Guidance for the Use of Latest Planning 

Assumptions in Transportation Conformity 

Determinations 

Revision to January 18, 2001 Guidance 

Memorandum 

December 2008 
Guidance for the Use of Latest Planning 


Assumptions in Transportation Conformity 


Determinations


Revision to January 18, 2001 Guidance 


Memorandum


Transportation and Regional Programs Division 

Office of Transportation and Air Quality 

U.S. Environmental Protection Agency 

Office of Natural and Human Environment


Federal Highway Administration


U.S. Department of Transportation


Office of Planning and Environment


Federal Transit Administration


U.S. Department of Transportation


EPA420-B-08-901


December 2008

Table of Contents 

 Page 

Executive Summary ...................................................................................................1
 

Section 1: Introduction...........................................................................................2
 

1.1 	 Why are EPA and DOT providing this guidance now? 

1.2 	 What is transportation conformity? 

1.3 What conformity requirements are covered by this guidance? 

1.4 	 What latest planning assumption requirements apply in my nonattainment or 

maintenance area? 

1.5 	 Who can I contact for more information? 

1.6 	 Does this guidance create any new requirements? 

Section 2: Latest Planning Assumption Requirements.......................................6
 

2.1 	 What are the conformity requirements for use of latest planning assumptions? 

2.2 	 What do we mean by “latest” planning assumptions? 

2.3 	 When should planning assumptions be updated? What process should be used 

for updates? 

2.4 	 What vehicle registration assumptions must be used in emissions modeling for 

conformity determinations? What process should be used to update these 

assumptions? 

2.5 	 If a state government or other agency collects vehicle registration data on a 

regular basis, should the most recent year of collected data be considered the 

“latest” planning assumptions for the current conformity determination? 

2.6 	 What are the planning assumption requirements for SIP development, and how 

do they relate to the conformity rule’s requirements? 

2.7 	 What if the assumptions used in a submitted or approved SIP are not the most 

current and best information for conformity determinations? 

2.8 	 What are the requirements for periodic emissions inventory updates, and how do 

they relate to the conformity rule’s requirements for latest planning assumptions? 

2.9 	 Are there any assumptions used in conformity analyses that must be consistent 

with those in the SIP? 

2.10 	 What are the requirements for taking credit in conformity analyses for approved 

SIP control measures, such as I/M programs and TCMs? What should be 

considered when determining the latest planning assumptions in conformity 

regarding the implementation of approved SIP control measures? 

2.11 	 Can an area that does conformity for several different pollutants rely on different 

planning assumptions in its conformity determination? 

2.12 	 Does a revision in latest planning assumptions trigger a new conformity 

determination? 

i 
2.13 	 Can an MPO rely on a previous regional emissions analysis even if planning 

assumptions have changed? 

2.14 	 How do the latest planning assumption requirements apply to project-level 

conformity determinations and hot-spot analyses? 

Section 3: 	 Implementation of the 

Latest Planning Assumption Requirements.......................................17 

3.1 	 How should state and local agencies define the “time the conformity analysis 

begins?” 

3.2 	 To support a valid conformity determination, what planning assumption 

information should be documented? 

3.3 	 If new data becomes available after the analysis begins, should it be included in 

the conformity determination? 

3.4 	 Must an area wait to start the conformity analysis until anticipated changes in 

planning data become available? 

ii 
Executive Summary 

The Clean Air Act requires that transportation investments be based on the most 

recent information that is available in order to protect public health over the long-term. 

As such, conformity determinations must be based upon the most recent planning 

assumptions in force at the time the conformity analysis begins (40 CFR 93.110(a)). 

The Environmental Protection Agency (EPA) and Department of Transportation (DOT) 

encourage state and local agencies to review and update their planning assumptions 

regularly. Areas are strongly encouraged to review and update planning assumptions at 

least every five years, especially population, employment, and vehicle registration 

assumptions. Areas with network-based travel models should review their assumptions 

and data used in model validation through the consultation process, and newer 

assumptions and data must be used whenever available. Conformity determinations 

that are based on assumptions that are older than five years should include written 

justification for not reviewing and updating assumptions at least every five years. For 

areas where updates are appropriate, the conformity determination should include an 

anticipated schedule for updating assumptions for future conformity determinations. Air 

quality and transportation agencies should use the consultation process to ensure that 

the latest available planning assumptions are used in conformity determinations and 

state implementation plan (SIP) development. 

1 
 
Section 1: Introduction 

1.1 Why are EPA and DOT providing this guidance now? 

This update is necessary because the July 1, 2004 final conformity rule changed 

the requirements regarding the use of the latest planning assumptions (69 FR 40052-

40056). Some examples of planning assumptions include population, employment, and 

vehicle fleet information. This guidance supersedes the January 18, 2001 guidance 

memorandum1 to address changes in the July 2004 conformity rule. All other aspects 

related to conformity requirements and SIP development are consistent with the 2001 

guidance. 

In the July 2004 conformity rule, EPA changed 40 CFR 93.110(a) to allow a 

conformity determination to be based on the latest planning assumptions in force at the 

time the conformity analysis begins, rather than at the time of DOT’s conformity 

determination2. EPA changed this provision so it is more workable for implementers 

while remaining consistent with Clean Air Act requirements. Allowing conformity 

determinations based on the latest information available at the time the conformity 

analysis begins provides certainty for implementers and is a reasonable approach to 

ensuring that conformity determinations are based on accurate and available 

information. 

EPA and DOT are issuing this guidance jointly to clarify our expectations for 

implementing the transportation conformity rule’s latest planning assumptions 

requirements. This guidance also reiterates EPA’s expectations for using latest 

planning assumptions in the development of motor vehicle emissions budgets in SIPs. 

1.2 What is transportation conformity? 

Transportation conformity is required under Clean Air Act section 176(c) (42 

U.S.C. 7506(c)) to ensure that federally supported highway and transit project activities 

are consistent with (“conform to”) the purpose of the SIP. Under this statute, conformity 

to the purpose of the SIP means that transportation activities will not cause new air 

quality violations, worsen existing violations, or delay timely attainment of the relevant 

national ambient air quality standards. EPA’s transportation conformity rule (40 CFR 

Parts 51 and 93) establishes the criteria and procedures for determining whether 

transportation plans, transportation improvement programs (TIPs), and projects conform 

to the SIP. Transportation conformity applies to areas that are designated 

nonattainment, and those redesignated to attainment after 1990 (“maintenance areas”) 

1

 “Use of Latest Planning Assumptions in Conformity Determinations,” memorandum from Robert 

Perciasepe, Assistant Administrator for EPA’s Office of Air and Radiation, Kenneth Wykle, FAA 

Administrator and Nuria Fernandez, FTA Acting Administrator to FHWA Division Administrators, Federal 

Lands Highway Division Engineers, FTA Regional Administrators, and EPA Regional Administrators, 

Region I-X, January 18, 2001. 

2

 It is important to note that in certain situations, the previous requirements still apply. See Question 1.4 

for further details. 

2 
 
for transportation-related criteria pollutants: carbon monoxide, ozone, nitrogen dioxide, 

PM2.5, and PM10. 

1.3 	 What conformity requirements are covered by this guidance? 

This guidance document clarifies the implementation of the latest planning 

assumption requirements in 40 CFR 93.110, which must be met at the time conformity 

analysis begins as described later in this guidance. There are other transportation 

conformity requirements that are not covered by the latest planning assumption 

requirements that continue to be required to be met on the date of DOT’s conformity 

determination. 

EPA discussed some examples of requirements that are not considered part of 

latest planning assumption requirements in the preamble to the July 1, 2004, final rule 

(69 FR 40054-40055). For example, DOT’s conformity determination must be based on 

whatever adequate or approved SIP motor vehicle emissions budgets apply at that time, 

even if such budgets became available after the MPO’s conformity determination (40 

CFR 93.109 and 93.118). In addition, DOT’s conformity determination must be based 

on a regional emissions analysis that accurately reflects control measures in the SIP, 

transportation plan/TIP, or through written commitments (40 CFR 93.122(a)(1)-(4)), as 

described further in Question 2.10. 

1.4 	 What latest planning assumption requirements apply in my nonattainment or 

maintenance area? 

The latest planning assumption requirements for a conformity determination in a 

given nonattainment or maintenance area can vary depending upon whether a state has 

an EPA-approved conformity SIP3, and if so, when it was approved: 

•	 If a state does not have an approved conformity SIP or if it has an approved 

conformity SIP based on the July 2004 final rule, transportation planners and DOT 

are required to meet the current conformity rule’s requirement (40 CFR 93.110(a)) 

that latest planning assumptions be determined at the time the conformity 

analysis begins. 

•	 If a state has an approved conformity SIP based on the previous conformity rule, 

transportation planners and DOT are required to meet that previous requirement. 

3

 A conformity SIP contains the state’s criteria and procedures for interagency consultation. Unlike a 

reasonable further progress SIP, an attainment demonstration, or maintenance plan, a conformity SIP 

does not contain motor vehicle emissions budgets, emissions inventories, air quality demonstrations, or 

control measures. A conformity SIP can be developed as a state rule, a memorandum of understanding 

(MOU), or a memorandum of agreement (MOA). In general, once approved, the conformity SIP governs 

the conformity process instead of the federal conformity rule for any provisions included in the conformity 

SIP. 

3 
 
In such a case, conformity determinations would be required to be based on the 

latest planning assumptions available at the time of DOT’s final conformity 

determination. 

Section 51.390(b) of the conformity rule requires conformity determinations in a 

given nonattainment or maintenance area to meet any applicable requirements in an 

EPA-approved conformity SIP. Any out-dated requirements, such as the previous rule’s 

latest planning assumptions requirements, would no longer apply when the state either 

withdraws or updates such requirements, and EPA approves the revised conformity 

SIP. Section 3 of today’s guidance document does not apply to areas operating under 

approved conformity SIPs that include the previous latest planning assumptions 

requirements. 

1.5 Who can I contact for more information? 

For specific questions concerning a particular nonattainment or maintenance 

area, please contact the transportation conformity staff person responsible for your state 

at the appropriate EPA regional office, Federal Highway Administration (FHWA) division 

office or Federal Transit Administration (FTA) regional office: 

•	 A listing of EPA regional offices, the states they cover, and contact information 

for conformity staff can be found at the following website: 

http://www.epa.gov/otaq/stateresources/transconf/contacts.htm, 

•	 Contact information for FHWA division offices can be found at: 
 

http://www.fhwa.dot.gov/field.html, and 
 

•	 Contact information for FTA regional offices can be found at: 
 

http://www.fta.dot.gov/regional_offices.html. 
 

General questions about this guidance can be directed to: 

•	 Patty Klavon of EPA’s Office of Transportation and Air Quality, 
 

klavon.patty@epa.gov or at (734) 214-4476, 
 

•	 Cecilia Ho of FHWA’s Office of Natural and Human Environment, 
 

cecilia.ho@dot.gov or at (202) 366-9862, or 
 

•	 Abbe Marner of FTA’s Office of Planning and Environment, 
 

abbe.marner@dot.gov or at (202) 366-4317. 
 

Readers can also find this guidance document, the complete transportation 

conformity regulation, and other materials on the EPA’s website at: 

http://www.epa.gov/otaq/stateresources/transconf/conf-regs.htm 

1.6 Does this guidance create any new requirements? 

No, this guidance is based on the requirements for latest planning assumptions 

contained in Clean Air Act section 176(c) and 40 CFR 93.110, and does not create any 

new requirements. This guidance merely explains how to implement the latest planning 

assumption provisions and related requirements. 

4 
 
The statutory provisions and the EPA regulations described in this document 

contain legally binding requirements. This document is not a substitute for those 

provisions or regulations, nor is it a regulation itself. Thus, it does not impose legally 

binding requirements on EPA, DOT, states, or the regulated community, and may not 

apply to a particular situation based upon the circumstances. EPA and DOT retain the 

discretion to adopt approaches on a case-by-case basis that may differ from this 

guidance, but still comply with the statute and conformity and SIP regulations. Any 

decisions regarding a particular area’s use of the latest planning assumptions will be 

made based on the statute and regulations, after appropriate public input and 

rulemaking procedures where applicable. This guidance may be revised periodically 

without public notice. 

5 
 
Section 2: Latest Planning Assumption Requirements 

2.1 What are the conformity requirements for use of latest planning assumptions? 

Section 176(c)(1)(B)(iii) of the Clean Air Act states that "[t]he determination of 

conformity shall be based on the most recent estimates of emissions, and such 

estimates shall be determined from the most recent population, employment, travel, and 

congestion estimates as determined by the MPO or other agency authorized to make 

such estimates." The Clean Air Act requires that transportation investments be based 

on the most recent information that is available, in order to protect public health over the 

long-term. 

The conformity regulations at 40 CFR 93.110 provide the requirements for using 

the latest planning assumptions in transportation conformity determinations. The 

requirements apply to all assumptions used in demonstrating conformity, including 

assumptions that are used in transportation demand and emissions modeling. 

Examples of assumptions are land use, vehicle age and fleet mix, and the most recent 

information regarding the implementation of control measures in approved SIPs (e.g., 

inspection and maintenance (I/M), fuels programs, and transportation control 

measures). Section 93.110(a) of the transportation conformity rule requires conformity 

determinations to be based on the “most recent planning assumptions in force at the 

time the conformity analysis begins.” Section 93.110(a) of the conformity rule also 

requires that, “New data that becomes available after an analysis begins is required to 

be used in the conformity determination only if a significant delay in the analysis has 

occurred, as determined through interagency consultation.” See Section 3 of this 

guidance for further discussion on when a “conformity analysis begins,” and for what 

may be considered “a significant delay.” 

As applicable, specific latest planning requirements are further outlined in 40 

CFR 93.110 (b)-(f): 

“(b) Assumptions must be derived from the estimates of current and future 

population, employment, travel, and congestion most recently developed by the 

MPO or other agency authorized to make such estimates and approved by the 

MPO. The conformity determination must also be based on the latest 

assumptions about current and future background concentrations. 

(c) The conformity determination for each transportation plan and TIP must 

discuss how transit operating policies (including fares and service levels) and 

assumed transit ridership have changed since the previous conformity 

determination. 

(d) The conformity determination must include reasonable assumptions about 

transit service and increases in transit fares and road and bridge tolls over time. 

6 
 
(e) The conformity determination must use the latest existing information 

regarding the effectiveness of the transportation control measures (TCMs) and 

other implementation plan measures which have already been implemented. 

(f) Key assumptions shall be specified and included in the draft documents and 

supporting materials used for the interagency and public consultation required by 

§93.105.” 

EPA, DOT, and state and local transportation and air quality agencies participate 

in the interagency consultation process to review and comment on assumptions (40 

CFR 93.105), and the latest planning assumption requirements must be met before 

DOT can make a conformity determination (40 CFR 93.109(b)). 

As assumptions are expected to change over time, it is important that state, local, 

and federal transportation and air quality planners evaluate on a regular basis existing 

assumptions and new information as it becomes available. Conformity determinations 

that are based on assumptions that are older than five years should include written 

justification for not using more recent information. See Question 2.3 for more details on 

when latest planning assumption updates are needed. 

2.2 What do we mean by "latest" planning assumptions? 

Using the "latest" planning assumptions means that the conformity determination 

is based on the most current information that is available to state and local planners 

(e.g., the MPO or other agency can obtain the information from another agency, the 

information is appropriate for the current conformity determination, the information is 

readily transferable for use in transportation and/or emissions modeling, etc.). 

Latest planning assumptions must be derived from the population, employment, 

travel, and congestion estimates that have been most recently developed by the MPO 

(or other agency authorized to make such estimates), and approved by the MPO (40 

CFR 93.110(b)). Once approved, these estimates must be used for determining the 

latest planning assumptions (40 CFR 93.110(a)). In areas using network-based travel 

models, scenarios of land development and use must be consistent with the future 

transportation system for which emissions are being estimated, and the distribution of 

employment and residences for the transportation system must be reasonable (40 CFR 

93.122(b)(1)(iii)). 

The interagency consultation process must be used to determine which planning 

assumptions are considered the latest and best assumptions for conformity 

determinations. The conformity rule specifically requires that the interagency 

consultation process be used to evaluate and choose assumptions to be used in 

conformity analyses (40 CFR 93.105(c)(1)(i)). 

7 
 
The consultation process should be used to evaluate assumptions for quality and 

accuracy as needed prior to use in conformity. Whenever Highway Performance 

Monitoring System (HPMS) data is used for current and future years in conformity 

analyses, the most recently available HPMS estimates of vehicle miles traveled (VMT) 

must be used (40 CFR 93.110(a)). Historical trends and other factors should be 

considered as a primary source of information from which planning assumptions should 

be evaluated (e.g., population, employment). If assumptions are used that contradict 

historical trends, the conformity determination should include an explanation regarding 

why the assumptions are appropriate. This explanation should be included when the 

conformity determination is provided for public comment. The consultation process 

should not be used to unduly delay or exclude the use of new information or to 

selectively employ it for the convenience of the conformity process. 

Areas that rely on the U.S. Census for certain planning assumptions must use 

the most recent estimates available from the Census Bureau (40 CFR 93.110(a)). 

Areas that are using assumptions based on data collected through local or state 

surveys or other mechanisms should use the consultation process to determine whether 

older state or local data is more appropriate than the most recently available U.S. 

Census. 

2.3 	 When should planning assumptions be updated? What process should be used 

for updates? 

EPA and DOT encourage nonattainment and maintenance areas to review and 

update their planning assumptions on a regular basis. Although regular updates of 

assumptions are not required by the transportation conformity rule, areas are strongly 

encouraged to review and update planning assumptions at least every five years, 

especially for population, employment, and vehicle registration assumptions, or to justify 

in the conformity determination why planning assumptions have not been reviewed and 

updated at least every five years. 

As planning assumptions are expected to change over time, it is important that 

state, local, and federal transportation and air quality planners evaluate, on a regular 

basis, existing assumptions and new information as it becomes available. This may be 

especially important in areas that are experiencing higher population and employment 

growth than is reflected in current planning assumptions. Conformity determinations that 

are based on information that is older than five years should include written justification 

for not using more recent information. For areas where future updates are determined 

to be necessary, the conformity determination should include an anticipated schedule 

for updating assumptions. 

Planning assumptions must be reviewed through the interagency consultation 

process to determine whether they are adequate for conformity purposes (see 40 CFR 

93.105(c)(1)(i)). The review of latest planning assumptions typically occurs in 

conjunction with transportation plan and TIP conformity determinations. The results of 

8 
 
the review of the planning assumptions and consultation process would be documented 

in the conformity determination. The interagency consultation process is also the forum 

for evaluating and choosing assumptions that are used in conformity determinations in 

isolated rural nonattainment and maintenance areas (40 CFR 93.105(c)(1)(vi)). 

EPA and DOT should be proactively involved in the interagency consultation 

process in all nonattainment and maintenance areas so that potential issues and 

problems can be identified and addressed early on in the conformity process. If it is 

determined through interagency consultation that certain planning assumptions need to 

be updated, state and local transportation and air quality agencies should commit to a 

schedule to update those assumptions for future conformity determinations. Different 

schedules can be established for updating different assumptions. The level of 

resources dedicated to updating assumptions may vary depending upon planning 

priorities in a given area. State and local agencies can prioritize these updates along 

with other planning needs. See below for specific information regarding our 

expectations for vehicle registration assumptions. 

Areas required to use network-based travel models for conformity purposes 

should also review their assumptions and data used in model development and 

validation through the consultation process, and the latest assumptions and data must 

be used whenever available (40 CFR 93.110). Since emission estimates are sensitive 

to vehicle speed changes, EPA and DOT recommend that areas using network-based 

travel models compare the speeds estimated in the validation year with speeds 

empirically observed during the peak and off-peak periods. The significant sensitivity of 

emissions to highway speeds emphasizes the need to monitor and maintain the ability 

of the transportation model to provide accurate speed estimates. Nonattainment and 

maintenance areas using network-based travel models are encouraged to establish 

criteria for updating the observed speed data that are used to validate the speeds 

predicted by the transportation model. The criteria should identify the schedule on 

which speed data will be collected given the pace of growth in the urban area, the 

magnitude of changes to the highway system, and any fundamental changes in speedrelated conditions such as a change in the federal or the state’s maximum speed limit.4 

2.4 	 What vehicle registration assumptions must be used in emissions modeling for 

conformity determinations? What process should be used to update these 

assumptions? 

As stated above, MPOs must use the most recent vehicle registration data that is 

available for conformity analyses at the time the conformity analysis begins (40 CFR 

93.110(a)). State and local transportation and air quality agencies should make an 

effort to obtain recent registration data files and should establish a schedule for 

updating local vehicle registration data on a regular basis. 

4

 In December 1995, the federal speed limit law was changed to remove the mandatory 55 mph speed 

limit and allow states to set limits using their own discretion. 

9 
 
The composition of vehicle fleets can vary significantly over time and from area 

to area. It is critical for areas to use the interagency consultation process to examine 

new vehicle registration data (i.e., vehicle type and age) as it becomes available and 

consider whether the current data and assumptions are still appropriate for conformity 

purposes. It is especially important for areas to update assumptions if the 

characteristics of the fleet have significantly changed from the assumptions that are 

currently being used for conformity. 

As a general rule, local data should be used whenever available. If local data is 

not currently available and cannot reasonably be made available for use in impending 

conformity determinations, areas may rely on the national defaults related to vehicle 

registration data in the latest EPA-approved motor vehicle emissions factor model. 

However, areas should explore whether more recent local vehicle registration data can 

be made available in a useable format for the next conformity determination. Once 

such data becomes available before a conformity analysis begins, it must be used in the 

conformity analysis (40 CFR 93.110(a)). When EPA releases a new motor vehicle 

emissions factor model and associated guidance for use in conformity determinations, 

some areas will need to use the interagency consultation process to determine whether 

older local data should be used instead of newer national defaults. 

EPA also notes that the SIP process can be a valuable source for vehicle 

registration data for conformity determinations, and many state and local air agencies 

play a lead role in reviewing this type of data. Another example may be when the state 

air agency has quality assured newer vehicle registration data from what is included in a 

currently approved SIP. 

2.5 	 If a state government or other agency collects vehicle registration data on a 

regular basis, should the most recent year of collected data be considered the 

"latest" planning assumptions for the current conformity determination? 

Yes, if that information is available for conformity purposes at the time the 

conformity analysis begins (40 CFR 93.110(a)). However, most state governments 

collect information, such as vehicle registration data, on an annual basis for purposes 

other than conformity. This data includes information about vehicle age and fleet mix, 

for example, which are important inputs for emissions modeling. Although this 

information may be sufficient for some state government purposes not related to air 

quality (e.g., state tax or insurance purposes), the most recent year of collected raw 

data may not necessarily be the best available information for use in the current 

conformity analysis. Of course, if the latest state or local vehicle registration data has 

been quality-assured and is being used for emissions modeling, then it must also be 

considered the latest data for conformity (40 CFR 93.110(a)). 

The interagency consultation process should be used to determine whether the 

most recent vehicle registration data is available for conformity. State and local 

transportation and air quality agencies should consider whether the data are accurate 

10 
 
and appropriate for conformity purposes, as well as whether the data are readily 

transferable for use in emissions modeling for the specific nonattainment or 

maintenance area. 

If the most recent vehicle registration data are not available for use in the current 

conformity determination, the consultation process should be used to determine 

whether such vehicle registration data should be reviewed and used for the next 

determination. The conformity determination should include an explanation of why 

recent state or local vehicle registration data was not considered available for 

conformity purposes, if concerns are raised during the consultation or public 

involvement processes. EPA and DOT recommend that if the new data can be quality 

assured and/or formatted for emissions modeling purposes, it should be used. 

2.6 	 What are the planning assumption requirements for SIP development, and how 

do they relate to the conformity rule’s requirements? 

Motor vehicle emissions budgets in SIPs must be based on the most current 

information available at the time that the SIP is developed (see Clean Air Act section 

172(c)(3)). As such, planning assumptions, including VMT, socioeconomic variables, 

and emissions modeling inputs (including vehicle registration by age and type) must be 

based on the latest information available at the time that the SIP is developed and be 

consistent with SIP inventory and emissions modeling requirements. 

As stated in the July 2004 final rule, state and local agencies should consider the 

availability of new planning assumptions when determining their conformity schedules 

(69 FR 40054). The consultation process would be used to determine the most recent 

planning assumptions available for SIP development, since the conformity rule provides 

for such consultation in SIP development (40 CFR 93.105). If a new SIP budget is 

found adequate or approved prior to the MPO or DOT’s conformity determination, 

conformity to the new budget is required. In this case, transportation planners should 

use the more recent planning assumptions that are in the new budget. State and local 

air agencies should inform transportation agencies of new planning assumptions as 

they become available. 

Transportation planners are encouraged to review and comment on motor 

vehicle emissions budgets as well as share more current assumptions with air quality 

agencies so that they can be included in the SIP. This is especially important since air 

quality and transportation agencies usually provide different types of planning 

assumptions to SIP and conformity processes. For example, transportation planners 

are more likely to have the latest VMT estimates for a given nonattainment or 

maintenance area while air agencies are more likely to have the latest available vehicle 

registration data. It is critical that transportation and air agencies exchange information 

on a regular basis so that the latest assumptions are used in SIP and conformity 

processes. 

11 
 
2.7 	 What if the assumptions used in a submitted or approved SIP are not the most 

current and best information for conformity determinations? 

As described above, the Clean Air Act and conformity regulations require the 

latest planning assumptions to be used in conformity determinations, even if they differ 

from those used in the SIP (40 CFR 93.110(a)). In many cases, the MPO may have 

developed more recent assumptions for the conformity process than those included in a 

submitted or approved SIP. For example, the MPO may have adopted new population, 

employment, and/or socioeconomic projections or updated transportation models since 

the SIP was submitted. In this case, an MPO would use the latest planning 

assumptions based on the newer projections and model improvements for conformity. 

The interagency consultation process should be used to ensure that air quality and 

transportation planning processes anticipate such changes in planning assumptions. It 

is expected that planning assumptions in the conformity process will change over time 

from those used in the SIP. 

For further information, see the preamble to the July 1, 2004, final rule (69 FR 

40053) and the November 24, 1993, final rule (58 FR 62210). 

2.8 	 What are the requirements for periodic emissions inventory updates, and how do 

they relate to the conformity rule’s requirements for latest planning assumptions? 

Clean Air Act section 172(c)(3) requires that periodic emissions inventories be 

done in nonattainment areas. There are also specific requirements for periodic 

inventories under individual subparts to part D of Title 1, such as section 182(a)(3) for 

ozone and section 187(a)(5) for carbon monoxide (CO). According to Clean Air Act 

section 172(c)(3), periodic inventory updates for SIPs must be based on the most 

current information available at the time that the inventory is developed. Examples of 

assumptions used in both inventory updates and conformity are VMT, socioeconomic 

variables, and vehicle fleet characteristics. Although periodic inventory updates do not 

establish motor vehicle emissions budgets, they can be a valuable source of more 

recent and better planning assumptions for use in the conformity process. 

Air quality and transportation agencies should work together to ensure that the 

latest planning assumptions are used in SIP inventory updates and conformity 

determinations. As discussed above, transportation planners should share more 

current assumptions with air quality agencies so that they can be included in the SIP. 

Air quality agencies should also provide more recent assumptions to transportation 

planners for conformity determinations. The interagency consultation process required 

by 40 CFR 93.105 should be used to determine whether the assumptions used in 

periodic SIP inventory updates are more recent and better than the assumptions used 

previously for conformity determinations. 

12 
 
2.9 	 Are there any assumptions used in conformity analyses that must be consistent 

with those in the SIP? 

Yes. The transportation conformity rule (40 CFR 93.122(a)(6)) requires that 

ambient temperatures in conformity analyses must be consistent with those used in the 

SIP’s motor vehicle emissions budgets. Other assumptions - or factors - that must be 

consistent with those used in the SIP’s motor vehicle emissions budget could also 

include humidity, altitude, the fraction of travel in a hot stabilized engine mode and 

annual mileage accumulation rates over the time frame of the transportation plan (40 

CFR 93.122(a)(6)). These types of factors do not typically change in future years and 

could significantly impact emissions; EPA generally believes that it is appropriate to 

require such factors to be consistent between conformity analyses and the SIP budgets. 

However, under certain circumstances, it may be appropriate to use alternative 

factors instead of certain SIP assumptions, if it is determined through the interagency 

consultation process that these factors should be modified. For example, such 

modifications in these types of factors may be appropriate where additional or more 

geographically specific information is incorporated or a logically estimated trend in such 

factors beyond the period considered in the SIP is represented. EPA does not expect 

changes in the SIP’s factors to occur often, and they could only occur after consultation 

among all local, state, and federal agencies. All planning assumptions used in a 

conformity analysis must be documented in the conformity determination that is 

released for public comment (40 CFR 93.105(e)); see the July 1, 2004, final rule for 

further information (69 FR 40056). 

2.10 	 What are the requirements for taking credit in conformity analyses for approved 

SIP control measures, such as I/M programs and TCMs? What should be 

considered when determining the latest planning assumptions in conformity 

regarding the implementation of approved SIP control measures? 

There are several conformity requirements that apply when reflecting approved 

SIP control measures in conformity determinations. First, DOT’s conformity 

determination must be based on a regional emissions analysis that reflects the actual 

implementation of approved SIP control measures (40 CFR 93.122(a)). For example, 

section 93.122(a)(2) of the conformity rule states: 

“The emissions analysis may not include for emissions reduction 

credit any TCMs or other measures in the applicable implementation 

plan which have been delayed beyond the scheduled date(s) until such 

time as their implementation has been assured. If the measure has 

been partially implemented and it can be demonstrated that it is 

providing quantifiable emission reduction benefits, the emissions 

analysis may include that emission reduction credit." 

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If a control measure in an approved SIP is delayed or changed in whole 

or in part (e.g., the legislative authority of a program has changed), then 

conformity determinations must reflect such a delay or change including 

situations in which a measure is cancelled or changed after the conformity 

analysis begins. Section 93.122(a)(3) also requires that DOT’s conformity 

determination be made only when regulatory control programs have been 

assured and will be implemented as described in the SIP. 

In addition, regional emissions analyses must be based on the latest 

information available at the time the analysis begins regarding the 

effectiveness of SIP programs that are being implemented. Section 93.110(e) 

of the conformity rule states, "[t]he conformity determination must use the 

latest existing information regarding the effectiveness of TCMs and other 

implementation plan measures which have already been implemented." See 

EPA’s July 1, 2004, final rule for further information on 40 CFR 93.110 and 

93.122 requirements (69 FR 40054-40055). 

The interagency consultation process should be used to determine the latest 

planning assumptions regarding implementation of approved SIP measures. It is critical 

that all parties understand the impacts of altering the implementation of approved SIP 

measures. State and local air agencies or other agencies that are responsible for air 

quality planning should inform transportation planners and provide technical assistance 

in quantifying the impact when the implementation of approved SIP measures could 

alter the emission reduction benefits assumed in previous conformity determinations. In 

addition, state and local air agencies should consider how SIP implementation issues 

can be addressed. They should immediately discuss how to make up for the emissions 

shortfalls and remedy the SIP, possibly through addressing implementation delays. The 

interagency consultation process should also be used to consider whether delays in the 

SIP’s control measures necessitate a SIP revision to incorporate other measures. EPA 

commits to share information through the interagency consultation process as 

appropriate when the implementation of an approved SIP measure changes and to 

address the implementation issues through the SIP process. 

Local, state, and federal agencies are also encouraged to be proactive in 

considering the impacts on the conformity process before the implementation of an 

approved SIP measure is changed. It is generally expected that air quality agencies 

would assist transportation agencies in quantifying the impacts of a change in approved 

SIP measures. The details of how this is accomplished should be determined through 

the interagency consultation process. 

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2.11 	 Can an area that does conformity for several different pollutants rely on different 

planning assumptions in its conformity determination? 

Planning assumptions that are applicable to conformity determinations for 

different pollutants must be consistent (40 CFR 93.110)). For example, local estimates 

of population, employment, travel, and congestion must be consistent among analyses 

for different pollutants and precursors. Of course, there may be assumptions that would 

vary due to the inherent differences in analyzing pollutants (e.g., temperatures may be 

different for ozone and CO analyses due to the seasons in which these pollutants are 

formed). Where there are differences in planning assumptions for different pollutants, 

these differences should be explained in the documentation for the conformity 

determination. 

2.12 	 Does a revision in latest planning assumptions trigger a new conformity 

determination? 

No. A new regional emissions analysis and conformity determination would not 

be required solely to incorporate revised planning assumptions (40 CFR 93.122(g)). 

Although revisions to planning assumptions would not trigger a conformity 

determination, future conformity determinations and supporting analyses must include 

the revised assumptions (40 CFR 93.110(a)). 

2.13 	 Can an MPO rely on a previous regional emissions analysis even if planning 

assumptions have changed? 

Yes. MPOs can rely on a previous emissions analysis if the requirements for 40 

CFR 93.122(g) are met (e.g., no regionally significant projects have changed, no more 

than four years have elapsed since the last regional emissions analysis). An MPO can 

rely on the previous analysis when no new transportation or emissions modeling is 

otherwise required (e.g., when a TIP is updated and the transportation plan has not 

changed) (58 FR 3778). Conformity determinations that rely on a previous analysis do 

not restart the four-year conformity requirement (40 CFR 93.122(g)(3) and 69 FR 

40060).5 

5

 It is important to note that the conformity rule requires a new transportation plan and TIP conformity 

determination and regional emissions analysis at least every four years (40 CFR 93.104(b)(3) and (c)(3)). 

These determinations are done in many areas when the transportation plan is updated. The regional 

emissions analysis must be based on a new planning horizon of at least 20 years and the latest planning 

assumptions. 

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2.14 	 How do the latest planning assumption requirements apply to project-level 

conformity determinations and hot-spot analyses? 

In general, section 93.109(b) of the conformity rule requires project-level 

conformity determinations to meet the latest planning assumptions requirements, but 

there are some additional requirements and considerations that also need to be taken 

into account when a hot-spot analysis is required. Hot-spot analysis assumptions for 

individual projects must be consistent with those assumptions used in the regional 

emissions analysis for those inputs which are required for both analyses (40 CFR 

93.123(c)(3)). However, a future transportation plan and TIP update and conformity 

determination which incorporates revised planning assumptions in the regional 

emissions analysis would not trigger a new hot-spot analysis for a project included in 

the prior transportation plan and TIP unless there has been a significant change in the 

project’s design concept and scope; three years elapse since the most recent major 

step to advance the project; or initiation of a supplemental environmental document for 

air quality purposes. See section 40 CFR 93.104(d) of the conformity rule for more 

details. The interagency consultation process must be used to determine what planning 

assumptions are appropriate for hot-spot analyses (40 CFR 93.105(c)(1)(i)). 

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Section 3: Implementation of the Latest Planning Assumption Requirements 

3.1 	 How should state and local agencies define the “time the conformity analysis 

begins”? 

As described previously, the conformity rule requires conformity determinations 

to be based on the latest planning assumptions available at the time the conformity 

analysis begins. Section 93.110(a) of the conformity rule states, “[t]he ‘time the 

conformity analysis begins’ is the point at which the MPO or other designated agency 

begins to model the impact of the proposed transportation plan or TIP on travel and/or 

emissions.” Any information and assumptions that become available before actual 

modeling for a conformity determination has commenced would be required to be 

considered in that conformity determination (69 FR 40053). In addition, section 

93.110(a) of the conformity rule requires that “New data that becomes available after an 

analysis begins is required to be used in the conformity determination only if a 

significant delay in the analysis has occurred, as determined through interagency 

consultation.” 

Section 93.105(c)(1)(i) requires the interagency consultation process to be used 

to decide which planning assumptions and models are available for use by the MPO or 

other designated agencies responsible for conducting conformity analyses. This point 

should be defined through interagency consultation and used consistently for all future 

conformity determinations. EPA encourages the MPO or other designated agency to 

use the interagency consultation process to inform other involved agencies of when a 

conformity analysis has started for a given conformity determination. The interagency 

consultation process would also be used to determine whether a significant delay has 

occurred and whether new data that becomes available during a delay should be 

incorporated. 

The preamble to the July 1, 2004, final rule (69 FR 40053) provides several 

examples for when the conformity analysis begins including: 

•	 The point at which travel demand modeling begins to generate the VMT and 

speed data that will be used to calculate emissions estimates for the conformity 

determination. 

•	 For smaller MPOs and rural areas that do not use a travel demand model the 

point at which VMT projections necessary to run the emissions model are 

calculated based on the most recent HPMS, population and employment data 

that are available at that time. 

The preamble to the July 1, 2004, final rule also provides several examples for 

when the conformity analysis does not begin including: 

• When the initial list of projects for the plan and TIP have been developed or 

before those projects have been coded into the transportation network; 

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•	 If travel or emissions modeling is conducted to preliminarily examine the impact 

of several potential projects or project alternatives on travel or emissions in the 

area; or, 

•	 When an initial schedule for completing an analysis is developed during an 

interagency consultation meeting. 

3.2 	 To support a valid conformity determination, what planning assumption 

information should be documented? 

When an MPO or other designated agency conducts a conformity determination, 

it should document in its determination the “time the conformity analysis begins” as 

determined by interagency consultation, the date on which the analysis was started, and 

the planning assumptions that were used. 

To support a valid conformity determination, the MPO or other appropriate 

agency should document the following specific information: 

•	 How the “time the conformity analysis begins” has been defined through 
 

interagency consultation; 
 

•	 The calendar date that the conformity analysis began; and, 

•	 The planning assumptions used in the conformity analysis. 

Documenting this information in the actual conformity determination informs the 

public of decisions regarding the use of latest planning assumptions, and will record 

when an analysis has begun, so that commenters can address any issues related to 

these decisions (40 CFR 93.105(e)). See the July 1, 2004 final rule preamble for more 

information (69 FR 40054). 

3.3 	 If new data becomes available after the analysis begins, should it be included in 

the conformity determination? 

New data (e.g., population or fleet data) that becomes available after the 

conformity analysis begins is not required to be incorporated into the current analysis if 

the analysis is on schedule, although an area could voluntarily include the new 

information at any time, as appropriate. Section 93.110(a) of the conformity rule 

requires that new data that becomes available after an analysis begins be used in the 

conformity determination only if a significant delay in the analysis has occurred, as 

determined through interagency consultation. An example of a significant delay in the 

conformity analysis would include the case where an MPO starts a conformity analysis 

and begins generating VMT estimates from the travel demand model and the MPO’s 

analysis is then delayed for six months (69 FR 40054). In such cases where a 

significant delay occurs, EPA believes that it is reasonable to expect that the MPO must 

incorporate new planning information that became available during the six-month delay 

period (40 CFR 93.110(a)). 

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In cases when there has not been a significant delay in the analysis, interagency 

consultation parties have discretion to decide if data available after the analysis begins 

should be incorporated, and should base this decision on whether substantial work has 

been completed. Although including new data that becomes available after the analysis 

begins is not automatically required, EPA and DOT encourage agencies to include such 

data if the schedule of the conformity determination can accommodate it. 

3.4 	 Must an area wait to start the conformity analysis until anticipated changes in 

planning data become available? 

The Clean Air Act and transportation conformity rule do not require state and 

local planners to delay their conformity analyses to incorporate anticipated data that is 

not yet available for conformity purposes (69 FR 40056). Section 93.110(a) requires 

conformity determinations to incorporate the only most recent planning information 

available at the time the conformity analysis begins although the consultation process 

should not be used to unduly delay or exclude the use of new information or to 

selectively employ it for the convenience of the conformity process. 

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