Emissions Impact of Elimination of the 55 tnph Speed Limit
http://www.epa.gov/OMS/reports/envspoms.htTTi
L/8/96
Information from the EPA Office of Mobile Sources
Emissions Impact of Elimination of the National
55 mph Speed Limit
President Clinton recently signed into law a bill that includes a provision eliminating 55/65 mph speed
limits as a prerequisite for Federal highway funding.Ill Several States have already acted to increase speed
limits on limited-access highways, and others are considering such action. This has led to questions
concerning the emissions impact of elimination of the national speed limit. This statement provides an
initial look at those impacts, and discusses activity in progress at EPA to better quantify those impacts.
Also, this statement from EPA's Office of Mobile Sources (OMS) provides reaction on the part of OMS to
the estimates of emission impacts made in an earlier memo from another EPA office. In response to
questions regarding the impact on emissions from highway vehicles of eliminating the national speed limit,
EPA's Office of Policy, Planning, and Evaluation (OPPE) released a memo dated November 30, 1995,
"Environmental Impacts of Removing National Speed Limit Requirements." This memo was provided to
EPA Regional Offices (Air Directors, Air Branch Chiefs, Air Section Chiefs, and Transportation Staff).
Available information
Estimating the overall emissions impact of elimination of the national 55/65 mph speed limits depends on a
number of assumptions, including: for each specific highway on which the speed limits is raised, what was
the pre-repeal speed limit and what is the new speed limit; what volume of traffic (vehicle miles traveled,
or VMT) is carried on highways with newly increased speed limits, and what proportion of total VMT in a
given area does this represent; and how much actual average speeds on each specific highway increase
after the change in the applicable speed limit (many highways that have been posted as having 55 or 65
mph limits are in reality characterized by higher speed traffic). These factors complicate the
characterization of the emissions impact of repeal of national speed limits.
Using the M0BILE5a highway vehicle emission factor model 121 to estimate exhaust emission factors for
calendar year 1996 at average speeds of 55 and 65 mph (with 8.7 psi RVP gasoline and summer
temperatures), the following trends are observed. Note that these estimates are for light-duty gas vehicles
(passenger cars) only; the effects for other vehicle types are not necessarily similar, as discussed below.
The difference in emission factors estimated for 55 and 65 mph is not directly applicable to rural highways,
where speed limits in many areas were already 65 mph and may now be increased further; however, it is
useful for looking at the effects of this change in urban areas, where speeds have been limited to 55 mph
and now may be increased to 65 mph in some areas.
Finally, these emission factors are for 100% stabilized operation (in other words, no "cold-starts" or
"hot-starts" are assumed — all traffic is assumed to consist of fully warmed-up vehicles). This is logical, in
that the roadways affected by elimination of the national speed limits are all limited access highways
(mostly interstates), and traffic on such roadways is characterized by virtually 100% stabilized operation.
Emissions
at 55 mph
Emissions
at 65 mph
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Emissions [mpacl of Elimination of the 55 mph Speed Limit
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Pollutant
(g/mi)
(g/mi)
% change
VOC
CO
NOx
0 . 59
7.60
2 .19
0 . 92
19.24
2 .40
+ 55.9
+ 153.0
+ 9.6
These increases are in the in-use fleet emission factor for automobiles actually traveling at the stated
average speeds. Estimating the impact on overall emission levels for any specific area and timeframe
requires assuming how much of total traffic (VMT) in the given area and timeframe will actually be subject
to the increased speed. For example, nationally about 13.5% of all VMT is on urban interstates (where
speed limit increases from 55 to 65 mph might be expected). If traffic on all such urban interstate highways
increased in average speed from 55 to 65 mph, the increase in total vehicle emissions would be about
13.5% of the values shown (in this case, increases in national total emissions from highway vehicles of
about 7.5% for VOC, 20.7% for CO, and 1,3% for NOx would be estimated as resulting from the speed
limit change on urban interstates).
EPA has only very recently begun to collect data from testing of vehicles at even higher speeds, up to 80
mph. Analysis of such data, which is almost entirely from relatively new, current technology automobiles,
is not complete. Indications are that emissions will continue to increase as speeds increase to above 65
mph. Data are not currently available for estimating the emissions impact of eliminating the national speed
limit for older cars and trucks.
The "real-world" impact of eliminating the national speed limit will depend in large part on the actual
increase in average traffic speeds on affected roadways, which is very difficult to estimate. Many States
had already raised rural speed limits to 65 mph under the 1987 law permitting this. In some specific cases,
average speeds are already so much above applicable 55/65 mph speed limits that they are unlikely to
increase much due to the elimination of the national speed limit; in such cases, if emissions estimates have
been based on currently posted speed limits, then those estimates are probably underpredicting actual
current emissions, but the increase in those emissions (actual, not modeled) due to the speed limit change
will be minimal. EPA's guidance to States has been to estimate emissions based on actual observed speeds
rather than posted speed limits wherever possible, so for States that have developed emission estimates
following this guidance, there may be only relatively limited changes (increases) in estimated total
emissions due to the elimination of national speed limits.
Also worth noting is the fact that the emission increases estimated for other vehicle types would not be the
same as those presented here for automobiles. The behavior of emissions as a function of average speed
varies by vehicle type, emission control technology, fuel delivery system (carbureted vs. fuel-injected
engines), and pollutant. Emissions from diesel vehicles (including heavy-duty diesel trucks), as presently
modeled, decrease slightly if average travel speeds for those vehicles is increased. (Although the speed
limits for heavy-duty trucks are typically lower than those for automobiles and light trucks, it is reasonable
to assume that increasing the speed limit for most traffic will increase average speeds on affected
highways, including the average speed of heavy-duty truck traffic.) As can be expected, there are
considerable uncertainties in this modeled response of heavy-duty truck emissions to speed limit changes.
Based on cruising operation at high speeds, rather than on average trip speeds, emissions from all types of
vehicles would be expected to increase if travel speeds increase to greater than 65 mph.
EPA will continue to collect and analyze data on emissions from vehicles at high speeds, and may issue an
updated statement on the effects of elimination of the national speed limit after collecting and analyzing
more relevant data.
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Emissions Impact of Elimination of ihc 55 mpli Speed Limit
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Comments on OPPE Estimates
1.	"Speed limit increases will raise NOx emissions by at least 5%"
The OPPE analysis was based on MOBILE5a emission factors and data from the Federal Highway
Administration (FHWA) on VMT by raodway type. The other assumptions made (e.g., what average
speeds are assumed before and after the national speed limit requirement is eliminated, what fraction of
traffic and in what areas is affected by this change, what temperatures and fuel volatilities are assumed,
etc.) are important in determining the accuracy of this estimate. The OPPE estimates assume that urban
area speed limits remain at 55 mph, which does not correspond to many state actions as reported in the
recent press. These estimates also assume that the rural speed limit goes to 65 mph in most states, though it
is unclear whether the base average speed for such roads was assumed to be 55 or 61 mph (cited as the
average speed for rural areas). This statement is certainly within the range of reasonable estimates, based
on the increase in the emission factors for automobiles presented above. It is unclear whether OPPE's
estimates are based only on automobiles, or on all vehicles.
2.	"State-by-State increases may be much higher"
OPPE's memo notes that the increase in NOx emissions could be "as much as 9% in portions of the 1-95
corridor from Virginia to Maine." OMS has not attempted to recreate the OPPE analysis, but it is apparent
that the effects of this change will vary by State, and some States will see larger emission increases than
others.
3.	"Speed limit increases will raise CO emissions"
Based on the emission factors presented above, the increases in CO will in fact be far larger than the
increases in NOx or VOC emissions.
4.	"Speed limit increases will raise C02 (greenhouse gas) emissions"
As fuel economy falls, fuel consumption rises, and C02 emissions increase. As in the other specifics, a
number of assumptions had to be made to develop such an estimate and those assumptions are not
provided in the memo. OMS agrees that directionally, higher speeds will lead to lowered fuel economy and
hence to increases in C02 emissions.
Footnotes
ULL The 55 mph national speed limit was introduced January 1, 1974 in response to the OPEC oil embargo and subsequent
"energy crisis." While this speed limit was not mandated by the Federal Government, highway funding was linked to adoption of
these speed limit, and its enforcement, by the States. In 1987, these provisions were revised to allow maximum speed limits of
65 mph to be adopted on limited-access highways outside of defined urban areas. Prior to the adoption of these limits, most
States had speed limits of 70 mph on limited-access highways, with some less densely populated States (primarily in the West)
having limits up to 75 mph and Montana and Wyoming having no specific speed limit in daytime hours. Only New York State
had a 55 mph speed limit before 1974.
121 EPA's official model for the estimation of in-use highway vehicle emission factors, currently MOBILE5a, allows emission
factors to be estimated for average speeds up to 65 mph. The model and the emissions data that support its estimated emission
factors are based on laboratory tests of vehicles operated over driving cycles of differing average speeds. Each of these driving
cycles represents a kind of trip, in that all driving cycles start and end at idle (0 mph), and include varying amounts and rates of
acceleration and deceleration and travel at different speeds; the average speed of a driving cycle is defined as total distance
traveled divided by total time elapsed. The higher the average speed of a driving cycle, the less idle time is included in the cycle.
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Flmissioris ImpLicL of Elimination of ihe 55 mph Speed Limit
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and the more driving at and above the average speed of the cycle is included within the cycle. The fact that MOBILE model
emission estimates are based on data from such trip-based cycles also complicates estimating the impacts of eliminating national
speed limits. Since cycle- based emission factors include accelerations and decelerations, and each cycle includes some travel at
speeds from zero to well above the average speed of that cycle, it is possible that emission increases resulting from the speed
limit change will be different than are estimated from the MOBILE model.
For example, the Highway Fuel Economy Test (HEET) cycle has an average speed of 48 mph, and 60 percent of the time of the
cycle is spent at speeds of 47 mph or greater. The highest average speed of a driving cycle for which EPA has significant data is
64.6 mph (California's ARB4 cycle); this cycle includes only 4 seconds of idle (0.6% of the total cycle time), and more than 60
percent of the cycle time is spent at speeds of 67.5 to 77.5 mph. Data from testing over these and other cycles form the basis of
the MOBILE model's estimation of average in-use emission factors as a function of average speed. It is worth noting that based
on the above driving cycle statistics, emission factors estimated by MOBILE5a for 65 mph average speed actually include
significant vehicle operation at speeds well over 65 mph; thus, the estimates provided below are more applicable to the situation
after an increase in the speed limit than might be apparent at first glance.
rOPPE Memo! IFulI Report I [QMS Home Pa gel
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Page 1 of 4
Date: November 30, 199 5
MEMORANDUM
Subject: Environmental Impacts o£ Removing National Speed Limit
Requirements
To: Regional Air Directors
Regional Air Branch Chiefs
Regional Air Section Chiefs
Regional Transportation Staff
From: Bob Noland, OPPE, 202-260-2418
Laura Gottsman, OPPE, 202-260-9247
Will Schroeer, OPPE, 202-260-1126
cc:	Robin Miles-McLean
Maryann Froehlich
Steve Lipman
Steve Cochran
Lucy Audette
Several people have been asked how the elimination of the
federal speed limit, recently signed into law as part of the
National Highway System Bill, will affect air quality. We have
done an analysis of the effect on auto emissions, and hope that
it is useful to you. You may want to share this information
with the state air quality agencies with whom you work. The
MOBILEBa runs on which the analysis is based are available upon
request.
Background
The 5 5 mph national speed limit was originally introduced on
Jan. 1, 1974, in response to the Arab oil embargo and subsequent
energy crisis. While this restriction was not mandated by the
Federal government, highway funding was linked to the adoption
of the speed limit (and its enforcement) by the states. In
1987, this was modified to allow maximum speed limits on rural
interstate freeways to increase to 65 mph. Prior to the
adoption of these limits, most states had speed limits of 70 mph
(with some having limits as high as 75 mph and Montana and
Wyoming having no maximum limits). Only one state, New York,
had a 55 mph speed limit prior to 1974, Speed limit increases
will raise NOx emissions by at least 5%
The National Highway System Bill just signed by the President
eliminates the federal national speed limit requirements for
non-commercial vehicles. Since emissions of the ozone pre-cursor
NOx increase as vehicle speeds increase above about 48 mph,
speed limit changes may have important consequences for ozone
nonattainment areas. Raising speed limits will affect the
ability of some areas to reach attainment status, and of other
areas to stay in compliance,
EPAs MOBlLESa emissions model shows that national NOx emissions
would increase at least 5 percent in the following scenario:
urban speed 'limits remain unchanged and rural speed limits
increase to 65 mph except that those states with limits below 65
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mph before 1974 would maintain those lower limits.
There are various reasons to believe this is a low estimate of
the impact. New York state, for example, recently increased its
rural speed limits to 65 mph, exceeding their pre-1974 maximum
speed limit. Several states have already increased their speed
limits to 70 and 75 mph. More states may follow suit; before
1974, most states had limits higher than 65 mph.
The 65 mph limit was modeled primarily because of the
1 imitations of MOBILE5a, which is only capable of analyzing
emissions at speeds up to 65 mph. In addition, actual average
driving speeds may exceed 65 mph, as is discussed further below.
State-by-state increases may be much higher. Under the above
scenario, MOBILE5a shows NQx emissions may increase as much as 9
percent in portions of the 1-95 corridor from Virginia to Maine.
* Increased NOx emissions may make it more difficult to meet
attainment deadlines, and increase costs of compliance with
NAAQS
Even if NOx emissions were only to increase in rural areas,
those emissions could hamper efforts of nonattainment areas to
reach attainment because NOx emitted in an attainment region one
day often migrates to a nonattainment region the next day.
If states decide that these increases in mobile NOx sources are
acceptable, they may face other costs to reduce ozone formation
and meet or maintain National Ambient Air Quality Standards
(NAAQS). For example, they may have to increase controls on
industrial sources of NOx, including utility and industrial
boilers. These controls will be more costly than maintaining
current speed limits. Employers may also have to consider more
stringent Traffic Control Measures to increase vehicle occupancy
levels for work trips.
* Speed limit increases will raise CO emissions
Increased motor-vehicle speeds are likely to also increase CO
emissions (Pechan, 199.2) . These emissions also result from the
combustion process and will increase at speeds above 4 8 mph.
Based upon similar reasoning, one could also expect increases in
particulate matter (PM).
* Speed limit increases will raise C02 (greenhouse gas)
emissions
Vehicle fuel economy decreases as vehicle speeds increase, and
markedly so above speeds of about 50 mph. The removal of
current speed limits would significantly increase fuel use for
the same amount of national vehicle travel, making Greenhouse
Gas reduction targets more difficult to meet. EPA analysis
indicates that carbon emissions would increase by 6-15 million
metric tons of carbon equivalent (mmtCe) per year, or about
6-15% of the amount needed to return U.S. emissions to 1990
levels in the year 2000.
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* Driver response is uncertain, but may contribute to
progressively higher travel speeds land therefore emissions)
over time
One unanswered question is how drivers will respond to higher
speed limits. Currently, a majority of drivers exceed the
posted speed limit (FHWA, 1994). Average travel speed on urban
interstates is about 59 mph, while on rural interstates it is
about 61 mph. Prior to the setting of national speed limits in
1974, the average on rural interstates was 65 mph (for
uncongested travel).
Generally, drivers prefer to maintain speeds similar to others
traveling near them. It can be anticipated that the removal of
legal restrictions against higher speeds (i.e., removal of the
cost of being cited for speeding} will result in an average
increase in speeds.
Increases in average speeds has been a consistent trend over the
last 20 years. This has been fueled partly by better and safer
road designs and by the design of safer automobiles. It is far
more likely that an automobile driver will survive an accident
today than 2 0 years ago. Therefore, the risks associated with
higher speeds are not as great as they once were, and hence, all
else equal, we can expect average speeds to exceed levels that
existed prior to 1974.
Faster automobile traffic will have other effects which are
likely to magnify the direct increase in emissions. Travel
times will be reduced due to higher speeds. This will encourage
people to use private automobiles rather than other modes of
travel (such as public transit). While travel times probably
will not be reduced .much in congested areas, many newly
developing areas not affected by congestion will see an increase
in motor-vehicle travel. Reduced travel times will also
encourage increased low density development. Both of these will
result in future increases in NQx emissions.
Increased speed limits on arterial roads (which many states are
expected to implement) will also increase the risk of travel to
both bicyclists and pedestrians. The impact will be to reduce
the use of these environmentally beneficial modes.
Many states will, in fact, be required by their own Speed limit
statutes to increase speed limits on arterial roads. These
speed limits are generally set by the 85th percentile rule.
This specifies that speed limits must be set at the speed at
which the 85th percentile of the traffic is moving. Higher
freeway speeds are likely to induce drivers to travel at higher
speeds on arterial roads (due to a decreased perception of their
actual speed when leaving a freeway). This will force many
states to increase existing speed limits on these arterials, if
they intend to abide by their own statutes.
The only indirect impact that may somewhat reduce any increase
in emissions is the higher cost of traveling at higher speeds,
due to decreased fuel efficiency. This is not expected to be a
major off-setting factor, due to the relatively low cost of
gasoline.
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References
FHWA, 1994; Federal Highway Administration, Highway Statistics
1993, Washington, DC 1994.
Pechan, 1992; E.H. Pechan & Associates, Sensitivity Analysis of
M0BILE4.1 Emissions Factors, Prepared for EPA Ozone/Carbon
Monoxide Programs Branch, Springfield, Virginia, July 1992.
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Analysis of the EITecls of Eliminating the National Speed Limit 011 NOx Emissions
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This report was prepared by E. H. Pechan and Associates for the U.S. Environmental Protection Agency,
Office of Policy, Planning, and Evaluation. For further information, please contact Bob Noland at
202-260-2418.
Analysis of the Effects of Eliminating the National
Speed Limit on NOx Emissions
Introduction
Highway vehicles contribute approximately one third of the oxides of nitrogen (NOx) emissions released to
the atmosphere in the United States annually (EPA, 1994). Since highway vehicles contribute such a large
percentage of NOx emissions, proposed modifications in the National Highway System bill are examined
in this analysis to determine their effect on this important source category. Specifically, the U.S. Senate has
recently proposed to abolish the national maximum speed limit. Such a policy modification may have a
significant impact on the magnitude of NOx emissions from motor vehicles. Motor vehicle NOx emissions
result from combustion processes and tend to increase with increasing speeds above 48 miles per hour
(mph) (Pechan, 1992). This analysis examines the potential consequences of the proposed Senate changes
to the National Highway System bill on highway vehicle NOx emission levels. The Senate recently voted
to repeal the national maximum speed limit on federally financed highways. The national maximum speed
limit rule currently restricts vehicle speed limits to 65 mph on rural freeways, and 55 mph for all other
corridors. The Senate proposal to repeal the national speed limit passed on June 19, 1995 with the
stipulation that federal speed limits still apply to commercial vehicles such as trucks and buses. If the bill
passes through the House of Representatives, States would have the authority to determine the maximum
allowable speed limits for automobiles within their State boundaries. Trucks and buses would still be
subject to the 55 mph urban and 65 mph rural limits. The national maximum speed limit was established in
1974; it restricted speed limits to 55 mph. The impetus for this regulation was to conserve fuel during the
1973 oil embargo and subsequent energy crisis. Prior to the national limit, States determined speed limits
for all corridors within their jurisdictions. Table 1 presents the speed limits maintained by States in January
1973, prior to implementation of the national maximum speed limit. Also presented in Table 1 are current
speed limits for automobiles and trucks, by Stale. In 1987, the maximum speed limit on rural freeways was
increased to 65 mph. Rural freeways are defined as freeways located in designated "rural" areas, with a
population less than 50,000. Freeways are "controlled access facilities," which means that access is by
ramp only and freeways are divided highways which usually consist of four lanes. The likely effect of the
recent Senate proposal abolishing the national speed limit will be increases in rural freeway speed limits
for automobiles. Since most States adhered to the 55 mph speed limit in urban areas before the national
maximum speed limit was established, changes in speed limits around metropolitan areas are expected to
be minimal (FHWA, 1995). As shown in Table I, speed limits on rural freeways before the national
maximum limit of 65 mph did not exceed 75 mph, with the exception of Montana and Nevada - which did
not establish maximum limits. Most maximum speed limits for rural corridors ranged between 65 and 75
mph with the majority of States setting rural speed limits of 70 mph.
This analysis assumes that Slates are most likely to raise the maximum speed limits on their rural freeways
to limits established prior to the national maximum speed limit and retain the 55 mph limit in urban areas.
Current Effects of Speeds Limits
Befoie estimating the likely effects that abolishing the national speed limit will have on interstate speeds, it
is helpful to examine the effectiveness of the 55 mph speed limit. Based on data from the Federal Highway
Administration (FHWA) published in the 1993 Highway Statistics report that analyzes trends in speeds on
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Analysis of the Effects of Eliminating the National Speed Limit on NOx Emissions
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highways signed for 55 mph, a majority of vehicles exceed this posted limit (FHWA, 1994).
In 1993, 70 percent of drivers exceeded 55 mph on urban interstate highways and 78 percent exceeded the
speed limit on rural interstate highways. Only highways with a posted 55 mph speed limit are included in
this average. The average speed recorded on the urban interstate highways was 59 mph and approximately
61 mph on rural interstate highways. In addition, 18 percent of drivers on urban interstates and 24 percent
of drivers on rural interstates were already exceeding 65 mph on 55 mph roads. Prior to the setting of the
national speed limit, FHWA reports that the average speed of free-moving traffic on level, straight,
uncongested sections of the rural interstate system reached a peak of approximately 65 mph in 1973. This
compares to a current average speed on rural interstates posted with 55 mph limits of 61 mph. It is
importani to consider that the average speed in 1973 was calculated at ideal conditions whereas the current
average speed includes all vehicle travel on the rural interstate system, not just the straight, uncongested
portions analyzed for the 1973 data.
Analysis Methods
This analysis was performed within the constraints of the MOBILE5a model. Therefore, the maximum
speed that could be examined was 65 mph. Two cases were compared with the 1994 highway vehicle NOx
emission inventory prepared for EPA's Emission Trends report-a maximum effects case and a likely
effects case.
In the base case analysis, the speeds modeled for each road type and vehicle type were determined using
data from FHWA's 1987 through 1990 HPMS impact analyses. Speeds varied less than 1 mph over this
time period for any given vehicle type/road type combination. Therefore, the 1990 data were used and
aggregated to determine average speeds for three vehicle classifications (light duty vehicles, light duty
trucks, and heavy duty vehicles).
Table 2 presents the national distribution of VMT by vehicle class and road type. Table 3 summarizes the
speeds modeled in the four road classes of interest (rural interstates, rural principal arterials, urban
interstates, and urban other freeways and expressways) in this analysis. Emissions were estimated at the
county, monthly, road type level. As seen in Table 2, these four road types account for 39 percent of the
total VMT. To get an estimate of the maximum effects of abolishing the national speed limit, speeds for all
vehicle types on all of the road types listed in Table 3 were modeled at a speed of 65 mph. This assumes
that urban speed limits would be increased as well as the rural speeds and that heavy duty vehicles would
also be able to increase to 65 mph. Due to factors such as congestion from heavy peak hour volumes, it is
unlikely that urban speeds would ever reach an average of 65 mph even if the speed limits on these roads
were increased. However, modeling this case gives an estimate of the upper bounds of possible emission
increases. Other than the change in speeds for these four road types, all other modeling was done
identically to the base case. A more realistic case was also examined. In this case, the base case speeds
were modeled for the heavy duty vehicles.
This corresponds with the version of the bill that has passed the Senate, in which these vehicle classes
would be subject to the current limits of 55 mph or 65 mph. In addition, it was assumed that urban speeds
for all vehicle types would remain unchanged from the base case, reflecting the reality of the lower average
speeds observed on urban interstates. Finally, the nine States shown in Table 1 to have maximum limits of
65 or lower prior to the setting of the national speed limit were modeled at the same speeds used in the
base case. As seen in Table 2, light-duty VMT on rural roads accounts for only 18 percent of the total 1994
VMT. With nine States modeled at base case speeds, the percentage of VMT modeled in this case at 65
mph would be even less than 18 percent. Again, all inputs other than speed used in the base case were
modeled in this case as well.
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Analysis of the Effects of Eliminating the National Speed Limit on NOx Emissions
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Results
Tables 4, 5, and 6 present the NOx emissions calculated for the base case, the maximum effects case, and
the likely effects case, respectively. These results are presented at the State level, and with emissions from
heavy duty vehicles separated from emissions from light duty vehicles. In this manner, results from the
three cases can easily be combined in other configurations if changes occur in the present status of
disallowing heavy duty vehicles from the higher speeds, or if urban speeds in certain States are likely to
approach the higher rural speeds. Table 7 compares the results from the likely case with the base case
results.
As seen in this table, NOx emissions are projected to increase by 5.2 percent with the elimination of the
national speed limit.
States such as Montana, North Dakota, and South Dakota that have high rural interstate VMT relative to
urban VMT show the greatest increases in NOx. In contrast. States like New Jersey, California, and Illinois
that have high urban VMT relative to rural VMT show relatively small emission increases resulting from
the change in speeds.
Related Issues
This analysis was completed using MOBILE5a.
The MOBILE model is designed to model vehicle speeds up to 65 mph. Therefore, it was not possible to
use to the MOBILE model to estimate emissions resulting from vehicle speeds in the 70 to 75 mph range.
Forty-two States posted speed limits above 70 mph prior to the maximum national speed limit set in 1974.
It is, therefore, realistic that the majority of States will post speed limits in this range again. A rough
estimate using the State of Texas as a sample indicates an increase in NOx emissions of an additional 11
percent for light duty vehicles on rural corridors if emissions are modelled using an emission factor
reflecting emission rates applicable to vehicles travelling at 70 mph. This example was calculated
assuming a linear increase in the emission factor with speed after 48 mph. This example illustrates that the
effects of repealing the maximum national speed limit on NOx emissions may be more dramatic than the
results in this analysis ~ using MOBlLESa — imply. Although this analysis examines the potential increase
in highway vehicle NOx emissions as a result of increased automobile speeds on rural highways, carbon
monoxide (CO) emissions are likely to increase as well.
Both CO and NOx emissions result from combustion processes. At lower speeds, around 15 mph, motor
vehicle emissions of CO and NOx decrease with increases in vehicle speed as a result of more efficient
combustion. However, after 48 mph, increases in vehicular speeds are accompanied by increases in
emissions of both CO and NOx (Pechan, 1992). Thus, increases in highway vehicle CO emissions are also
likely as a result of the proposed changes to the National Highway System bill. Using the same reasoning,
particulate matter (PM) emissions may also increase.
Conclusions
The results of this analysis indicate a 5 percent increase in motor vehicle NOx emissions annually on a
national level. Current controls on NOx emissions are focused in ozone nonattainment areas where NOx
and HC emissions react with sunlight to form tropospheric ozone. Although NOx emissions will increase
on a national level as a result of repealing the maximum national speed limit on rural corridors, the net
effect on ozone levels will likely be much less significant. Ozone nonattainment areas are predominantly
urban areas where automobile speed limit changes are not as likely to occur. Despite the effect of transport,
the increases in NOx shown in this analysis on a national level may not necessarily contribute to parallel
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Analysis ol'thc Effects of Eliminating the National Speed Limit on NOx Emissions
http://www.epa.gov/OMS/reports/env-spds.htm
increases in ozone formation. In order to more accurately assess the implications on air quality of the
proposed Senate rule, a more thorough analysis should be conducted which analyzes increases in highway
vehicle CO and PM emissions as well. Moreover, emission estimates should be determined using an
appropriate correction factor to estimate emissions generated at 70 mph speeds. The effect of increased
motor vehicle NOx emissions resulting from light duty vehicle speed increases on rural corridors on ozone
nonattainment areas also deserves more in depth attention.
References
EPA, 1994; U.S. Environmental Protection Agency, "National Air Pollutant Emission Trends, 1900-1993"
OAQPS, Research Triangle Park, North Carolina, October, 1994.
FHWA, 1994; Federal Highway Association, "Highway Statistics 1993," Washington, DC, 1994.
FHWA, 1995; Federal Highway Association. Personal Communication with Ms. Julie Cirillo, June, 1995.
Pechan, 1992; E.H. Pechan & Associates, "Sensitivity Analysis of MOBILE4.1 Emission Factors,"
Prepared for EPA Ozone/Carbon Monoxide Programs Branch Springfield, Virginia, July 1992.
Table 1 Maximum State Speed Limits
State
State Speed Limit
Prior to 1974
(Jtuph)
Current
Speed Limits
(mph)
Comments
Automobiles Trucks
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
DC
Florida
Georgia
Hawai i
Idaho
111inois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
70
70
75
75
70
70
60
60
60
70
70
70
70
70
70
75
75
70
70
70
70
65
65
65
65
65
65
65
55
55
55
65
65
55
65
65
65
65
65
65
65
65
55
65
65
65
65
65
55
65 Trucks lower in mountains
55
55
55
65
65
55
65
55
60
65
65
65
65
65
55 65 mph on July 1, 1995
65 65 mph on Mass. Tnpk
only. All other
locations 55 mph
Michigan
Minnesota
70
65
70
70
65
65
65
65
55
65
65
60
Mississippi
Missouri
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Montana
unlimited
65
65
Nebraska
75
65
65
Nevada
unlimited
65
65
New Hampshire
70
65
65
New Jersey
70
55
55
New Mexico
70
65
65
New York
55
55
55
North Carolina
70
65
65
Worth Dakota
75
65
65
Ohio
70
65
55
Oklahoma
70
65
65
Oregon
75
65
55
Pennsylvania
65
55
55
Rhode Island
60
55
55
South Carolina
70
65
65
South Dakota
75
65
65
Tennessee
75
65
65
Texas
70
65
60
Utah
70
65
65
Vermont
65
65
65
Virginia
70
65
65
Washington
70
65
60
West Virginia
70
65
65
Wisconsin
70
65
65
Wyoming
75
65
65
65 raph on August 1, 1995
- all vehicles
65 mph July 13, 1995
Trucks limited to 60 mph
daytime and 55 mph
nighttime
Source: Department of Transportation/Federal Highway Association
Table 2 1994 Vehicle Miles Travelled (VMT) by Road Type and Vehicle Class
Road
Type
RURAL
Total VMT
Percent
of Total
VMT
(Heavy
Duty)
Percent
of Total
VMT
(Light
Duty)
Percent o£
Total VMT
Interstate
Other Principal Arterial
Minor Arterial
Major Collector
Minor collector
Local
Total Rural
214,757
209,017
153,503
183,281
49,932
105,430
915,919
4.5
8.1
7.9
5.8
6.9
1.9
4.0
34.6
9.1
8.9
6.5
7.8
2.1
4.5
39.0
URBAN
Interstate	322,023
Other Freeways &	144,284
Expressways
Other Principal Arterial	360,146
Minor Arterial	279,419
Collector	122,536
Local	202,918
Total Urban	1,431,32 5
0 . 6
0.3
0.7
0.5
0.2
0.4
2 . 8
13.1
5 . 9
14 . 6
11.4
5 . 0
8.3
58 .2
13 ,
6 ,
15,
11,
5 .
8 .
61. 0
TOTAL
2,347,244
7.2
92.8
100 . 0
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Table 3 Average Speeds by Road Type and Vehicle Type Modeled for the Base Case
Vehicle Type
Light Duty
Vehicles
Light Duty Trucks
Heavy Duty
Vehicles
Rural
Interstate
60 mph
55 mph
40 mph
Urban Other
Principal
Arterial
45 mph
45 mph
35 mph
Interstate
45 mph
45 rrtph
3 5 mph
Freeways
& Expressways
45 mph
45 mph
35 mph
Table 4
1994 Highway Vehicle NOx Emissions
with Current Speed Limits (Base Case)
1994 Base Case NOx Emissions (tons)
State
Light-Duty-
Vehicles
Heavy-Duty-
Vehicles
All
Vehicles
Alabama
108
578
53,320
161,898
Alaska
10
843
4, 500
15,343
Arizona
78
660
35,363
114,023
Arkansas
57
374
30,694
88,067
California
500
401
213,317
713,718
Colorado
75
598
32,384
107,982
Connecticut
64
465
23,245
87,710
Delaware
15
142
6, 902
22,044
District of Columbia
7
737
2, 286
10,023
Florida
252
135
108,530
360,665
Georgia
179
504
81,615
261,119
Hawaii
16
470
6, 983
23,453
Idaho
28
555
14,902
43,457
Illinois
213
507
83,066
296,573
Indiana
147
618
68,419
216,037
Iowa
63
270
31,458
94,727
Kansas
58
571
27,933
86,504
Kentucky
93
233
45,802
139,035
Louisiana
76
049
38,144
114,193
Maine
30
784
16,103
46,887
Maryland
98
724
38,949
137,673
Massachusetts
108
300
36,795
145,095
Michigan
206
614
83,471
290,085
Minnesota
105
734
45,987
151,721
Mississippi
62
248
35,023
97,270
Missouri
131
072
59,523
190,594
Montana
22
571
11,854
34,425
Nebraska
36
876
18,277
55,152
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Nevada
25,996
11,843
37,839
New Hampshire
26,392
12,325
38,717
New Jersey
129,949
47,339
177,288
New Mexico
44,555
22,952
67,507
New York
255,046
98,126
353,172
North Carolina
162,538
78,311
240,850
North Dakota
16,118
8, 190
24,308
Ohio
233,350
97,841
331,190
Oklahoma
83,252
39,856
123,108
Oregon
68,076
32,079
100,156
Pennsylvania
211,943
92,727
304,670
Rhode Island
16,431
5,371
21,802
South Carolina
83,695
43,879
127,574
South Dakota
19,791
10,472
30,263
Tennessee
123,994
56,331
180,325
Texas
379,434
160,823
540,257
Utah
41,066
16,572
57,637
Vermont
15,528
7, 698
23,227
Virginia
151,518
70,200
221,718
Washington
111,867
43,970
155,838
West Virginia
40,811
21,793
62,603
Wisconsin
120,804
55,905
176,709
Wyoming
17,972
9, 503
27,475
Total
5,230,757
2,298,948
7,529,705
Table 5
1994 Highway Vehicle NOx Emissions
with Maximum Effects of New Speed Limit
(Max Case)
1994 Max Case NOx Emissions (tons)

Light-Duty
Heavy-Duty
All
State
Vehicles
Vehicles
Vehicles
Alabama
127,531
68,944
196,476
Alaska
12,626
5, 977
18,603
Arizona
91,844
46,780
138,623
Arkansas
70,699
42,242
112,941
California
659,405
296,651
956,056
Colorado
93,065
45,069
138,134
Connecticut
84,009
31,378
115,387
Delaware
18,844
9, 096
27,939
District of Columbia
9, 211
2, 709
11,920
Florida
299,930
144,682
444,612
Georgia
217,807
107,692
325,499
Hawaii
20,091
8, 159
28,250
Idaho
33,742
19,813
53,555
Illinois
253,580
107,783
361,363
Indiana
174,579
89,146
263,725
Iowa
76,480
43,497
119,978
Kansas
71,893
37,959
109,851
Kentucky
113,245
61,522
174,767
Louisiana
89,839
49,970
139,809
Maine
35,515
20,660
56,175
Maryland
126,817
53,138
179,955
Massachusetts
137,296
48,748
186,044
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Michigan
252
893
109,273
362,166
Minnesota
132
464
62,094
194,558
Mississippi
72
498
45,020
117,518
Missouri
167
001
83,125
250,126
Montana
27
322
17,026
44,348
Nebraska
43
766
24,731
68,497
Nevada
30
988
16,321
47,309
New Hampshire
32
351
16,897
49,248
New Jersey-
159
460
61,426
220,886
New Mexico
52
269
31,958
84,227
New York
317
253
126,359
443,612
North Carolina
192
285
100,044
292,330
North Dakota
19
531
11,508
31,039
Ohio
282
603
126,366
408,969
Oklahoma
100
295
52,658
152,953
Oregon
84
930
45,058
129,988
Pennsylvania
254
841
121,159
376,000
Rhode Island
20
489
S, 998
27,487
South Carolina
98
405
58,821
157,226
South Dakota
23
804
14,612
38,416
Tennessee
148
259
75,407
223,667
Texas
476
931
216,639
693,570
Utah
50
521
23,425
73,946
Vermont
18
131
10,162
28,293
Virginia
182
925
93,046
275,971
Washington
142
421
60,383
202,804
West Virginia
48
686
29,510
78,196
Wisconsin
144
875
74,397
219,271
Wyoming
21
408
13,445
34,853
Total
6,417j
650
3,069,488
9,487,138
Modeled with 65 mph speeds on rural interstates and principal arterial roads and on urban interstate and
other freeways and expressways for all vehicle types.
Table 6
1994 Highway Vehicle NOx Emissions
with Likely Effects
of New Speed Limit
(Likely Case)
1994 Likely Case NOx Emissions (tons)

Light-Duty-
Heavy-Duty
All
State
Vehicles
Vehicles
Vehicles
Alabama
119,344
53 , 320
172,664
Alaska
11,747
4, 500
16,247
Arizona
84,239
35,363
119,602
Arkansas
66,001
30,694
96,694
California
527,430
213,317
740,748
Colorado
82,742
32,384
115,125
Connecticut
64,465
23 , 245
87,710
Delaware
15,142
6, 902
22,044
District of Columbia
7, 737
2 ,286
10,023
Florida
274,081
108,530
382,611
Georgia
193,385
81,615
275,000
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Analysis of the Effects of Eliminating the National Speed Limit on NOx Emissions
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Hawaii
16,561
6,
983
23
544
Idaho
32,405
14
902
47
307
Illinois
226,292
83
066
309
359
Indiana
161,397
68
419
229
816
Iowa
73,464
31
458
104
922
Kansas
66,126
27
933
94
059
Kentucky-
104,264
45
802
150
065
Louisiana
83,044
38
144
121
188
Maine
34,433
16
103
50
536
Maryland
105,618
38
949
144
567
Massachusetts
103,300
36
795
145
095
Michigan
222,395
83
471
305
866
Minnesota
105,734
45
987
151
721
Mississippi
69,836
35
023
104
859
Missouri
146,705
59
523
206
227
Montana
26,957
11
854
38
811
Nebraska
42,201
18
277
60
478
Nevada
28,532
11
843
40
375
New Hampshire
29,945
12
325
42
270
New Jersey
136,708
47
339
184
047
New Mexico
50,320
22
952
73
271
New York
255,046
98
126
353
172
North Carolina
177,367
78
311
255
678
North Dakota
19,168
8
190
27
358
Ohio
248,014
97
841
345
855
Oklahoma
91,459
39
856
131
315
Oregon
77,448
32
079
109
528
Pennsylvania
211,943
92
727
304
670
Rhode Island
16,431
5
371
21
802
South Carolina
93,254
43
879
137
132
South Dakota
23,296
10
472
33
768
Tennessee
135,117
56
331
191
448
Texas
407,912
160
823
568
734
Utah
44,536
16
572
61
108
Vermont
15,528
7
698
23
227
Virginia
165,109
70
200
235
309
Washington
120,682
43
970
164
653
West Virginia
46,365
21
793
68
158
Wisconsin
136,212
55
905
192
118
Wyoming
20,931
9
503
30
434
Total
5,623,370
2,298,
948
7,922,
318
Modeled with 65 mph speeds on rural interstates and principal arterial roads for light duty vehicles and
trucks only and only in states with former limits above 65.
Table 7
Comparison of Base Case and Likely Case
1994 Highway Vehicle NOx Emissions
State
Alabama
1994 Total NOx Emissions
from All Vehicle Types
(tons)
Base Case Likely Case
161,898	172,664
Increase in
1994
NOx
Emissions
(tons)
10,766
Percentage
Increase
in Emissions
6 . 65
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Alaska
15
, 343
16
, 247

904
5
. 89
Arizona
114,
, 023
119
, 602
5
, 579
4
. 89
Arkansas
88,
, 067
96
, 694
8
, 627
9
.80
California
713,
, 718
740
, 748
27
, 030
3
.79
Colorado
107,
, 982
115
, 125
7
, 143
6
.62
Connecticut
87
,710
87
, 710

0
0
.00
Delaware
22,
, 044
22
, 044

0
0
. 00
District of Columbia
10,
, 023
10,
, 023

0
0
. 00
Florida
360,
, 665
382
, 611
21
,946
6
.08
Georgia
261,
, 119
275,
, 000
13
, 881
5
.32
Hawa i i
23,
, 453
23
,544

91
0
.39
Idaho
43,
,457
47,
, 307
3,
, 850
8
.86
Illinois
296,
, 573
309,
, 359
12,
,786
4
.31
Indiana
216,
, 037
229,
, 816
13,
, 779
6
.38
Iowa
94,
, 727
104,
, 922
10
, 195
10
.76
Kansas
86,
, 504
94,
, 059
7,
, 555
8
.73
Kentucky-
139,
, 035
150,
, 065
11,
, 030
7
.93
Louisiana
114,
, 193
121,
, 188
6,
, 996
6 ,
. 13
Maine
46,
, 887
50,
, 536
3,
, 649
7 ,
. 78
Maryland
137,
, 673
144,
, 567
6,
, 895
5,
. 01
Massachusetts
145,
, 095
145,
, 095

0
0
. 00
Michigan
290,
, 085
305,
,866
15,
, 781
5 ,
.44
Minnesota
151,
, 721
151,
, 721

0
0.
. 00
Mississippi
97,
, 270
104,
, 859
7,
, 588
7 .
.80
Missouri
190,
, 594
206,
,227
15,
, 633
8
.20
Montana
34,
,425
38,
,811
4,
, 386
12,
.74
Nebraska
55,
152
60,
,478
5,
, 325
9 ,
, 66
Nevada
37,
, 839
40,
, 375
2,
, 536
6 ,
,70
New Hampshire
38,
717
42,
270
3,
, 553
9 .
. 18
New Jersey
177,
288
184,
, 047
6,
, 758
3 .
,81
New Mexico
67,
507
73,
,271
5,
, 764
8 ,
.54
New York
353,
172
353,
172

0
0,
00
North Carolina
240,
850
255,
678
14,
, 828
6 ,
, 16
North Dakota
24,
308
27,
358
3,
050
12 .
55
Ohio
331,
190
345,
855
14,
, 664
4.
43
Oklahoma
123,
108
131,
315
8,
207
6 .
67
Oregon
100,
156
109,
528
9,
372
9 ,
36
Pennsylvania
304,
670
304,
670

0
0 .
00
Rhode Island
21,
802
21,
802

0
0 .
00
South Carolina
127,
574
137,
132
9,
559
7 .
49
South Dakota
30,
263
33,
768
3,
505
11.
58
Tennessee
180,
325
191,
448
11,
123
6 .
17
Texas
540,
257
568,
734
28,
477
5 .
27
Utah
57,
637
61,
108
3,
471
6 .
02
Vermont
23,
227
23,
227

0
0 .
00
Virginia
221,
718
235,
309
13,
591
6 .
13
Washington
155,
838
164,
653
8,
815
5 .
66
West Virginia
62,
603
68,
158
5,
554
8 .
87
Wisconsin
176,
709
192,
118
15,
409
8 .
72
Wyoming
27,
475
30,
434
2,
959
10.
77
Total
7,529,
705
7,922,
318
392,
613
5.
21
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