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Transportation Conformity Alert

August 2, 2000

A number of issues have surfaced in recent weeks that have potentially large impacts on conformity. We would like to take the opportunity to alert you to these issues and recommend that you give careful consideration to each of them as you work with our partners to develop conformity determinations.

Peak and Off-Peak Speeds

The Transportation Conformity Rule (40 CFR Parts 51 and 93) in section 93.122(b)(1)(iv) requires that emissions estimates be based on both peak and off-peak speeds in serious, severe and extreme ozone nonattainment and serious CO nonattainment areas with a population over 200,000. It is important that these speeds be based on recent data to reflect changes in the highway network, including changes to speed limits and completed capacity improvements.

PM Qualitative Analysis

The conformity rule also requires PM10 hot-spot analysis for projects in PM10 nonattainment areas. Section 93.123(b)(4) of the rule states that the requirements for "quantitative" hot spot analysis will not take effect until EPA releases modeling guidance. However, this does not eliminate the need for qualitative analysis on projects. NEPA documents for projects in PM10 nonattainment areas must include a qualitative analysis that demonstrates project-level conformity. We are currently working on guidance to assist in the development of these qualitative analyses.

Latest Planning Assumptions

Section 93.110 of the conformity rule requires that the latest planning assumptions be used in conformity determinations. It is essential that the most currently available estimates of population, employment, travel, and congestion be used in conformity determinations to accurately predict emissions. As emissions estimates are sensitive to the vehicle fleet mix, which may change over time, the conformity determination should be based on recent vehicle registration data (preferably no older than 5 years). We recommend that you work through interagency consultation to establish mechanisms to review and update planning assumptions and data on a regular basis. We are working with EPA to develop guidance on this issue. In the meantime, please confer with our office prior to relying on any vehicle fleet data older than 5 years.

Motor Vehicle Emissions Budgets in SIPs

Some nonattainment areas are discovering that mobile source emissions budgets are not clearly defined in their State Implementation Plan (SIP). Without a clearly indicated motor vehicle emissions budget, the SIP's estimate of future highway and transit emissions used in the milestone or attainment demonstration becomes the budget. In order to avoid conformity problems, it is essential that transportation agencies work closely with State and local air agencies to clearly document in the SIP, the motor vehicle emissions budgets to be used for transportation conformity purposes. When developing a conformity determination, interagency consultation should be used to ensure there is agreement on the applicable motor vehicle emissions budget.

TCMs in SIPs

Transportation control measures (TCMs) should be clearly defined in the SIP. Transportation agencies should work with State and local air agencies to ensure that any TCMs in the SIP are distinct, well-defined, and can be implemented according to schedule. This is important as Transportation Plans and TIPs must provide for timely implementation of TCMs in the SIP. Failure to provide for timely implementation of TCMs in the SIP may result in a conformity lapse, and the imposition of highway sanctions. The State DOTs and MPOs should also strongly encourage State and local air agencies to clearly identify the emission reduction benefits of the TCMs, and to credit such benefits toward the rate-of-progress and attainment demonstrations.

Partial Implementation of TCMs

Section 93.122(a)(2) indicates that the conformity emissions analysis may not take full emissions reduction credit for TCMs that have only been partially implemented. If measures such as inspection and maintenance (I/M) programs or reformulated gasoline (RFG) programs are modified from that which was approved in the SIP, the MPO and the State DOT should request State and local air agencies to identify the emissions consequences, and to develop additional measures to make up for any emissions shortfalls through a SIP revision. If the State and local air agencies fail to act, then the MPO will have to make corrective adjustments in any future conformity determinations to reflect incomplete implementation of TCMs.

Updated: 7/6/2011
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