Acknowledgement

This study was requested by the American Association of State Highway and 

Transportation Officials (AASHTO), and conducted as part of National Cooperative 

Highway Research Program (NCHRP) Project 08-36. The NCHRP is supported by annual 

voluntary contributions from the state Departments of Transportation. Project 08-36 is 

intended to fund quick response studies on behalf of the AASHTO Standing Committee on 

Planning. The report was prepared by ICF Consulting, with assistance from Sarah J. 

Siwek and Associates. The work was guided by a task group chaired by Calvin Legget, 

North Carolina Department of Transportation which included Cecilia Ho, Federal Highway

Administration; David Hyder, North Carolina Department of Transportation; Meg Patulski, 

U.S. Environmental Protection Agency; and Lynn Soporowski, Kentucky Transportation 

Cabinet. In addition, Angela Spickard from the US Environmental Protection Agency also 

provided extensive feedback on the methodology and draft report. Appendix B lists the 

many practitioners who provided the substance of this report by describing their 

experiences with the rural transportation conformity process. The project was managed by

Ronald D. McCready, NCHRP Senior Program Officer. 

Disclaimer

The opinions and conclusions expressed or implied are those of the research agency that 

performed the research and are not necessarily those of the Transportation Research Board 

or its sponsors. This report has not been reviewed or accepted by the Transportation 

Research Board's Executive Committee or the Governing Board of the National Research 

Council. 
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TABLE OF CONTENTS

1 INTRODUCTION ................................................................................................................1

1.1 Background .............................................................................................................1

1.2 Study Purpose..........................................................................................................1

1.3 Research Approach..................................................................................................2

1.4 Report Organization ................................................................................................3

2 AN OVERVIEW OF TRANSPORTATION CONFORMITY REQUIREMENTS AND ISOLATED 

RURAL AREAS.........................................................................................................................4

2.1 Types of Rural Areas...............................................................................................4

2.2 What Is Transportation Conformity?.......................................................................5

2.3 Transportation Planning and Conformity Requirements in Metropolitan Areas ....6

2.4 Conformity Requirements for Donut Areas............................................................7

2.5 Conformity Requirements for Isolated Rural Areas................................................7

3 STATE OF THE PRACTICE................................................................................................15

3.1 Frequency of Conformity Analysis.......................................................................15

3.2 Role of State DOT and Local Agencies................................................................18

3.3 Outcomes...............................................................................................................19

3.4 Issues Encountered in the Conformity Process.....................................................22

3.5 Preparations for New Standards............................................................................25

4 LESSONS LEARNED ........................................................................................................27

4.1 Transportation Staff Need to be Involved in Development of the SIP..................27

4.2 Build Trust Between DOT and Air Agency Staff .................................................27

4.3 Be Aware of Data Limitations and Improvements to Address Them ...................28

4.4 Build Institutional Memory and Expertise ............................................................30

4.5 Conclusion.............................................................................................................31

5 RESOURCES / REFERENCES ............................................................................................32

APPENDIX A: INTERVIEW GUIDE...........................................................................................34

APPENDIX B: CONTACT INFORMATION .................................................................................36
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1 INTRODUCTION

1.1 BACKGROUND 

The Clean Air Act (CAA) of 1990 requires that transportation plans, programs and projects not 

cause or contribute to violations of the health based air quality standards. This requirement is met 

through the transportation conformity process, which helps ensure that transportation activities are 

consistent with, or are in “conformity,” with regional air quality plans in air quality nonattainment 

and maintenance areas.1 

Although air quality is often thought of as primarily a concern for urban areas, a number of rural 

locations are designated as nonattainment or maintenance areas. Furthermore, EPA has 

promulgated a new eight-hour ozone standard and fine particulate matter (PM-2.5) standard that 

are expected to result in additional rural areas being designated as nonattainment areas. These rural 

areas will be subject to the conformity process for the first time, and thus will need to understand 

the technical and procedural requirements.

The conformity process is critically important to transportation agencies because a conformity 

determination must be made before advancing Federally funded or approved transportation 

projects. The conformity process is complex, however, and many state and local transportation 

agencies in rural areas have limited experience on conformity requirements and how to meet them. 

A rural area is defined as an area with an urbanized population of less than 50,000. Federal 

metropolitan transportation planning requirements do not apply to these areas. This study focuses 

on isolated nonattainment and maintenance rural areas (isolated rural areas), which are rural 

areas not located within a metropolitan nonattainment or maintenance area. These areas are 

different from so-called “donut” areas, which are areas located inside a metropolitan nonattainment 

or maintenance area boundary but outside the metropolitan planning boundary (i.e., outside a 

Metropolitan Planning Organization’s boundary). Isolated rural areas face different conformity 

requirements from metropolitan areas and typically have limited access to resources and technical 

tools. 

1.2 STUDY PURPOSE

The purpose of this report is to provide practical information to help transportation staff better 

understand the conformity requirements in isolated rural areas and the actions that need to be taken 

to meet them. 

The objectives are threefold:

1. Describe the conformity requirements for isolated rural areas, and how these differ 

from requirements in metropolitan areas and “donut” areas;

 

1 A nonattainment area is a region that does not meet (or that contributes to ambient air quality in a nearby area that 

does not meet) the national ambient air quality standard (NAAQS) for any of the six criteria pollutants. The area 

must take specified actions within a certain time frame to reduce emissions and attain the standard. A maintenance 

area is a region previously designated as nonattainment, and subsequently redesignated to attainment.
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2. Describe the experience of isolated rural areas with the conformity process, including 

challenges faced, based on a limited set of interviews with transportation and air 

quality staff in these regions; and 

3. Highlight state of the practice and lessons learned from these experiences.

1.3 RESEARCH APPROACH 

This study builds on a number of previous efforts to address air quality issues in rural areas and the 

conformity process in general. The research approach involved, first, a review of relevant 

documents, case studies, and guidance on the conformity process, particularly as it relates to 

isolated rural areas. This information was used to develop a background guide on the conformity 

process focused on isolated rural areas.

The research team then conducted a series of interviews with transportation and environmental 

agency staff in seven states containing isolated rural areas. The interviews were designed to collect 

information both on existing experience with conformity in isolated rural areas and actions being 

undertaken to prepare for conformity in anticipated new isolated rural nonattainment areas under 

the eight-hour ozone or fine particulate matter standards. 

The following seven states were selected for this study:

 Colorado

 Kentucky

 Maine

 Oregon

 Ohio

 Pennsylvania

 South Carolina.

These states were selected using the following criteria:

1. Each state contains one or more existing isolated rural nonattainment or maintenance areas 

for ozone, carbon monoxide, or particulate matter; some areas also anticipate new isolated 

rural nonattainment areas under the eight-hour ozone and/or PM-2.5 standards.

2. Each of the states has conducted at least one conformity analysis within the past five years 

or is undertaking substantial efforts to prepare for new rural nonattainment areas; this was 

an important criteria since many isolated rural areas have not undertaken a conformity 

analysis in recent years, according to a recent survey by FHWA.2

3. The states represent a geographic diversity in terms of regions of the U.S., including all 

four AASHTO regions and six different EPA regions.

The research team began the interview process with state department of transportation (DOT) staff 

because state DOTs typically take the lead in conformity for isolated rural areas. State DOT staff 

also could describe experience with more than one isolated rural area. Following the state DOT 

interviews, the research team interviewed staff from the state air quality agency in each of the 

selected states. In some cases, the research team also interviewed staff from a regional EPA or 

FHWA division office to supplement the interviews. 

 

2 Dye Management. Transportation/Air Quality Issues in Rural Areas. Prepared for Federal Highway 

Administration, June 2003.
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1.4 REPORT ORGANIZATION

This report contains the following sections:

Section 1: Introduction

This section describes the motivation and purpose of this report, as well as the study approach.

Section 2: A Guide to Conformity Requirements for Isolated Rural Areas

This section defines what is meant by an isolated rural area, and includes a summary of the 

conformity requirements for isolated rural areas and how they differ from requirements of 

metropolitan areas. It is intended to provide a simple description of what state and local 

transportation practitioners in isolated rural areas need to know about conformity requirements. 

Section 3: State of the Practice 

This section highlights findings from interviews conducted in seven states, in terms of their 

experience with conformity in isolated rural areas, practices used, and activities to prepare for 

potential new nonattainment areas. These findings should help to inform existing and new isolated 

rural areas about what to expect with the conformity process. 

Section 4: Lessons Learned

This section summarizes key lessons to be taken from the experience of the seven states. It 

highlights practices that were characterized as particularly successful and provides 

recommendations. 

Section 5: Resources and References

This section provides information on resources that transportation staff should be aware of to help 

prepare for conformity evaluations in isolated rural areas. 
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2 AN OVERVIEW OFTRANSPORTATION CONFORMITY

REQUIREMENTS AND ISOLATED RURALAREAS 

Regulations governing transportation conformity are found in Title 40 of the Code of Federal 

Regulations (40 CFR Parts 51 and 93). The specific requirements for isolated rural areas are 

described in 40 CFR §93.109(g). Given the unique issues associated with isolated rural areas, the 

different requirements for these areas compared to metropolitan areas, and the complexity of 

federal regulations, transportation practitioners who are new to the conformity arena may need 

assistance to decipher the requirements in these areas. Many states have a small number of isolated 

rural non-attainment and maintenance areas, and thus, have not had to conduct conformity analyses 

in isolated rural areas often, or on a regular cycle.

This section is intended to help transportation and other agencies understand the conformity 

requirements for isolated rural areas. It is designed to provide a simple and concise summary of the 

key requirements. It defines isolated rural areas and describes how the conformity process differs 

between isolated rural areas, metropolitan areas, and “donut” areas. This section draws from 

federal guidance on the conformity process, including Transportation Conformity: A Basic Guide 

for State and Local Officials and the Transportation Conformity Reference Guide. Readers are 

urged to consult these resources, and other references listed in Section 5 for more detailed 

information. 

2.1 TYPES OF RURAL AREAS 

The U.S. DOT (FHWA/FTA) defines a rural area as an area having an urbanized population of less 

than 50,000. U.S. DOT’s metropolitan planning requirements3including those related to the 

development of transportation plans and Transportation Improvement Programs (TIPs), do not 

apply in these areas.

For conformity purposes, nonattainment and maintenance areas that are rural fit into one of two 

categories: 

1) An isolated rural area is a rural area that is not located within or adjacent to a metropolitan 

nonattainment or maintenance area; 

2) A “donut” area is a rural area that is located inside a metropolitan nonattainment or 

maintenance area boundary, but is outside the metropolitan transportation planning 

boundary. 

Exhibit 1 provides a visual diagram of the geographic differences between these two types of areas. 

 

3

23 CFR 450 Part 613, 58 FR 58040, Oct. 28, 1993.
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Exhibit 1: Difference between Isolated Rural Area and Donut Area

Although both types of rural areas may be similar in character, isolated rural areas and donut areas 

are very different when it comes to conformity analysis. An isolated rural area is not part of the 

emissions analysis or transportation demand modeling analysis of any Metropolitan Planning 

Organization’s (MPO) transportation plan or TIP. On the other hand, donut area projects must be 

incorporated into the metropolitan area’s regional emissions analyses and generally must follow the 

more standardized metropolitan area process.

2.2 WHAT IS TRANSPORTATION CONFORMITY?

Transportation conformity is a way to ensure that transportation plans, programs, and projects that 

receive Federal funding and approval are consistent with Federal air quality goals. It ensures that 

these transportation activities do not worsen air quality or interfere with the “purpose” of the State 

Implementation Plan (SIP), which is to meet the national ambient air quality standards (NAAQS) 

set by the U.S. Environmental Protection Agency (EPA). Meeting these standards often requires 

emission reductions from on-road mobile sources.

According to the Clean Air Act (CAA), transportation plans, programs, and projects cannot:

 Cause new violations of the Federal air quality standards; 

 Increase the frequency or severity of existing violations of the standards; or 

 Delay timely attainment of the standards. 

The CAA requires that transportation plans, programs, and projects in nonattainment or 

maintenance areas that are funded or approved by the Federal Highway Administration (FHWA) or 

Federal Transit Agency (FTA) be in conformity with the SIP through the process described in the 

transportation conformity regulation. This requirement also applies to projects that are not 

federally funded if they have regional significance.

The foundation upon which a transportation conformity determination is based is the motor vehicle 

emissions budget contained in an approved SIP or a SIP budget that has been found adequate by 

EPA. It is this budget which establishes the ceiling on aggregate emissions allowed from 

transportation sources. The regional emissions analysis is the major analytical element of the 

Donut area

Isolated 

rural area

Nonattainment area boundary

MPO boundary
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conformity process. A regional emissions analysis must show that the emissions resulting from the 

planned transportation system will not exceed the budget for specific analysis years. 4

In areas where budgets are not required, emission reduction tests are used. There are two such tests: 

 “Less than baseline” test - shows that emissions are less than the baseline year emissions.5

 “Build/no build” test6- shows that emissions from implementing the plan (the action/build 

scenario) are less than if the plan were not implemented (the baseline/no build scenario). 

2.3 TRANSPORTATION PLANNING AND CONFORMITY REQUIREMENTS IN 

METROPOLITAN AREAS

Under U.S. DOT’s planning regulations,7 Metropolitan Planning Organizations (MPOs) must have 

transportation plans in place that present a 20-year perspective on transportation investments for 

their region. The MPO must also have a transportation improvement program (TIP), which is a 

multi-year prioritized list of projects (covering at least 3 years) including all projects that require 

funding or approval by FHWA or FTA. The transportation plan must be updated every three years 

in nonattainment and maintenance areas and the TIP every two years. The Clean Air Act section 

176(c) and conformity rule requires conformity to be demonstrated not less frequently than every 

three years. 

In general, the conformity process involves three components:

 Plan Conformity - The transportation plan must be shown to conform to the SIP, based on a 

regional emissions analysis for the plan;

 TIP Conformity – The TIP must be shown to be consistent with the transportation plan and SIP 

before it can be approved, based on a regional emissions analysis;8and

 Project Conformity - FHWA/FTA projects must be found to conform before they are adopted, 

accepted, approved, or funded. This means that transportation projects must conform to the 

following criteria: 1) they must come from a conforming transportation plan and TIP; 2) the 

design concept and scope of the project that was in place at the time of the conformity finding 

must be maintained through implementation; and 3) the project design concept and scope must 

be sufficiently defined to determine emissions at the time of the conformity determination. 

Regions that are designated as carbon monoxide (CO) or particulate matter (PM-10) nonattainment 

or maintenance areas must also show that new localized violations of those pollutants will not 

result from project implementation. Certain types of projects are exempt from conformity 

requirements (e.g. safety, landscaping, and other projects with neutral or minimal emissions 

impacts).

 

4

In addition, compliance with the planning requirements of Title 23 and 49 U.S.C. is integral to making a 

conformity determination. These requirements include demonstration of a fiscally constrained plan and TIP.

5

1990 is the baseline year for existing nonattainment and maintenance areas. A “less than 1990” test is used 

for moderate and above ozone nonattainment areas that are subject to the reasonable further progress 

requirements of CAA section 182(b)(1), in moderate CO areas with design value greater than 12.7 ppm, and 

in serious CO nonattainment areas. A “no greater than” (less than or equal to) 1990 test is used for other 

areas. For newly designated areas, 2002 has been proposed as the baseline. 

6 The “build/no build” test is referred to as the “baseline/action” test in 40 CFR 93.119. The term build/no 

build is used through this report.

7

23 USC 134 and 135 codified in 23 CFR 450.

8 This is the same analysis as the plan analysis.
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While conformity determination is required at least every three years, certain events, such as SIP 

revisions that establish or revise a transportation-related emissions budget, or add or delete 

transportation control measures (TCMs) also trigger new conformity determinations. Conformity 

analyses must also be conducted for each new project unless it is an exempt project.

Within the conformity process, the regional emissions analysis should reflect emissions of all 

travel, including all FHWA/FTA projects. Modeling must also include all regionally significant 

projects, regardless of project funding source.9Projects that are not regionally significant are not 

required to be modeled, but vehicle miles traveled (VMT) from such projects must be estimated in 

accordance with reasonable professional practice. Certain types of projects are exempt from 

conformity, such as safety projects, maintenance activities, rehabilitation of transit vehicles, and 

bicycle and pedestrian facilities, as long as they do not have potential adverse impacts on 

emissions.10

The regional emissions analysis must estimate total projected emissions for a number of different 

future years. Per §93.118(d)(2) of the transportation conformity rule, analysis years must include:

 Any years in the time frame of the transportation plan, provided they are no more than 10-years 

apart;

 The attainment year (if it is within the time frame of the transportation plan); and

 The last year of the transportation plan (at least 20 years in the future).

2.4 CONFORMITY REQUIREMENTS FOR DONUT AREAS

A donut area is a geographic area that falls within the boundary of a nonattainment or maintenance 

area that contains a metropolitan area, but falls outside of the metropolitan planning area boundary. 

Emissions in donut areas must be included in the metropolitan area regional emissions analysis for 

the Plan and TIP.11 As a result, in most cases transportation emissions in donut areas must be 

analyzed and documented on the same schedule as the conformity analysis for the MPO area. In 

some cases, the State DOT takes the lead in analyzing emissions in donut areas, in coordination 

with the MPO. The process for addressing the donut areas is subject to agreement through the 

interagency consultation process. 

2.5 CONFORMITY REQUIREMENTS FOR ISOLATED RURAL AREAS

An isolated rural nonattainment or maintenance area does not contain and is not part of any 

metropolitan planning area.12 Isolated rural areas do not have metropolitan transportation plans or 

 

9 Regionally significant project is defined as a transportation project (other than an exempt project) that is on 

a facility which serves regional transportation needs (such as access to and from the area outside of the 

region, major activity centers in the region, major planned developments such as new retail malls, sports 

complexes, etc., or transportation terminals as well as most terminals themselves) and would normally be 

included in the modeling of a metropolitan area's transportation network, including, at a minimum, all 

principal arterial highways and all fixed guideway transit facilities that offer an alternative to regional 

highway travel. See 40 CFR 93.101 (Definitions).

10 See 40 CFR 93.126 (Exempt projects), 40 CFR 93.127 (Projects exempt from regional emissions analyses) 

and 40 CFR 93.128 (Traffic signal synchronization projects).

11 The requirement that donut area emissions be included in the regional emissions analysis conducted by the 

associated MPO may not be applicable where there are sub-area emissions budgets for the donut area.

12 Metropolitan planning areas are designated under 23 U.S.C. 134 and 49 U.S.C. 5303.
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TIPs13 and do not have projects that are part of the emissions analysis of any MPO’s metropolitan 

transportation plan or TIP. Projects in such areas are instead included only in statewide 

transportation improvement programs and in some cases in the statewide transportation plan.

As in other areas, the conformity process is intended to ensure that transportation projects conform 

to the SIP. However, isolated rural areas do not have a federally required metropolitan 

transportation plans or TIPs. As a result, the conformity frequency requirements for plans and 

TIPs14 (including the three-year conformity update requirement) do not apply. 

Federal transportation projects in isolated rural areas must be consistent with the Statewide 

Transportation Plan and incorporated into the Statewide Transportation Improvement Program 

(STIP) prior to any federal approval. A regional conformity analysis must be conducted, therefore, 

only when a project sponsor seeks FHWA or FTA approval or funding for a regionally significant, 

non-exempt project, or if there is a change in the design concept or scope of a regionally significant 

project that was in place at the time of the conformity finding. In addition, conformity must be 

redetermined for any FHWA/FTA project if three years have elapsed since the most recent major 

step to advance the project occurred. Major steps are defined as the following: NEPA process 

completion; start of final design; acquisition of a significant portion of the right-of-way; or 

approval of the plans, specifications and estimates.

Overview of Isolated Rural Area Requirements /Differences from Metropolitan Areas

In general, the conformity requirements for 

isolated rural areas are similar to 

metropolitan nonattainment or maintenance 

areas except that:

1. frequency requirements for updating 

conformity in metropolitan areas do not 

apply, and

2. some flexibility is available to isolated 

areas for the years after the attainment 

date or the last year of the maintenance 

plan.

The state DOT often plays a key role in determining conformity in isolated rural areas. This is 

because isolated rural areas ordinarily do not have transportation planning agencies or regional 

modeling capabilities. The state DOT usually performs the analytical work necessary to make a 

conformity determination in such areas, in conjunction with the state environmental agency and 

local or county transportation agencies. 

Exhibit 2 contains a flowchart that shows the general process and conformity requirements for 

isolated rural areas. The steps in this process are described below.

 

13 Requirements for metropolitan plans and TIPs are described under 23 U.S.C. 134 and 49 U.S.C. 5303 and 

5304.

14 These frequency requirements are described in 40 CFR 93.104.

Difference in Frequency of Conformity Analysis

In Metropolitan Areas:

Conformity determinations must be made at least 

every three years

In Isolated Rural Areas:

Conformity determinations must be made only 

prior to FHWA or FTA approval of a non-exempt 

project.
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Exhibit 2: Conformity Process in Isolated Rural Areas

Yes

Regionally significant

pro ject in isolated rural 

area?

No No conformity 

analysis required

Conduct regional emissions analysis

Does SIP include a motor 

vehicle emissions budget?

Budget test

For analysis years before the 

attainment date or end of the 

attainment / maintenance period 

For analysis years after the 

attainment date or end of the 

attainment / maintenance period

• Budget test

OR

• Build/no-build and/or 

Less-than 1990 test*

OR

• Air quality modeling 

No

Project (hot spot) analysis

Yes

Conformity Analysis 

Complete

Yes

 Must be consistent with statewide plan and included in STIP

 NEPA requirements must be met

 Must not interfere with TCM implementation

 Must satisfy latest planning assumptions and emissions model

 Must satisfy consultation requirements

Build/no-build and/or 

Less-than-1990 test

(according to area 

classification)*

Area is maintenance or

nonattainment for CO 

or PM-10?

No

* Exhibit 3 (below) contains more specific descriptions of which tests are required.

Regional Emissions Analysis

A regional emissions analysis is required that includes travel from the entire transportation system, 

including all Federal projects, and all regionally significant non-Federal projects in the isolated 

rural nonattainment or maintenance area. If there is no new regionally significant non-exempt 

project requiring Federal approval, no conformity analysis is required for that area. 

Projects in isolated rural areas must be included in a statewide transportation improvement program 

(STIP) prior to any Federal action to fund the project. Any Federally funded project must also meet 

National Environmental Policy Act (NEPA) requirements.

The required regional emissions analysis tests for isolated rural areas depend on whether the area 

has an emissions budget or not. If an area has an approved budget or an emissions budget found 

adequate by EPA, the emissions budget test must be used. If the area does not have an adequate 

emissions budget, emission reduction tests are used; the specific test that must be used depends on 
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the area’s classification. In addition, for the period after the attainment date15 or the end of the 

attainment or maintenance period, isolated rural areas have flexibility in the regional emissions 

analysis tests for those “out-years” whereas other areas do not have this flexibility. The 

requirements are shown below in Exhibit 3. 

Exhibit 3: Regional Emissions Analysis Tests Required for Isolated Rural Areas16

Type of Area

Analysis for years during time frame 

of last adequate SIP Analysis for years 

after time frame of 

last adequate SIP

No Emissions Budget

Adequate 

Emissions 

Budget

The following areas, which are 

required to submit a control strategy 

SIP containing an emissions budget: 

 Moderate or above ozone areas 

 Moderate CO areas with design 

value greater than 12.7 ppm 

 Serious CO areas

If no adequate or approved 

budget is submitted, a regional 

emissions analysis must be 

performed that meets the 

following emissions reduction 

tests: 

build/no-build and less-than 1990

Conduct regional 

emissions analysis 

meeting emissions 

budget test (as long 

as the budget has 

been found adequate 

or approved by 

EPA). 

Projects must satisfy one of the 

following: 

1) Regional emissions analysis 

meeting emissions budget test;

2) Build/no-build and less than 

1990 test including NOx in 

ozone areas, or

3) Air quality model as used in 

last adequate or approved SIP

 Rural PM10 areas (which are 

required to submit a control 

strategy SIP containing an 

emissions budget)

 Any rural NOx area. If there is noadequate or 

approved budget, a regional 

emissions analysis must be 

performed that meets the 

build/no-build or no-greater-than1990 tests

Projects must satisfy one of the 

following: 

1) Regional emissions analysis 

meeting emissions budget test;

2) Build/no-build or nogreater-than 1990 test 

including NOx in ozone areas, 

or

3) air quality model as used in 

last adequate or approved SIP

Areas not required to submit a control 

strategy SIP containing an emissions 

budget:

 Rural transport ozone area

 Marginal and below ozone area

 Incomplete data ozone area

 Moderate CO area with design 

value of 12.7 ppm or less

 Unclassified CO area

Source: Based on Exhibit 37 of the Federal Highway Administration Transportation Conformity Reference Guide. Revised July 31, 

2001. 

Regional Emissions Analysis Tests for Periods Before Attainment Date or End of 

Maintenance Period

For analysis years before the attainment date or end of the maintenance period, the tests that must 

be applied depend on the classification of the area and whether it has an adequate emissions 

budget.

 

15 The attainment date is the date by which an area is required to meet air quality standards.

16 This table, and other parts of the report occasionally refer to areas that are required to submit SIPs. 

Technically speaking, it is the State that is required to submit the SIP.
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Tests for Areas Required to Submit SIPs

Areas that are required to submit control strategy SIPs are: moderate and above ozone areas; CO 

areas classified as serious or as moderate with a design value of greater than 12.7 ppm; and PM-10 

areas. In addition, other areas can choose to voluntarily submit a motor vehicle emissions budget.

As shown in Exhibit 3 above, isolated rural areas that have SIPs in place with adequate or approved 

motor vehicle emissions budgets must meet the budget test for the period of the SIP or maintenance 

plan. This means that emissions from on-road mobile sources cannot exceed the budget for specific 

analysis years including the attainment year or, in a maintenance area, the last year of the 

maintenance plan. If there is no adequate or approved emissions budget, the regional emissions 

analysis must use both the build/no-build and the less-than-1990 tests, with the exception of PM-10 

areas, which may use either the build/no build or the no-greater-than-1990 test. 

Tests for Areas Not Required to Submit SIPs 

Areas that are not required to submit control strategy SIPs include the following:

 Rural transport ozone nonattainment areas,

 Marginal ozone areas,

 Submarginal ozone areas,

 Incomplete data ozone areas,

 Moderate CO areas with a design value of 12.7 ppm or less, and

 Not classified CO areas.

In order to demonstrate conformity, projects located in one of the above areas that is an isolated 

rural nonattainment or maintenance area must meet one of the emission reduction tests in 40 CFR 

93.119 for all applicable pollutants or pollutant precursors (including NOx in ozone areas) within 

the nonattainment area. 

The emissions reduction test that is applied can be either the build/no-build test or the no-greaterthan-1990 emissions level test. The emissions budget test is not applied in the above areas unless 

the State voluntarily submits an attainment demonstration and accompanying motor vehicle 

emissions budget(s) or maintenance plan. In such a case, the budget test replaces the above 

emissions reduction test once the EPA finds the budget adequate for conformity purposes. 

Regional Emissions Analysis Tests Beyond the Attainment Year or Last Year of 

Maintenance Plan

For the years beyond the attainment year or last year of the maintenance plan, an isolated rural area 

can choose among three tests:

 Budget test - The area can use the budget test to demonstrate that on-road mobile source 

emissions, including those from all regionally significant projects, will not exceed the 

motor vehicle emissions budget for the attainment year or the end year of the maintenance 

plan, 

 Emission reduction test(s) - The area can use the emissions reduction tests that apply to 

that classification of nonattainment area to demonstrate conformity for the years beyond 

the attainment date or end year of the maintenance plan. As shown in Exhibit 3: 
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o For moderate or above ozone areas, moderate CO areas with design value greater 

than 12.7 ppm, and serious CO areas, both the build/no-build and the less-than1990 tests are required; 

o For other areas, either the build/no-build or no-greater-than-1990 test is required, 

or,

 Air quality modeling - The area can demonstrate through air quality modeling used in the 

attainment SIP or maintenance plan that the project in combination with all other 

regionally significant projects expected in the area in the time frame of the statewide 

transportation plan, will not:

o Cause or contribute to any new violation of any standard in any areas; 

o Increase the frequency or severity of any existing violation of any standard in any 

area; or,

o Delay timely attainment of any standard or any required interim emissions 

reductions or other milestones in any area.

In choosing which option to use for regional emissions analysis beyond the attainment date or end 

date of the maintenance plan, the state DOT must use the interagency consultation process 

established in accordance with the conformity rule. 

Exhibit 4 provides an example of how an isolated rural area may conduct a regional emissions 

analysis using different tests for different analysis years.

Exhibit 4: Example of Regional Emissions Analysis – Ozone Maintenance Area

Situation:

An isolated rural area has been redesignated to maintenance from its original classification as a 

marginal ozone nonattainment area. The rural area has a maintenance plan for ozone, with an 

end year of 2007. The maintenance plan includes an emissions budget for NOX and VOC 

emissions for 2007. A non-exempt project has been included in the STIP within the maintenance 

area, requiring a conformity determination in 2003.

Regional Emissions Analysis Years:

A regional emissions analysis must be conducted for the following years:

 2007 – end year of maintenance plan; 

 2015 – interim analysis year

 2023 – analysis year 20 years in the future

Regional Emissions Analysis Tests:

For the 2007 analysis, the budget test is required because there is an adequate budget.

For the 2015 and 2023 analyses, a choice of tests can be made:

 Budget test using 2007 budget, OR

 Build/no build test OR no-greater-than-1990 test, OR

 Air dispersion or other air quality modeling used in the attainment demonstration or 

maintenance plan and agreed to through the interagency consultation process. 
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Regional Analysis Without a Travel Demand Model

If there is no network-based travel demand model in the isolated rural area, estimates of vehicle 

miles traveled (VMT) based on FHWA’s Highway Performance Monitoring System (HPMS) or 

locally-approved traffic counts will need to be developed for vehicle travel in the area. These data 

can then be used to generate regional emissions estimates for each analysis year, which in turn will 

be used to perform the required motor vehicle emissions budget or emissions reduction tests.

Project Level (Hot Spot) Emissions Analysis

Project level analysis is required in CO and PM-10 nonattainment and maintenance areas.

In rural CO nonattainment or maintenance areas, projects that require Federal approval or funding 

must also meet the CO hot spot tests as required under the conformity rule (40 CFR 93.116 and 

93.123). In CO nonattainment areas, it must be shown that the project will improve air quality and 

in CO maintenance areas, it must be shown that the project will not create new violations. For CO 

nonattainment areas, quantitative analysis is required for certain projects such as those that are 

listed in the SIP as having the worst level of service (LOS) in the nonattainment area. For all other 

projects, a qualitative analysis is required.

For PM-10 nonattainment areas, qualitative analysis is required to ensure that new PM-10 hotspots 

are not created and that projects will not delay progress toward attainment; in PM-10 maintenance 

areas, qualitative analysis is required to show that the project will not create new violations. The 

projects to be analyzed and analysis tools to be used are subject to agreement through the 

interagency consultation process.17

General Requirements

As with metropolitan areas, some general requirements apply for all conformity analyses. Some of 

the key requirements are highlighted below.

Latest Planning Assumptions

As part of the regional emissions analysis, the latest planning assumptions requirements apply. 

This means that the latest demographic information (e.g., population, employment, land use) and 

travel information (e.g., vehicle classification, age and turnover of fleet, vehicle miles traveled) 

must be used in the regional emissions analysis.

Latest Emissions Model

The latest EPA-approved emissions model18 must be used in estimating emissions in isolated rural 

nonattainment and maintenance areas. In isolated rural areas, the model is ordinarily run by either 

the state DOT or the state environmental agency because isolated rural areas rarely have the 

resources to conduct emissions analysis. 

Interagency Consultation

The interagency consultation process that applies to metropolitan areas also applies to isolated rural 

areas. The state DOT or rural area agency should initiate the process and ensure that key 

assumptions are agreed to. In isolated rural areas, the lead is likely to be taken by the state DOT 

 

17 FHWA’s PM10 qualitative analysis guidance can be found at: 

http://www.fhwa.dot.gov/environment/conformity/hspotmem.htm

18 The current EPA-approved emissions model is MOBILE6 in all states except California.
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but interagency consultation also includes the state environmental agency, local public works 

departments, local transit agencies, and other local agencies as appropriate.

Timely Implementation of TCMs 

In all cases, projects that are not from a plan or TIP (such as those projects implemented in isolated 

rural areas) must not interfere with the implementation of any transportation control measures in a 

State Implementation Plan (SIP). 
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3 STATE OF THE PRACTICE

This section summarizes experiences with transportation conformity in isolated rural areas, drawn 

from interviews with the following seven states: Colorado, Kentucky, Maine, Oregon, Ohio, 

Pennsylvania, and South Carolina. Given the small sample of states, the results cannot necessarily 

be extrapolated to the nation as a whole. However, the findings do provide an indication of the 

ways in which conformity analysis has been undertaken in isolated rural areas, the types of issues 

encountered, and the perceived ramifications for transportation and air quality. This section also 

reports on what types of actions states are taking to prepare for new nonattainment areas that will 

be designated under the eight-hour ozone and fine particulate matter standards. This information, in 

turn, is designed to help practitioners better understand the current state of the practice in isolated 

rural areas and actions that can be taken to prepare areas for conducting transportation conformity 

analyses.

The key lessons learned from these experiences and recommendations are summarized in Section 4 

of this report.

This section is divided into the five sub-sections, which address the following questions: 

1. Frequency of Conformity Analysis

 How often were conformity analyses conducted?

2. Role of State DOT and Local Agencies

 Who was involved?

3. Outcomes 

 How many areas had a problem meeting conformity?

 What types of effects has conformity had on transportation decisions?

4. Issues Encountered in the Conformity Process

 What types of issues or challenges arose, and how were these resolved?

 How has interagency consultation worked?

 What data and tools were used for emissions analysis?

5. Preparation for New Standards

 How are the new eight-hour ozone and fine particulate matter standards expected to 

affect the number of isolated rural areas subject to transportation conformity? 

 And what is being done to prepare for the new standards?

3.1 FREQUENCY OF CONFORMITY ANALYSIS 

Isolated rural areas are required to conduct conformity 

analyses only when there is a new non-exempt project or if 

there are significant changes to the scope or timing of a nonexempt project. Many of the areas interviewed for this 

research stated that there have been few projects in isolated 

rural areas, and consequently few conformity analyses have been conducted. 

Exhibit 5 summarizes the isolated rural areas and conformity analyses that have been conducted for 

the seven states interviewed for this study. 

Conformity analyses are 

conducted infrequently in 

most isolated rural areas.
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Exhibit 5: Conformity Analyses Conducted 

State Number, Location, and Current Designation of 

Isolated Rural Areas:

Conformity Analyses Conducted

Colorado Six (6) areas:

 Aspen - PM-10 maintenance 

 Canon City – PM-10 maintenance 

 Lamar – PM-10 maintenance

 Pagosa Springs – PM-10 maintenance 

 Steamboat Springs – PM-10 maintenance

 Telluride – PM-10 maintenance 

Only one conformity analysis has ever 

been conducted - Entrance to Aspen 

project on State Highway 82 (1995/96).

Kentucky Two (2) areas:

 Paducah (Marshall Co. and Part of Livingston 

Co.)– ozone maintenance

 Edmonson County – ozone maintenance

Conformity analyses have been 

conducted in both of the isolated rural 

areas in the state within the past five 

years: Edmonson in 1999; Paducah 

failed to determine conformity in 1998, 

but eventually was able to make a 

conformity determination in 2002 after 

revising its motor vehicle emissions 

budget in the SIP. 

Maine Three (3) areas:

 Hancock and Waldo Co. – ozone maintenance

 Knox and Lincoln Co. – moderate for ozone 

 Presque Isle – PM-10 maintenance (exempt 

from regional emissions analysis requirements 

because mobile sources are insignificant)

No significant projects requiring a 

conformity determination have been 

initiated within past five years but rural 

conformity analyses are conducted on a 

routine basis as part of the STIP cycle. 

Ohio Four (4) areas:

 Clinton Co. –ozone maintenance

 Columbiana Co. –ozone maintenance

 Jefferson Co. – PM-10 maintenance (exempt 

from regional emissions analysis requirements 

because mobile sources are insignificant)

 Preble Co. – ozone maintenance

Conformity analyses have been 

conducted in Clinton (about 

1999/2000) and Columbiana (not 

within past five years). 

Oregon Five (5) areas:

 LeGrande (Union Co.) – moderate PM-10 

 Lakeview (Lake Co.) – moderate for PM-10

 Oakridge (Lane Co.) – moderate for PM-10 

(exempt from regional emissions analysis 

requirements because mobile sources are 

insignificant)

 Klamath Falls (Klamath Co.) – maintenance 

for CO, moderate for PM-10 (EPA 

redesignation and maintenance plan approval 

pending)

 Grants Pass (Josephine Co.) – maintenance for 

CO, moderate for PM-10 (EPA redesignation 

and maintenance plan approval pending) 

Conformity analyses have been 

conducted in Lakeview, Klamath Falls, 

and Grants Pass. 
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State Number, Location, and Current Designation of 

Isolated Rural Areas:

Conformity Analyses Conducted

PennsylvaniaEleven (11) areas:

 Crawford Co. – ozone incomplete data 

 Franklin Co. – ozone incomplete data 

 Juniata Co. – ozone incomplete data 

 Lawrence Co – ozone incomplete data 

 Northumberland Co. – ozone incomplete data 

 Pike Co. - ozone incomplete data 

 Schuylkill Co. – ozone incomplete 

 Snyder Co. – ozone incomplete data 

 Susquehanna Co – ozone incomplete data

 Warren Co. – ozone incomplete data 

 Wayne Co. – ozone incomplete data 

Rural conformity analyses are 

conducted on a routine basis as part of 

the STIP cycle. In generally, isolated 

rural areas each had between two and 

five projects over of the past 4 years.

South 

Carolina

One (1) area:

 Cherokee Co.– ozone maintenance

One conformity determination has been 

made. Regional conformity analysis 

included four projects in Cherokee 

County, generally associated with 

widening I-85 and approach routes.

As shown in the table above, the states we interviewed each contain from one to six isolated rural 

non-attainment areas, with the exception of Pennsylvania, which has eleven ozone areas, classified 

as incomplete data areas. Most states have had only a handful of regionally significant non-exempt 

projects requiring conformity determinations. For example, within the six isolated rural PM-10 

areas in Colorado, only one conformity analysis has ever been conducted. Within the four isolated 

rural areas (three for ozone, one for PM-10) in Ohio, only one conformity analysis has been 

conducted within the past five years. In Oregon, conformity analyses have been conducted in three 

of five isolated rural areas. PennDOT staff estimated that there were an average of two to five nonexempt projects in each area over the past four years. This somewhat higher frequency of nonexempt projects may be due to the fact that each of Pennsylvania’s isolated rural maintenance and 

non-attainment areas encompasses an entire county, while other states isolated rural areas tend to 

be smaller.

Two of the sample states – Pennsylvania and Maine – conduct conformity analyses for areas on a 

routine basis as part of their STIP cycle, despite the fact that conformity analyses in isolated rural 

areas are only required when there are regionally significant non-exempt projects. In the case of 

Pennsylvania, the state DOT indicated that even where no new significant projects have been 

added, other changes might cause emissions calculations to change (e.g., fleet mix, registration 

data, etc.). The state DOT chooses to conduct conformity analyses to incorporate such new data 

even when no new projects are planned for a given area; however, the conformity regulation does 

not require a new analysis and determination in these cases. In the case of Maine, the state DOT 

staff indicated that they believed analyses were required at least every three years, and so went 

through the process of conducting an analysis regularly. 

Some states such as Oregon and Pennsylvania expect even fewer rural conformity determinations 

in the future. This is partly because these states expect to continue current trends that have shifted 

funds toward system preservation and safety enhancement activities and away from activities 

involving new infrastructure development and capacity expansion. This shift may be most 

significant in slower growth states, but in some cases it represents a more general shift in 

investment priorities. More immediately, over the next several years many states foresee budget 

shortfalls that may preclude regionally significant transportation projects in rural areas. 
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3.2 ROLE OF STATE DOT AND LOCAL AGENCIES

Within all the states we interviewed, the State DOT played the lead role in conducting conformity 

analyses for isolated rural areas.19 In some cases, 

local agencies20 did not play any substantive role in 

the conformity process. In none of the cases did 

local agencies conduct the technical work for the 

regional emissions analysis. They also typically did 

not participate in interagency consultation activities, and in some cases probably had no idea that 

conformity analyses were being conducted, according to state DOT staff. 

Of the seven states, Colorado, Oregon, and Pennsylvania indicated that local governments played a 

role in the conformity process. The level of involvement and role in the process differed:

 In Colorado, the local governments in the City of Aspen / Pitkin County worked with 

Colorado DOT and the Colorado Air Pollution Control Division to provide input on control 

measures as part of a conformity analysis in the Aspen area. While the state agencies work 

with local agencies throughout the Colorado, the Aspen area is known for a high level of local 

involvement, and works more closely with CDOT than most rural communities in the state. It 

is also somewhat unique in terms of air pollution controls, since paid parking in Aspen is a 

legally enforceable control measure in the SIP. 

 Oregon had a relatively high degree of local involvement in Grants Pass, Lakeview, and 

Klamath Falls. In Oregon, a number of local and regional air pollution organizations had been 

formed to deal with air quality concerns. Oregon DOT involved these organizations in the 

conformity process. Since local residents trusted these organizations, these groups turned out 

to be a valuable means of communicating with public constituencies.

 Pennsylvania involves Local Development Districts (LDDs) in the conformity meetings at the 

early stages to discuss schedules and ensure that all regionally significant projects have been 

identified. Once the data and report are assembled, they are sent to the LDD approval body. 

The approved report comes back to PennDOT, which assembles all the pieces for the 

statewide conformity document. Each LDD has a technical committee and an approval body 

that endorses the report.21

 

19 Although donut areas were not the focus of our study, we also found that State DOTs played a lead role in 

conformity analyses in rural donut areas in North Carolina, Ohio, Kentucky, and Pennsylvania. This was 

somewhat surprising since the conformity requirements stipulate that regionally significant projects in donut 

areas must be incorporated into the MPO’s regional emissions analysis. In these cases, through the 

interagency consultation process, it was agreed that the State DOT would analyze emissions associated with 

the projects outside the MPO boundary and combine these results with the MPO analysis. 

20 A local agency, in this context, refers to any government agency whose jurisdiction is below the state level. 

This is not intended to include local branches of state agencies such as DOT district or regional offices.

21 Pennsylvania has six regional planning and development organizations in rural areas. These are known as 

local development districts (LDDs). LDDs were initiated through the state DOT’s rural transportation 

planning program in FY 1992-93 to complement the ongoing metropolitan activities. Each LDD has a 

technical committee that reviews conformity plans, and an approval body that receives a recommendation 

from the technical committee. LDDs also assist the state DOT with long-range planning, Congestion 

Management System planning, and collection of Highway Performance Monitoring System data, and assist 

in advancing projects through needs assessments, environmental studies and public involvement.

State DOTs tend to play the lead in 

conducting conformity analyses in 

isolated rural areas; local agencies 

have played a role in only some areas.
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There are a number of reasons why state DOT staff members play the key role in conformity for 

rural areas. Most state DOT staff indicated that local transportation agencies and governments have 

limited understanding of the conformity requirements and lack the technical expertise to undertake 

such analysis. Moreover, most projects in isolated rural areas are state highway projects; as such, 

the state DOT includes the projects in the STIP and conducts environmental assessments. 

Moreover, the state DOT is in a better position to handle interagency consultation involving the 

state air agency, FHWA, FTA, and EPA. 

Many state DOTs felt that local officials typically only take an interest in conformity at the point 

where a desired project is threatened. In two specific cases where local agencies and officials 

actually were involved, it occurred as a result of project delays. In Oregon’s Grants Pass area, local 

officials became involved because problems meeting conformity caused delay for a downtown 

street improvement project. In Paducah, Kentucky, local officials involved themselves once they 

learned that a highway project was stopped because of challenges in satisfying conformity 

requirements.

3.3 OUTCOMES 

Meeting Conformity: Problems Encountered and Solutions 

The seven states interviewed had a wide 

range of experience in terms of ease of 

meeting conformity requirements. Many of 

the states interviewed expressed that they 

had not encountered any significant 

problems in meeting the emissions budget 

or emissions reduction tests in isolated 

rural areas. However, there have been a 

number of cases where a state was not able 

to initially demonstrate conformity for a project, causing project delays. Moreover, there were 

procedural difficulties, stemming from limited experience with conformity, which hindered the 

ease of demonstrating conformity in some cases. 

Exhibit 6 provides a summary of outcomes of conformity analyses that have been conducted.

In many isolated rural areas, meeting 

conformity has not been a problem, largely 

due to federal or state emission reduction 

measures. In several cases, however, states 

faced project delays and procedural 

difficulties; many of these related to lack of 

familiarity with conformity requirements. 
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Exhibit 6: Outcomes of Conformity Analyses

State Conformity Analyses 

Conducted

Outcomes

Colorado Aspen - Entrance to Aspen project 

on State Highway 82. 

Initial project was not able to meet build-no build 

test. Conformity process influenced design of 

project: added busway/light-rail component. 

[However, project is on hold due to other issues].

Kentucky Edmonson 

Paducah 

Paducah – failed to determine conformity in 1998. 

Eventually was able to make a conformity 

determination in 2002 after revising its motor 

vehicle emissions budget in the SIP.

Maine Rural conformity analyses are 

conducted on a routine basis as 

part of the STIP cycle. 

No significant projects. Problem has been 

identifying projects to conduct build/no build test.

Ohio Clinton 

Columbiana 

No problems meeting conformity have been 

encountered.

Oregon Lakeview

Klamath Falls

Grants Pass

Grants Pass – Experienced difficulty in 

demonstrating conformity; however, the project 

was delayed long enough that fleet turnover, as 

reflected in an upgraded MOBILE model, allowed 

the area to demonstrate conformity.22

PennsylvaniaConformity analyses are 

conducted for the eleven isolated 

rural counties during each twoyear STIP cycle. There have been 

two to five projects in isolated 

rural areas in over the past two 

STIP cycles. 

No projects have been delayed, but project 

components have been modified in order to 

demonstrate conformity. Conformity demonstration 

was challenging with the build/no-build test but 

there have been no problems meeting conformity 

using the “less than 1990” test. 

South 

Carolina

Cherokee County Conformity analysis for project was initiated in 

1995 but only completed in 2000, due to a variety 

of technical and procedural issues, not issues 

associated with meeting the emissions budget. 

To a large degree, state DOT staff indicated that federal or state actions have been successful in 

reducing emissions. These actions have enabled areas to demonstrate conformity, particularly in 

terms of meeting the “less than 1990” test. For example, in Pennsylvania, where isolated rural areas 

do not have an emissions budget (as ozone incomplete data areas), the state has been able to show 

conformity using the “less than 1990” test. In the future, new federal requirements are expected to 

help many isolated rural areas reduce emissions further.23

In states in the sample with emissions budgets, there were generally few problems meeting the 

emissions budget limits. For example, Ohio and South Carolina indicated that they have never had 

problems staying within their emissions budgets. 

 

22 Note the approved emissions models predict the fleet mix for future years, so although the vehicle fleet 

might be modernizing while a project is delayed; this would not generally affect future year emissions 

projections. However, the project delay and concurrent fleet modernization could help when an area is not 

meeting the budget for the current year.

23 A South Carolina assessment of rural communities, for example, found that Tier II and low sulfur fuel 

standards are likely to eliminate many ozone and PM violations in the longer-term (sometime after 2007). 

The greatest concern is the near-term during which increasing VMT and the new ozone and PM-2.5 

standards will create new violations before Tier II and low sulfur fuel standards have their full, mitigating 

effect.
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Only one state in the sample – Kentucky - indicated that it had faced a problem meeting conformity 

that was severe enough to require a revision to the SIP emissions budget. In Paducah, Kentucky, a 

project on U.S. Route 68 was included in the STIP, but was not able to meet the emissions budget 

test. The emissions budget for the isolated rural area did not allow for travel growth over time, and 

the Kentucky Transportation Cabinet (KYTC) could identify no feasible means to meet the test, 

despite several attempts, including developing vehicle speeds for roadway classifications and 

investigating whether the hot and cold start fractions were appropriate. KYTC made the case to the 

Kentucky Division for Air Quality (DAQ) that revising the SIP motor vehicle emissions budget 

was necessary because the inventory on which the budget was developed was outdated and had 

underestimated future growth. The process of convincing the DAQ of the need for a SIP revision 

was difficult, as the DAQ was hesitant to undergo the potentially complex process involving 

reexamination of emissions contributions from all sources. Ultimately, the SIP was revised to 

increase the emissions budget. However, the affected segment of the project was delayed from 

early 2000 until the resolution of the conformity issue in late spring 2002.

In two other cases, states initially had a problem meeting conformity that caused project delay, but 

these were resolved without requiring a revision of the SIP. In Oregon, a highway project in the 

Grants Pass carbon monoxide nonattainment area was not able to meet conformity initially, and the 

project was delayed as a result. However, the area was later able to meet conformity without any 

project changes due in part to vehicle fleet turnover, which resulted in lower emissions for the 

current year. In addition, new modeling techniques that were implemented in the transition from 

MOBILE4 to MOBILE5 lead to lower emissions estimates and helped to meet conformity. In 

Colorado, the Entrance to Aspen project on State Highway 82 did not meet conformity with the 

initial project design. However, by adding a busway/light rail component, the state was able to 

demonstrate conformity.

In addition to these challenges satisfying regional emissions analysis tests, several states 

experienced problems meeting conformity due to procedural issues. Many of these problems 

stemmed from lack of familiarity with the conformity process in isolated rural areas, and the 

“learning curve” involved in understanding conformity requirements. For instance: 

 In Maine, it appears that there have been misunderstandings in regard to the frequency 

requirements for conformity analysis in isolated rural areas. The DOT goes through the 

conformity process for all nonattainment areas, including isolated rural areas, every two years 

when they update the STIP. Because it has isolated rural areas that are moderate for ozone, but 

with no emissions budget, these areas must meet BOTH the less-than-1990 test AND the 

build/no-build test. Maine DOT staff indicated that they have no problems meeting the lessthan-1990 test, but have had problems meeting the build/no-build test when there are no 

projects in an isolated rural area. As a result, the DOT has tried to identify projects for these 

isolated rural areas, even when no project would otherwise have been planned, in order to pass 

the build/no build test. It appears that this problem would not exist if conformity analyses were 

only conducted when there are new non-exempt projects. 

 In Pennsylvania, PennDOT had problems when it began conducting conformity analyses using 

the build/no-build test in isolated rural areas. The “less than 1990” test is easier to meet 

because Federal and state emissions reduction measures that have reduced motor vehicle 

emissions. After it was recognized that the isolated rural areas could use either test, due to their 

status as ozone incomplete data areas (without a budget), the state has used the “less than 

1990” test, and has experienced no problems meeting conformity in isolated rural areas.

 In South Carolina, it took about a five-year timeframe, 1995 to 2000, in order to demonstrate 

conformity for a project to widen I-85 in Cherokee County, largely due to limited information 
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on conformity requirements and procedural issues involving the state air agency. The DOT 

submitted an EIS for the project in 1998, and was informed by the FHWA regional office that a 

conformity analysis needed to be completed before the EIS could be accepted. The conformity 

process was basically unknown by staff at the state level – both within the state DOT and air 

agency. Cherokee County is the only nonattainment/maintenance area in the state, so a 

conformity analysis had never been previously conducted, and there was a substantial learning 

curve in terms of addressing all of the conformity requirements. First, the SIP had to be revised 

to incorporate updated mobile emissions factors. And the mobile source budget had to be 

increased. While staff succeeded in demonstrating that the isolated rural area would be under 

the new emissions budget, the SIP still needed to be updated since it had been developed 

initially using an earlier version of the MOBILE model. There were also some issues 

associated with different interpretations of the regulations by the EPA regional office and 

FHWA, which caused delay. 

3.4 ISSUES ENCOUNTERED IN THE CONFORMITY PROCESS 

Time and Resources Involved in Comparison to Effects on Air Quality 

Although state DOT staff recognized the 

intent of the conformity process, a number 

of state DOTs expressed concern that the 

conformity process in isolated rural areas 

can take significant time and resources, 

without producing meaningful air quality 

benefits.

In many isolated rural ozone areas, 

transportation sources play a small role in 

contributing to air quality problems. For 

example, in Paducah, KY, and Cherokee 

County, SC much of the ozone problem was due to transport from other areas. Some state DOT 

staff questioned whether isolated rural areas had significantly different air quality from surrounding 

areas. In one case, the DOT staff felt that their designation as a nonattainment area, which prompts 

conformity requirements, had more to do with the presence of an air quality monitor than anything 

else. In another case, the DOT explained that an area reached attainment because of three 

consecutive years of good weather.

Moreover, the amount of time invested in rural conformity processes can be very large relative to 

the small amount of mobile source emissions associated with the project. In the case of Paducah, 

the U.S. 68 project, which was delayed as a result of conformity problems, involved a 15- to 17-

mile section of highway, including a replacement bridge over Kentucky Lake; only about 2,000 

feet of the project was located within the ozone maintenance area, but required considerable effort 

and resources to address. As noted earlier, in Grants Pass, Oregon, a project was delayed because it 

was not able to meet conformity initially; however, the area was later able to demonstrate 

conformity with exactly the same project because of vehicle fleet turnover and changes in the 

methodology applied within the emissions models. After a period of delay, the analysis found that 

the project no longer had a meaningful effect on air quality. Due to these issues, a number of 

agencies felt that rural conformity requirements should provide greater flexibility to address 

situations where required staff time and resources would be disproportionate to the air quality 

benefits possible through mobile source reductions.

State DOT staff expressed concern that 

conformity in isolated rural areas takes a 

disproportionate amount of time and 

resources compared to its actual effects on air 

quality. On the other hand, the transportation 

conformity process has motivated increased 

coordination between air agencies and DOTs 

and has fostered an awareness of the air 

quality effects of different types of projects.
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On the other hand, most state DOTs and air agencies felt that the transportation conformity process 

was an important motivator to increase interaction and coordination between the air agencies and 

the DOTs. Such interactions almost never took place before conformity was required. In this 

regard, the process has been a success. Pennsylvania DOT, for example, noted that the rural 

conformity process has had an effect on the mix of projects they select. There is now a greater 

focus on projects that improve traffic safety in rural areas, and there is less likelihood of new 

“megaprojects” that can seriously impact a rural area’s ability to achieve conformity. Conformity 

has also increased attention to signal coordination in rural areas as a mitigation strategy. Within a 

number of states, DOT staff expressed that air quality concerns are rarely a priority for public 

officials in rural areas and it is challenging to persuade public officials to take these concerns 

seriously at an early stage in the planning process. Therefore, by raising the possibility of stopping 

projects, the conformity process forces a serious consideration of these issues.

In some cases, experience with conformity in isolated rural areas has also informed efforts to 

reduce emissions in other parts of the state. For example, high PM-10 concentrations registered in 

Triple Creek, Colorado threatened to bring this area into nonattainment. Based on Colorado’s 

experience with nonattainment and maintenance areas, Colorado DOT (CDOT) examined roadsanding practices, which had been identified previously as a key problem in rural PM-10 

nonattainment areas. A review of road sanding practices found that between CDOT and local road 

crews, the roads were being significantly over-sanded. As a result, the state and local agencies 

coordinated to reduce sanding. CDOT’s six engineering regions each have regional environmental 

managers and maintenance superintendents that are now aware of potential PM-10 problems and 

are cooperative in the effort to manage these impacts. 

Feasibility of Reducing On-Road Emissions in Isolated Rural Areas 

Transportation agencies in some states expressed 

concern about the feasibility of reducing emissions 

from motor vehicles in isolated rural areas. 

Traditional TCMs to reduce the demand for vehicle 

travel have limited application in most rural areas. 

Population densities are not high enough to support 

transit, ridesharing, bicycling, or other demand 

management measures. As a result, options available 

to state DOTs to reduce emissions are severely constrained. In Paducah, Kentucky, for example, 

state DOT staff felt that there were few viable opportunities to reduce emissions of ozone 

precursors by an amount sufficient to meet the emissions budget.

On the other hand, there were some cases in which isolated rural areas had opportunities to reduce 

on-road emissions. For example Aspen, Colorado was able to demonstrate reduced emissions by 

adding a transit component. Although Aspen, Colorado has only about 6,000 residents, the 

population expands dramatically when all the workers who live down valley come into town and 

tourists add to traffic volumes. The plan to widen State Route 82 into Aspen would not meet 

conformity solely as a road widening project. However, the inclusion of a busway or light-rail 

component reduced the predicted VMT and mitigated community concerns about traffic. With the 

redesign, the project was able to meet conformity. 

In the case of isolated rural PM-10 areas, road dust is often a major source of emissions, and there 

are opportunities to reduce fugitive dust from rural transportation including paving commonly used 

dirt roads and switching from sand to chemical road de-icers. In Colorado and Oregon, where much 

In isolated rural ozone areas, there 

are limited options for reducing 

project-related emissions. 

However, there are some 

opportunities, particularly in PM10 areas and resort areas. 
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of the PM-10 problem was due to road dust, DOTs have successfully reduced emissions using 

these strategies. These experiences suggest that there are cases – such as in resort-type areas and 

PM-10 areas – where there are reasonable opportunities to reduce motor vehicle emissions.

Role of Interagency Consultation 

The interagency consultation process is the 

formal coordinating mechanism among 

transportation and air agency staffs. Although 

some states have experienced problems in the 

past, interview subjects generally felt that their 

procedures for interagency consultation were 

working effectively, and emphasized the importance of coordination. In many cases, states 

characterized their coordination with the air agency as being very effective. Several practitioners, 

however, noted that establishing an interagency consultation agreement takes significant time, 

preparation, development of professional trust, and legal expertise, and so significant time should 

be allotted for this aspect of the process. 

The actual mechanism of the interagency consultation process, in terms of number of meetings, 

appeared to vary considerably among states. Some states found it valuable to hold regular meetings 

between the State DOT, the State air agency, EPA, FHWA, and FTA while others felt that this 

exchange worked best if meetings were held on an as-needed basis. In most cases, states hold an 

initial kick-off meeting when a conformity cycle is beginning in order to discuss schedules, data, 

methodology, and subsequent communications protocol. Rural areas are discussed during these 

meetings as needed. 

Data and Technical Analysis Methods

In isolated rural areas, network-based travel 

demand modeling is not generally available, 

nor is it required by the transportation 

conformity regulation. Rural areas typically 

rely on the Highway Performance Monitoring 

System (HPMS) for VMT estimates, and 

MOBILE defaults for fleet mix and other 

factors, since these are often the only sources 

of data available for use in regional emissions 

analysis. A number of states, however, noted that HPMS and the MOBILE defaults have 

significant limitations for analyzing emissions in rural areas. 

The interviewees who expressed the most concern about the quality of HPMS data generally were 

those that had experienced some problems meeting conformity. These areas in particular had 

explored a number of improvements to better estimate VMT and speed at a local level.

Limitations of HPMS for VMT Projections

Several states noted the inadequacy of HPMS data for county level VMT forecasts. HPMS was not 

designed for use in county-level analysis. Rather, it was designed as a statewide tool and is used 

nationally as an indicator of road use and performance. As a result, VMT projections may be very 

inaccurate for a rural county. In the case of Paducah, KY, motor vehicle emissions budgets that 

were developed based on HPMS data significantly underestimated VMT growth, and updated 

The interagency consultation process 

appears to be working effectively, but in 

many cases took significant time and 

preparation to develop the process.

Simple sketch planning methods, using 

HPMS data and MOBILE default values, 

are commonly used for conformity analyses 

in rural areas. However, there are 

significant limitations to these sources, and a 

number of states have developed more 

refined methods, partly in response to 

potential conformity problems. 
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VMT estimates showed substantially higher VMT than assumed in the baseline. In response, 

Kentucky has undertaken a number of modeling improvements to better estimate VMT at the 

county level. Maine, Pennsylvania, and Ohio also have developed or maintain road management 

systems that provide detail beyond what is available from HPMS. 

Limitations of MOBILE Defaults for Rural Areas

Several states noted that defaults used within the MOBILE model (used to generate emissions 

factors) do not always accurately reflect “real-world” conditions in rural areas. In particular, the 

defaults for the share of VMT on different types of roadways within the MOBILE model can be 

inaccurate for rural areas. For example, according to staff at the Kentucky Transportation Cabinet, 

the MOBILE default values indicate that 8% of interstate VMT is on ramps, which is generally 

accurate for urban areas. However, in Kentucky’s rural areas, the ramps carry only about 1.5% of 

VMT. Several states also explained that vehicle mix default data used in MOBILE are often 

inaccurate for rural areas, and that in fact, the vehicle mix can vary substantially between rural 

areas within a single state. In response, a number of states have undertaken efforts to develop 

improved vehicle mix data for rural areas, which is particularly important if heavy-duty vehicles 

comprise a large portion of the vehicle travel.

The types of methodologies that states have used are described further in Section 4 on Lessons 

Learned.

3.5 PREPARATIONS FOR NEW STANDARDS

States are at different stages with regard to their preparation for the 8-hour ozone and PM-2.5 

standards. This situation was evolving during the course of this research project. At the time that 

interviews for this report were conducted many state DOTs were still uncertain about the number 

of isolated rural non-attainment areas that might be designated under new 8-hour ozone and PM2.5 standards. Some of this uncertainty was associated with how specific nonattainment 

boundaries would be drawn relative to new MPO boundaries. Several states such as Kentucky, 

South Carolina, and North Carolina are taking measures to prepare for new standards, by training 

staff on new models, educating rural officials about conformity and its potential impacts, and in 

some cases establishing early action compacts. Other states are waiting for requirements and 

designations to be complete before investing in staff preparations and/or education campaigns. 

Some of the preparatory actions being undertaken by sample states are described below:

Educating the Public and Local Officials about New Standards

The Kentucky Transportation Cabinet implemented a “Road Show” awareness tour for the 8-hour 

ozone standard. The purpose was to meet with a wide range of local officials to discuss the 

forthcoming changes, the benefits of good air quality, and the importance of being involved in the 

process. The state expects to see the rewards of this education effort when new areas are 

designated nonattainment under the 8-hour standard. The purpose is to familiarize local officials 

with the air quality requirements so they will better understand the implications of not meeting 

conformity and focus more on cooperatively working through the necessary steps, rather than being 

frustrated with the process.

Establishing New Interagency Agreements 

South Carolina has been particularly aggressive in pulling together various counties and state 

agencies for a Memorandum of Agreement (MOA) that defines agency relationships for the 

purpose of conformity. The concept is to include all MPOs, all potentially violating counties, and 

all pollutants within an MOA, in order to prevent delay if it is determined that some additional 
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region or pollutant must later be included in the conformity analysis. Getting buy-in from the 

necessary authorities and preparing the legal documents for this process can take longer than 

expected, and these MOAs are particularly important for rural areas where it may be difficult to 

predict where new violations occur.

Regional Information Sharing

Several states have increased their involvement with regional organizations that function to share 

professional practices. In particular, state agencies mentioned Northeast States for Coordinated Air 

Use Management (NESCAUM), the Midwest Regional Planning Organization, and the Lake 

Michigan Air Directors Consortium (LADCO). These organizations include experienced modelers 

and emissions experts that are able to assist one another when challenging issues arise. The state of 

Ohio specifically attributes its readiness to work with new emissions models and conformity 

requirements to the support provided by such organizations. 

Early Action Compacts

A number of states have entered into Early Action Compacts (EACs), which were designed to 

defer nonattainment designations under the 8-hour ozone standard in response to early actions to 

reduce emissions. Areas that have Early Action Compacts will not be required to conduct 

transportation conformity analyses under the 8-hour ozone standard so long as they continue to 

meet the required milestones. Nevertheless, several states are currently considering some variation 

of the conformity process in Early Action Compact areas, calling it, “conformity without the red 

tape.” This could have a significant impact on the EAC areas because one of the principal reasons 

some areas agreed to EACs was to get relief from conformity requirements.24

Efforts to Minimize Attainment Area Boundaries 

Several states are working with the U.S. EPA in an effort to minimize the size of new ozone nonattainment areas. Pennsylvania and Maine have been particularly focused on this goal. In many 

cases, an entire county has been designated as an isolated rural area, when monitored data and 

travel conditions imply that only a portion of the county has an air quality problem. Having 

countywide designations in these conditions may create a situation where conformity is required 

for projects that may have little or no relationship to the area where standards are being violated. 

This is largely the case in nonattainment areas that receive transported pollution from upwind 

areas. Nevertheless, because of the regional nature of ozone, some argue for larger ozone 

nonattainment areas. 

Air Agencies Encouraging Mobile Source Budget Restraint

Anticipating the challenges of meeting the new 8-hour ozone and PM-2.5 standards, a number of 

state air agencies have become more restrictive in providing safety buffers for mobile source 

emissions budgets. These air agencies believe that a large buffer may deter the state DOT from 

exploring the full range of options for minimizing mobile source emissions in rural areas, which 

may be necessary to meet the new standards. Where previously the state air agencies might 

consider only the emissions reduction needs from other sources when allowing for mobile source 

buffers, they now work to establish a compromise with state DOTs that accommodates anticipated 

growth in VMT, but also acknowledge potential for some mobile source reductions.

 

24 In addition, there may be questions about EPA’s legal authority to defer nonattainment designations, which 

is central to the point of EACs. This has not been litigated thus far; however, some professionals anticipate 

that there may be litigation over this matter.
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4 LESSONS LEARNED 

Based on the experience of states in conducting conformity analyses in isolated rural areas and 

preparing for potential new nonattainment areas, a few key lessons can be derived from this 

experience. This section summarizes our synthesis of these lessons for practitioners, and 

recommendations for areas that may soon be facing new isolated rural nonattainment areas.

4.1 TRANSPORTATION STAFF NEED TO BE INVOLVED IN DEVELOPMENT OF THE SIP

One of the key lessons coming out of the 

interviews in this study is the importance of 

having state DOT staff actively involved in and 

aware of SIP development activities. In most 

isolated rural areas, with the exception of PM10 areas, there are few options for reducing 

vehicle emissions in isolated rural areas. As a result, having an attainable emissions budget is 

critical to meeting conformity requirements for those rural areas that have budgets. 

The importance of the State DOT working closely with the state air agency to establish an 

emissions budget for motor vehicles that allows for traffic growth cannot be overemphasized. 

Failure to provide for adequate growth in VMT created a problem in meeting conformity in 

Paducah, KY, which ultimately required the state to undertake a time-consuming revision of the 

SIP budget. When the state DOT initially provided VMT projections to the air agency, it did not 

clearly understand how the projections would be used and the severe implications of these 

projections for the emissions budget.

Moreover, DOTs should work with air agencies to ensure that the emissions budget includes some 

safety buffer, if an emissions surplus exists, to account for changes in methodologies and data that 

may occur over time. In many rural areas, the majority of travel occurs on very few major 

facilities. A rural project subject to conformity would typically addresses capacity on one or more 

of these major facilities. Since rural areas have low population and VMT, a small underestimate in 

VMT forecasts for such facilities can lead to large percentage changes in the overall VMT for the 

area, which in turn can create a problem meeting the emissions budget. Consequently, reasonable 

safety buffers in mobile source emissions budgets should consider the potential for increases in 

VMT, which may account for a small amount of pollution but make an area unable to meet a 

budget. 

Most agencies described good coordination and flexibility on this matter. Several states – in 

particular Maine and Ohio – described cases where the DOT requested a safety buffer for mobile 

source emissions in the SIP. While the air agencies did not provide all of the requested mobile 

source budget increase, in both cases the agencies came to an informed agreement following a 

discussion about the constraints and other factors related to the SIP budget. 

4.2 BUILD TRUST BETWEEN DOT AND AIR AGENCY STAFF

Most states pointed to the importance of maintaining 

some type of consistent contact and cooperative ethic 

between transportation and air agencies. This is 

important both for resolving conformity problems as 

they arise and more importantly to prevent problems 

It is critically important for State DOTs 

to be involved in the development of 

SIPs and the motor vehicle emissions 

budget in rural areas.

It is important to build trust 

between state DOT and air 

agency staff in order to have an 

effective and collaborative 

process.
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from occurring in the first place. Across the sample states, cooperation and trust were achieved in a 

number of ways.

The exchange of professional staff between agencies is one factor that seemed to enhance 

interagency communication. In several of the sample states, practitioners had experience working 

in both the transportation and the air agency. This condition improved interagency understanding 

and cooperation. As such, agencies might consider a short-term exchange program for staff 

members that play a role in transportation conformity.

In some cases, for example, South Carolina, both the DOT and air agency have significant 

experience operating the MOBILE models. This appears to have the effect of building a common 

vocabulary and mutual understanding of analysis challenges and priorities that work in favor of 

effective interagency relations. In other cases, regular meetings among interagency staff helped to 

build relationships and facilitate good working partnerships.

Some air agency staff noted that relationships as well as project planning might be improved if air 

agencies were more involved in the project selection process. For example, staff at one air agency 

felt that they should have a larger role in the selection of CMAQ projects. This had historically 

been the case, but at some point the State DOT took complete responsibility for this task. Since 

these projects are expected to reduce emissions, it could be helpful to take into account air agency 

perspective and expertise. On the other hand, many states noted that air agencies have very limited 

resources, and that the state DOT takes on many responsibilities related to emissions modeling and 

training staff on the MOBILE model, which are within the responsibility of the air agency.

4.3 BE AWARE OF DATA LIMITATIONS AND IMPROVEMENTS TO ADDRESS THEM

Although the development of local data for factors 

such as speeds and vehicle mix data is not 

required in isolated rural areas, staff involved in 

the conformity process should be aware of the 

limitations of using HPMS data and MOBILE 

model defaults alone to generate emissions

estimates in isolated rural areas. As noted in 

Section 3, there are weaknesses to these methods. 

A number of states have developed methodologies or data systems to improve forecasting in rural 

areas, and these provide guidance on the types of activities that states can undertake to improve 

their emissions estimates. State and local agencies involved in the conformity process for isolated 

rural areas should decide through the interagency consultation process whether improvements in 

data and methodologies justify the costs of making such improvements. Several of these methods 

are discussed below.

VMT Forecasts

Since isolated rural areas typically do not have a travel demand model, improvements in VMT 

forecasts can be made by separately estimating local-VMT and through-VMT. Most of the sample 

states in this study used such a methodology. Local-VMT refers to travel by residents of the 

county, which generally increases in proportion to the population. Local-VMT is projected using 

per-capita VMT estimates along with population projections. An important factor in these 

projections is the growth rate in per capita VMT. Through-VMT refers to vehicles passing through 

the county, which generally increases in proportion to inter-county or even interstate travel. 

Staff should be aware of the limitations 

of HPMS data and MOBILE default 

values for use in isolated rural areas, and 

consider methods to develop more 

accurate emissions estimates where 

appropriate.
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In most rural counties, inter-county travel tends to increase faster than local travel, so strictly 

population based VMT growth rates would usually underestimate VMT. Where interstates are 

present in rural counties, it is wise to assess interstate VMT growth rates independently. For 

example, Kentucky found that VMT growth rates were nearly double in rural counties that had an 

Interstate compared to rural counties without an interstate. South Carolina has documented its 

methodology for VMT growth projections in rural areas, which may be useful to other practitioners 

(listed in Section 5 of this document). 

Vehicle Fleet Mix and Roadway Distributions

MOBILE model defaults may not always accurately reflect “real-world” conditions in rural areas 

and may not always assign the appropriate factors for vehicle fleet mix and VMT by roadway type. 

State DOTs may have a better source of data to develop these factors, since they should have 

access to relatively current and geographically specific vehicle mix data from the state motor 

vehicle department. With heavy-duty vehicles comprising an important portion of emissions, 

accurate identification of this component of the vehicle mix is vital. Ohio DOT makes use of their 

Statewide Economic Census Vehicle Inventory and Use Survey. This forms a critical component 

of their vehicle mix data since it presents the best source of geographic-specific heavy-duty vehicle 

information. Although most states do not have as detailed a heavy-duty vehicle inventory as Ohio, 

the national Vehicle Inventory and Use survey can be helpful where a rural county includes an 

important truck corridor. Information about the survey is available through the Census Bureau.25

Speed Estimation

Vehicle emissions predicted by MOBILE are sensitive to vehicle speed inputs.26 Consequently, 

accurate speed estimates should be a priority for regional emissions analysis. Speeds have been 

traditionally estimated based on posted speeds, or derived using methods described in the Highway 

Capacity Manual (HCM). These HCM methods use hourly traffic volumes and capacities by 

roadway functional classification.27 While such approaches provide reasonable estimates of speeds, 

speed studies could also be conducted in order to collect field data, which would be better than

published default values or posted speed limits. A number of states have undertaken efforts to 

correct generic speed categories with more extensive local field data. These efforts need not be 

resource intensive. For example, about 40 hours of staff time were required in South Carolina for 

data collection and analysis for a speed study on a sample of urban, suburban and rural highways in 

Cherokee County. Research by the South Carolina DOT found that in rural areas, improved 

estimates of free-flow mean speeds were much more important than improved estimates of 

congested speeds, and that applying a speed-flow formula to estimate “congested” speeds 

complicates the analysis with minimal change in predicted mean speed on many road 

classifications since most rural areas have limited congestion. 28 

Several resources are available to assist with speed estimation and other procedures. A good 

explanation focusing on rural applications can be found in a report titled Development of On-Road 

 

25 http://help.econ.census.gov/econhelp/vius/vius_FAQS.html

26 Sensitivity Analysis of MOBILE6 Motor Vehicle Emission Factor Model, Tianjia Tang, Mike Roberts, and 

Cecilia Ho, Federal Highway Administration Resource Center, Atlanta. 

<www.fhwa.dot.gov/resourcecenter/teamaq_pubs.htm>

27 A more complete summary of speed estimation techniques has been developed by the Kentucky 

Transportation Cabinet Office of Multimodal Planning <www.kytc.state.ky.us/Multimodal/doc/> under 

“Summary of Travel Model Speed Estimation.”

28 Vehicle Miles of Travel Projections and Speed Estimates for Rural Nonattainment and Maintenance Areas, 

John Gardner, AICP, South Carolina Department of Transportation.
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Mobile Source Emission Inventories for Rural Counties.29 Rural speed estimation is also covered 

in the National Highway Institute course titled, Estimating Regional Mobile Source Emissions.

30

Several states have had success in using equations described in NCHRP Report 387, Planning 

Techniques to Estimate Speeds and Service Volumes for Planning Applications.

31

 

Local Government Data

In some cases, data to help develop improved VMT, speed, and fleet characterization estimates 

may be available from local governments. Both Oregon and Colorado rely heavily on local 

agencies for lane-miles, speed, and VMT data. The availability of such data is probably related to 

the level of transportation planning that occurs in rural areas. In the case of Oregon, all counties 

with population above 25,000 are required to develop Transportation Systems Plans that necessitate 

collecting such data. In Colorado, Regional Planning Commissions prepare 25-year long range 

plans for all non-MPO areas. State DOTs should be aware of the extent to which rural 

transportation planning agencies may provide better data before using defaults. 

Statewide Travel Demand Forecasting Model

States may also wish to consider developing a statewide travel forecasting model. More advanced 

VMT forecasting techniques also take into account current and anticipated economic trends and 

anticipated changes in population age distribution for a range of household characteristics. Maine 

DOT incorporates these factors into their statewide travel demand forecasting model, which 

produces different VMT growth rates by county. 32 Ohio DOT is currently developing a statewide 

model that it believes will allow for more efficient and more accurate mobile source emissions 

estimates for conformity. 

4.4 BUILD INSTITUTIONAL MEMORY AND EXPERTISE

States agencies should prepare for the future by becoming 

familiar with the conformity regulations and flexibility 

available for isolated rural areas. The interviews for this 

research revealed that considerable effort was required in 

most of the seven sample states to understand how to 

undertake conformity analysis in isolated rural areas. In 

some cases, practitioners had difficulties understanding 

what types of tests needed to be conducted and when analyses needed to be conducted. Therefore, 

clearly understanding the conformity requirements, and flexibility available to isolated rural areas, 

is important, particularly in states with few existing nonattainment areas. As noted in Section 3, 

some states are actively preparing for potential new nonattainment areas by undertaking 

educational and outreach efforts and developing memoranda of agreement.

 

29 G. B. Dresser, D. G. Perkinson, Texas Transportation Institute. Development of On-Road Mobile Source 

Emission Inventories for Rural Counties. Available at 

<www.epa.gov/ttn/chief/conference/ei10/index.html#ses-6>, listed as 

30 <http://tmip.fhwa.dot.gov/clearinghouse/docs/newsletters/jan03_issue15.stm#nhi_course>

31 Dowling, R.; Kittelson, W; Zegeer, J.; Skabardonis, A.; National Cooperative Highway Research Report 

Program Report 387: Planning Techniques to Estimate Speeds and Service Volumes for Planning 

Applications; Transportation Research Board, 1997.

32 Maine’s VMT, speed forecasting approaches are effectively summarized in section five of the Maine DOT 

2002 - 2004 STIP Conformity Analysis, August 2001. Available on-line at 

<http://www.maine.gov/mdot/planning/air_qual/ca_docs.htm>

It is important to build institutional 

memory and expertise in order to 

reduce the potential for increased 

effort and delay in meeting 

conformity. 
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Moreover, given that conformity analyses take place infrequently in isolated rural areas, it is 

important to have mechanisms to build institutional memory and expertise. Several State DOTs 

emphasized the importance of being meticulous in documenting interagency agreements. They 

emphasized that agreements on seemingly minor issues may turn out to be critical, and that 

everything should be in writing. This serves not only for legal cover, but also for institutional 

memory. Staff turnover can be particularly problematic because of the infrequency of rural 

conformity determinations. This became evident even in discussing projects for this research. A 

number of staff did not have conformity determination reports available, and in many cases, the 

person that worked on the document was no longer at the agency. In this way, documentation is 

therefore perhaps more important in isolated rural areas than in metropolitan areas. 

4.5 CONCLUSION

Conformity requirements are complex and significant preparation time is needed to develop the 

expertise, tools, and interagency relationships that facilitate an efficient and effective process. In 

most cases, the state DOT is in the best position to lead conformity analysis, rather than rural 

counties. In our review of seven states’ experience with conformity in isolated rural areas, several 

state DOTs encountered significant technical and procedural challenges in conducting these 

analyses, while others had relatively few difficulties. In general, the difficulties did not relate to 

problems in reducing emissions in nonattainment areas. They typically stemmed from limited 

familiarity with the conformity requirements and limited coordination between transportation and 

air agencies. 

States agreed that good relationships between state DOTs, state air agencies, and federal partners 

are paramount to a successful process. In most cases, rural areas produce small levels of 

transportation related emissions and have very few opportunities to reduce motor vehicle 

emissions. As a result, it is extremely important for state DOTs and air agencies to work together to 

develop an emissions budget in the SIP that is attainable by accounting for potential future VMT

growth and potential changes in calculation methodologies. Coordination early on in the SIP 

development process will help to avoid potential problems in the future. There are also a number of 

steps that states can take to prepare for conformity in rural areas that do not have transportation 

models, including data collection to improve estimates of VMT and speeds in rural areas, 

developing interagency agreements, and educating public officials about conformity and air quality 

issues. 
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5 RESOURCES / REFERENCES

This section includes a list of useful resources that practitioners may wish to review in order to gain 

a better understanding of conformity requirements and methodologies for regional emissions 

analysis. It also includes recent research, including case studies, on the conformity process in rural 

and metropolitan areas.

Legislation/Requirements, Guidance, and Training

Conformity Requirements: Title 40 of the Code of Federal Regulations (40 CFR Parts 51 and 93)

<http://www.access.gpo.gov/nara/cfr/waisidx_99/40cfr93_99.html>

Transportation Conformity: A Basic Guide for State and Local Officials, U.S. Federal Highway 

Administration, Revised June 19, 2000. 

<http://www.fhwa.dot.gov/environment/conformity/con_bas.htm>

Transportation Conformity Reference Guide. Federal Highway Administration (last updated July, 

2001) <http://www.fhwa.dot.gov/environment/conformity/ref_guid/index.htm> 

Clean Air Act (42 USC 7401-7671q) <http://www.epa.gov/oar/caa/contents.html>

SIP Requirements <http://www.epa.gov/oar/caa/contents.html>

National Transit Institute (NTI) Course on Introduction to Transportation/Air Quality Conformity

<http://www.ntionline.com>

National Highway Institute (NHI) Course on Estimating Regional Mobile Source Emissions

<http://www.nhi.fhwa.dot.gov>

Other FHWA/EPA Conformity Resources, including Transportation Conformity Community of 

Practice

<http://www.fhwa.dot.gov/environment/conform.htm>

Methodologies

Development of On-Road Mobile Source Emission Inventories for Rural Counties, G. B. Dresser, 

D. G. Perkinson, Texas Transportation Institute, May 2001. 

<http://www.epa.gov/ttn/chief/conference/ei10/index.html#ses-6>

Highway Speed Estimation for MOBILE6 by Bob Bostrom and Jesse Mayes, Kentucky 

Transportation Cabinet.

Planning Techniques to Estimate Speeds and Service Volumes for Planning Applications, National 

Cooperative Highway Research Report Program Report 387, Transportation Research Board, 

Dowling, R.; Kittelson, W; Zegeer, J.; Skabardonis, A, 1997.
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Vehicle Miles of Travel Projections and Speed Estimates for Rural Nonattainment and 

Maintenance Areas, John Gardner, South Carolina Department of Transportation, presented at 

Southern Transportation and Air Quality Summit, October 2001, Atlanta, Ga.

Research

Exhausting Options: Assessing SIP-Conformity Interactions, Winston Harrington, Arnold Howitt, 

Alan J. Krupnick, Jonathan Makler, Peter Nelson, and Sarah J. Siwek, Resources for the Future, 

January 2003. <http://www.fhwa.dot.gov/environment/conformity/con_res.htm>

Linking Transportation and Air Quality Planning: Implementation of the Transportation 

Conformity Regulations in 15 Nonattainment Areas. Arnold M. Howitt and Elizabeth M. Moore, 

Prepared for U.S. Environmental Protection Agency (EPA 420-R-99-011), March 1999. 

<http://www.epa.gov/otaq/transp/conform/fullrpt.pdf>

Regulating Transportation in New Nonattainment Areas Under the 8-hour Ozone Standard, 

Jonathan Makler and Arnold M. Howitt, Transportation Research Board Annual Meeting, January 

2003. <http://www.ksg.harvard.edu/taubmancenter/research/trenv.html>

Transportation/Air Quality Issues in Rural Areas, prepared by Dye Management for the Federal 

Highway Administration, June 2003.
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APPENDIX A: INTERVIEW GUIDE

Introductory Questions: 

Can you briefly summarize the situation in your state regarding existing rural nonattainment or 

maintenance areas? 

Can you summarize the changes in attainment status that you anticipate for rural areas once the 

new 8-hour ozone and PM 2.5 requirements have taken effect? 

Interview Questions for Existing Nonattainment / Maintenance Areas:

1) Describe your experience with rural transportation conformity.

 How many conformity analyses have been conducted over the past 5 years?

 What was the role of the State DOT, the regional FHWA and EPA offices, the air quality 

planning agency, rural counties, and other agencies?

 [For those who have been involved in conformity analysis] Have you experienced 

difficulties in demonstrating conformity that resulted in the area not being able to 

implement/approve a new transportation project? If yes, for how long was this project 

delayed? 

 What have state and local agencies done to resolve any transportation planning, air quality 

planning, or conformity challenges related to specific projects?

2) For particular rural isolated areas:

 What federal actions/measures (e.g., federal vehicle emissions controls; fleet turnover) are 

helping the area attain or maintain the air quality standard(s)? 

 What are the projected population and VMT growth rates in this isolated rural area?

3) Describe the technical issues involved in establishing conformity in isolated rural areas?

 How did you prepare the necessary baseline data and projections for the conformity 

analysis?

 What were the sources for VMT, speed, lane-miles, and emissions data? Which agency 

provided such information? At what scale was it available? What data was easily 

obtained, what data was difficult to obtain, and why?

 What methods are available to estimate regional emissions in rural areas?

 Do you have the ability to assess localized impacts for CO and PM? How?

 How did you define projects that were still under development well enough to determine 

emissions?

4) Describe the interagency consultation process. How did interagency coordination occur? 

Which agency took the lead in interagency consultation?

 What opportunities or processes are in place to encourage coordination between state 

transportation officials, local transportation officials, and air quality officials?

 How often did you meet with state environmental agencies, FHWA and EPA division 

staff?

5) What agencies are involved in the approval process for conformity plans in rural areas?
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6) How do you currently engage with the SIP process? 

 What was the nature of your participation in development of the last SIP? What, if any, are 

the barriers to your involvement? How did conformity change the role of the State DOT or 

local government in SIP planning?

 What information is available to decision-makers in determining the most practical 

distribution of emissions reduction targets?

 What is currently known about the sources of criteria pollutants that are responsible for 

nonattainment? Is the role of on-road mobile sources well understood? If not, are there 

efforts underway to gather this information?

7) Did the conformity process change ways that you do air quality and long-term transportation 

planning in rural areas?

8) Describe how members of the public and other stakeholders were involved in the process.

9) If you were to provide guidance to other rural areas that will need to address conformity due to 

the new ozone and PM2.5 standards, what kind of advice or recommendations would you give 

them?

Interview Questions for Expected New Nonattainment Areas:

1) What are you currently doing to prepare for nonattainment status in rural areas?

 Do you have any training in doing conformity? 

 What are your technical capabilities in transportation and emissions planning and 

modeling?

 Have you defined what the role of the state DOT, air agency, and/or local planning agency 

will have when conformity applies?

 Who will be the lead agency for project conformity determinations and analyses?

 What is the current role of the State DOT?

 What is the current role of the State air agency? 

 Have you discussed what VMT, speed, and emissions data will be used in the event that a 

project requires NEPA approval and a conformity determination?

2) Based on existing information, what are the primary sources of pollution in your area? 

3) How many projects have you approved in the last 5 years? 

4) Do you anticipate having any projects that require NEPA approvals in the next 5 years?

5) What additional assistance would be most valuable in helping you prepare to meet conformity 

requirements? Have you heard of any steps in the state that sound like they are good examples 

of preparatory measures (e.g., training, data compilation, agency coordination and 

agreements)?
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APPENDIX B: CONTACT INFORMATION 

This section will include a list of people that were interviewed for this study. 

State Contacts

Colorado Jerry Piffer, 

Environmental Programs

Colorado DOT

Mike Silverstein

Colorado Air Pollution Control District

Department of Public Health and Environment

Kentucky Lynn Soporowski

Division of Multimodal Programs

Kentucky Transportation Cabinet

Jesse Mayes

Division of Multimodal Programs

Kentucky Transportation Cabinet

Charles Schaub

Division of Multimodal Programs

Kentucky Transportation Cabinet

Barry House

Division of Multimodal Programs

Kentucky Transportation Cabinet

John Gowins

Program Planning and Administration Branch

Kentucky Division for Air Quality

Maine Lori Brann

Bureau of Planning

Maine DOT

Duane Scott

Bureau of Planning

Maine DOT

Melissa Morrill

Air Bureau

Maine Department of Environmental Protection

Donald Cooke (brief questions)

Region 1

U.S. EPA 
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State Contacts

North Carolina* David Hyder

Office of Human Environment

North Carolina DOT

Edward Dancausse

Program Planning and Development

FHWA, North Carolina Division

Ohio Dave Moore

Office of Urban and Corridor Planning

Ohio DOT

Bill Spires

Division of Air Pollution Control

Ohio Environmental Protections Agency

Oregon Vince Carrow

Environmental Services

Oregon DOT

David Nordberg

Air Quality Division

Oregon Department of Environmental Quality

David Collier

Air Quality Division

Oregon Department of Environmental Quality

Ralph Johnston

Lane Regional Air Pollution Authority

Pennsylvania Michael Baker

Air Quality Section

Pennsylvania DOT

Arleen Shulman

Mobile Sources Division

Pennsylvania Department of Environmental Protection
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State Contacts

South Carolina John Gardner

Office of Planning

South Carolina DOT

Henry Phillips

Bureau of Air Quality

South Carolina Department of Health and Environmental Control

Mike Roberts

Resource Center

FHWA

*Note: Interviews were conducted in North Carolina as a test of the interview protocol. Findings 

were not highlighted in this report because North Carolina does not contain any isolated rural areas; 

it does, however, contain a number of rural donut areas.