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Related documents:
ENVSPOMS.TXT (this 1/8/96 OMS memo)
ENVSPMEM.TXT (11/30/95 OPPE memo)
ENV-SPDS.TXT or ENV-SPDS.W51 (E.H. Pechan report:
Analysis of the Effects of Eliminating the National
Speed Limit on NOx Emissions)
For further information on this issue, please contact:
Bob Noland, EPA/OPPE 202-260-2418.
Laura Gottsman, EPA/OPPE 202-260-9247
Will Schroeer, EPA/OPPE 202-260-1126
Terry Newell, EPA/OMS 313-668-4462
01.08.96
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.[1] 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 MOBILE5a highway vehicle emission factor model[2] 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 Emissions
at 55 mph at 65 mph
Pollutant (g/mi) (g/mi) % change
VOC 0.59 0.92 + 55.9
CO 7.60 19.24 + 153.0
NOx 2.19 2.40 +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.
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 I-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 CO2 (greenhouse gas)
emissions"
As fuel economy falls, fuel consumption rises, and CO2 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 CO2 emissions.
Footnotes
[1] 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.
[2] 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, 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 (HFET) 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.
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