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.