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Technical

Highlights

 Printed on Recycled Paper

United States

Environmental Protection

Agency

Air and Radiation EPA420-F-98-023

May 1998

Office of Mobile Sources

Clean Screening in Inspection and

Maintenance Programs

Background

Clean screening is the term used to describe methods that states can use

to excuse cars from a scheduled I/M emissions test. Typical I/M programs require every car to appear at a testing station once each year or

every other year for a test of the vehicle’s tailpipe emissions; some states

test other emissions components–such as the gas cap and the fuel lines–

as well. Over the last several years, vehicle emission systems have

become more reliable and durable, and many cars pass the I/M test.

Guidance Issued

EPA’s draft guidance has been distributed for state comment and independent scientific peer review, and EPA is accepting comments on the

guidance until July 11, 1998. Once EPA has reviewed the comments and

incorporated them as appropriate, the guidance will be finalized in the

fall of 1998.

EPA has recently completed a draft guidance document on the use of

clean screening in Inspection and Maintenance (I/M) programs. Clean

screening is designed to exempt certain cars from the I/M requirement,

on the strength of other evidence of the high probability that they are

clean enough to pass anyway. Based on a preliminary assessment of

data currently available, EPA believes it is possible to excuse up to onethird of cars from inspection each year, with only a 5 to 10 percent loss

in emission reductions.
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The options included in the draft guidance

are aimed at making I/M programs more

efficient and cost effective for states and

car owners, by focusing inspections on

vehicles more likely to be high emitters in

need of repair. There will be some increase

in vehicle emissions as a result of the

changes in these I/M programs. It is

important to note that the estimates included in the draft guidance are based on

the latest and best information currently

available. However, EPA expects at least

certain aspects of the estimates to change

somewhat before they are finalized. Once

the clean screening guidance is finalized,

EPA may revise the estimates from time to

time as better information and understanding of clean screening options becomes

available.

The draft guidance provides descriptions

and projected emission impacts for three

types of clean screening, which differ with

respect to the type of evidence required to

exempt cars from regular I/M. These are

listed below:

Remote Sensing Clean

Screening

Overview

In this concept, roadside remote sensing is

used to excuse specific cars from their next

annual or biennial I/M test because they

pass remote sensing. Obviously, this has

owner convenience and cost advantages.

After skipping one scheduled test, cars

would have to get the next one, unless they

again pass a remote sensing test. In practical application, cars would have to pass

two different remote sensing tests, at

different times or places, and within a

limited time window (e.g., no more than

twelve months before the scheduled I/M

test).

Effect on vehicle population

The number of vehicles excused from

testing depends on the stringency of the

remote sensing test and the fleet coverage

of the remote sensing program. Up to fifty

percent coverage of the fleet has been

demonstrated; more should be possible

with resources and experience. A 50 percent pass rate on the remote sensing test

and 80 percent fleet coverage for example

would mean that about 40 percent of the

fleet would be excused each year. This is

likely to be the highest pass rate and

coverage an area should consider. An

exemption fraction of one-third may be

more typical.

SIP credit ramifications

Use of remote sensing for clean screening

will typically reduce the credit ascribable

to the I/M program because some cars with

high tailpipe emissions may appear clean in

a remote sensing test and will be excused

from I/M tailpipe testing and repair for that

I/M cycle. Also, remote sensing cannot

identify low versus high emitting vehicles

with respect to evaporative HC emissions.

However, older cars have a much higher

incidence of evaporative problems than

newer cars, and remote sensing clean

screening tends to fail most older cars

because they so frequently have defective

parts causing high tailpipe emissions.

When such older cars with both high

tailpipe and high evaporative emissions

report for their regular I/M test, having

failed to pass the remote sensing tailpipe
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test, both problems can be identified and

fixed. Therefore, the loss of evaporative

HC credits is much less than the portion of

the fleet exempted, but may be as or more

significant than the loss of tailpipe credit.

 An analysis performed for one state example indicates that by using remote

sensing cutpoints that excuse 37 percent of

remotely tested vehicles with a fleet coverage of 80 percent, the loss in tailpipe HC

benefit is about 4 percent and the loss in

overall HC benefit is 5 to 8 percent. By

using remote sensing clean screening to

measure NOx with a tight NOx cutpoint,

NOx benefit losses can be limited to 6

percent, for the 80 percent coverage example. If only HC and CO cutpoints are

used, the NOx benefit loss can be as high

as 22 percent.

However, it is important to note that use of

remote sensing clean screening for NOx

reduces the number of cars that are excused

from regular testing. The effectiveness of

remote sensing varies among cars of

different model years, so the overall effect

will depend on the calendar year of interest

and the mix of cars on the road.

Vehicle Emissions Profiling

Overview

This is a method of ranking vehicles for the

likelihood that they need emissions repairs,

using statistics on the historic failure rate

of vehicles of very similar design. For

example, failure rates of vehicle models in

the Arizona IM240 program can be used to

predict whether a certain group of nearly

identical cars in another state will have a

high or low failure rate. This information

can be used in either of two ways, or in

both ways.

In Low Emitter Profiling, the ranking

would be used as a clean screening tool to

determine what cars should be excused

from testing. In High Emitter Profiling, the

ranking is used to require some type of

special testing regime for the cars most

likely to need repair. This special testing

could involve special testing stations,

annual instead of biennial testing, etc. It is

possible to add other information into the

ranking process. For example, remote

sensing readings or the most recent I/M

outcome of the specific vehicle in question

could be used, assuming such data is

available. Generally, extra information can

improve the ranking’s accuracy, and

thereby reduce the credit loss from screening out a given number of vehicles from

their next regular I/M test.

Effect on vehicle population

This varies and is dependent on a number

of factors, including the exemption fraction, the fleet mix, and the information

used in the vehicle ranking process. For

example, by expanding the ranking information used, states can fine-tune their

profiling. Thus, the effect on the vehicle

population can be customized to a certain

degree–dependent on the fleet mix and

sophistication of the vehicle ranking

process–within the goal of clean-screening

a specific percentage of the vehicle population. Credit losses increase quickly when

the exemption fraction is higher than 50

percent.
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Credit ramifications

Low emitter profiling carries a credit loss

due to the fact that some cars will be

incorrectly clean screened out of traditional

I/M testing. In general, recent studies

indicate that vehicle profiling can support

clean screening with roughly the same

immediate emission credit ramifications as

remote sensing (i.e., in the range of 5 to 8

percent loss in credit) for all pollutants.

This, however, is also dependent on the

specific profiling utilized by the state. The

draft guidance contains a special note

regarding the preliminary and evolving

nature of the estimates of credit losses with

low emitter profiling.

Model Year Exemptions

Overview

This approach exempts cars until they

reach a certain age, on the premise that

virtually all cars are clean when sold and

most remain clean for at least several years.

Many states now wait until a car is four or

five years old before the first required

inspection. Some give the owner the option

of inspection during these years. It is the

state’s choice whether to assess a program

fee on new cars while they are exempt

from testing.

Effect on vehicle population

Again, the effect varies, given the fleet mix

(i.e., vehicle age distribution) of a specific

area. EPA recommends that states exempt

cars from testing until they are at least four

years old.

Credit ramifications

The effect of model year exemptions in

specific I/M areas can be estimated using

existing features of MOBILE5b. States

with a simple system of scheduling inspections on the anniversary of a new vehicle

sales date should have no difficulty analyzing scenarios for themselves. Overall, the

loss of credit for exempting the four newest

model years is quite small, and EPA believes most states can find ways to compensate for it if needed to keep the SIP

approvable. States with more complex

scheduling may need to consult EPA.

For More Information

A copy of the draft guidance and related

information is available electronically from

the EPA Internet server at:

http://www.epa.gov/oms/models.htm

For further information on the draft guidance, please contact Joe Somers at:

U.S. Environmental Protection Agency

Assessment and Modeling Division

2000 Traverwood Drive

Ann Arbor, MI 48105

Phone: (734) 214-4321

somers.joseph@epa.gov