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Effect of speed reduction on emissions of heavy
duty lorries
L Int Panis, I De Vlieger, L Pelkmans, L Schrooten
Flemish Institute for Technological Research (VITO), Boeretang 200,
B-2400 Belgium
Abstract
In many European countries the speed limit for trucks is under discussion
or review. The speed limit for heavy trucks is 80 km h
-1
in most countries,
but 90 km h
-1
in Belgium. We investigated the effect of reducing the speed
limit on fuel consumption and emissions of CO
2
, NO
x
and PM.
To ensure robust conclusions under a strict deadline, our evaluation used
two existing, complementary approaches: the macroscopic emission model
TEMAT and the microscopic emission simulation model VeTESS. Both
models show a CO
2
reduction between 5 and 15 %. The results for NO
x
and PM were ambiguous.
Introduction
In June 2005 the Flemish transport Minister proposed to lower the maxi-
mum speed for trucks on highways from 90 to 80 km h
-1
. This resulted in
an enormous wave of critique from various stakeholders. Reference was
made to time losses, economic losses and serious doubts were cast over the
assumed environmental and safety benefits. Unfortunately most of the dis-
cussion was on the basis of ideology and prejudice. Scientific analysis was
either ignored or was unavailable for use in the discussion at that time.
Given the strict deadlines for the debate in parliament, VITO conducted a
2 Int Panis L, De Vlieger I, Pelkmans L, Schrooten L
fast screening to assess the possible effects on emissions of the proposed
policy. We defined the scope of the study to include only two scenarios:
1. A reduction of the maximum speed from 90 km h
-1
to 80 km h
-1
for
trucks (12-40 tonnes; equipped with a speed limiter) on motorways.
2. A reduction of the maximum speed of trucks having a gross weight
between 3.5 and 12 tonnes (those currently not equipped with a speed
limiting device) from 100 km h
-1
to 90 or 80 km h
-1
.
Only the direct effect of the speed reduction on the emissions of trucks
was taken into account. Second and third order effects on the speed distri-
bution of other vehicles and their emissions were not included in this
screening. For this reason the third question of the minister, about the ef-
fect of a complete ban on overtaking by trucks, was not answered because
it would require the use of sophisticated traffic simulation models which
was far outside the scope of the allocated budget.
Methodology
Scope, general approach and models used
Given the context it was decided to perform a screening based on existing
tools to ensure that existing scientific knowledge would be used in the po-
litical discussions. In this respect it reflects a common situation for scien-
tists involved in policy advice. Because there was broad agreement that the
emissions to air would be the dominant impact, it was decided to estimate
the emissions for the three most important pollutants in the transport sec-
tor: CO
2
NO
x
and PM [1,2]. To ensure robust conclusions the problem was
tackled from three angles simultaneously. We discuss the results of two
complementary models and compare them with results from an inquiry
among truck manufacturers.
The fleet based emission model TEMAT
Fleet emission factors (in grams per vehicle.km) were estimated with the
macroscopic model TEMAT [3]. The model essentially uses well estab-
lished COPERT III emission functions but was updated with information
from our own emission measurements [4] and results from the Emission
Factor Handbook [5, 6] the COST 346 report as well as the European
ARTEMIS project (mainly for NO
x
and PM) because earlier versions
Effect of speed reduction on emissions of heavy duty lorries 3
Formatted: Font: Bold
tended to underestimate the “real” on the road emission for some of the
more advanced engines (Euro 2 and later standards). The model was fed
with both theoretical maximum speeds as well as generalized average
speeds on highways (which are typically between 3 and 6 % lower).
There are two disadvantages to using the TEMAT model in this study.
1. A break at 12 tonnes, important from a policy viewpoint, was
unavailable from the TEMAT model (developed from a technical
perspective for optimal emission modelling cfr. COPERT III).
2. COPERT functions for heavy duty vehicles are inherently unreliable
at speeds above 100 km h
-1
.
The vehicle based VeTESS model
The same scenarios were also evaluated with a completely different model.
VeTESS is a microscopic model that estimates fuel consumption and
emissions for a single vehicle on the basis of specific (second by second)
speed profiles, gear choice and the efficiency of all elements of the power
train and other vehicle characteristics. VeTESS calculates total engine
power demand and uses 3D engine maps to estimate emissions.
The main disadvantage of this complex model is that detailed dynamical
engine maps are available for only three trucks. All three comply with the
Euro 2 standard. Nevertheless differences found between these trucks may
hint at the reliability of the results. The reader is referred to [7] for a de-
tailed description of the VeTESS model.
For the current situation the model was run with a compilation of speed
profiles measured on Flemish highways in normal traffic.
The maximum
speed is legally limited to 90 km h
-1
and the average real speed is approxi-
mately 86-87 km h
-1
. Small variations that occur between 85 and 90 km h
-1
can be attributed to the presence of other vehicles. The measured speed
profiles were then converted to lower speeds to reflect a change in the le-
gal speed limit. The speed variation was left unchanged whereas the aver-
age speed in scenario 1 was 77-78 km h
-1
.
Integrated analysis and uncertainty
To arrive at an integrated conclusion and test its robustness, emission re-
ductions for the entire fleet were converted to external environmental costs
using the methodology described in [8]. Uncertainty was assessed by
Monte Carlo techniques in which single value assumptions were replaced
by probability distributions. The assessment of uncertainty in external
Deleted: Effect of speed
reduction on emissions of heavy
duty lorries
4 Int Panis L, De Vlieger I, Pelkmans L, Schrooten L
costs was formalized by [9] and the use of MC techniques in the context of
transport policy is described in detail in [10].
Results
General context
Heavy trucks (32-40 tonnes) are the most important emitters for each of
the pollutants studied. Their emissions equal at least two thirds of the total
emissions (of trucks) because of their large share in the total mileage
driven on highways. Smaller trucks are driven less far, less frequently and
relatively more on other road types.
Table 1. Emissions at 80 km h
-1
relative to 90 km h
-1
(results from TEMAT)
Maximum speeds Real speeds
2005 2010 2020 2005 2010 2020
3.5-7.5 tonnes
86% 86% 86% 86% 86% 86%
7.5-
16 tonnes
86% 86% 86% 89% 89% 89%
16-
32 tonnes
92% 92% 92% 95% 95% 95%
32-
40 tonnes
91% 91% 91% 94% 94% 94%
CO
2
Fleet average 91% 91% 91% 94% 94% 94%
3.5-7.5 tonnes
84% 84% 84% 84% 84% 84%
7.5-
16 tonnes
89% 89% 89% 93% 93% 93%
16-
32 tonnes
94% 94% 94% 97% 97% 97%
32-
40 tonnes
105% 105% 105% 105% 105% 105%
NO
x
Fleet average 102% 102% 103% 103% 103% 103%
3.5-7.5 tonnes
97% 97% 97% 98% 98% 97%
7.5-
16 tonnes
102% 102% 102% 103% 103% 102%
16-
32 tonnes
97% 97% 98% 98% 98% 98%
32-
40 tonnes
106% 106% 106% 105% 105% 106%
PM
Fleet average 103% 104% 104% 103% 104% 104%
Effect of speed reduction on emissions of heavy duty lorries 5
Formatted:
Font: Bold
Results for policy scenario 1
In Table 1 the relative emissions under scenario 1 are given. Summarizing
we can say that the total CO
2
emission would decrease by 5 to 10 %. This
trend is consistent for all weight classes and years. Results from the de-
tailed vehicle based modelling (VeTESS) confirm the results for CO
2
(Ta-
ble 2) which makes the results more credible. In absolute numbers the CO
2
emission factors would on average drop by approximately 100 g km
-1
if
the policy resulted in a decrease from 90 to 80 km h
-1
. Using more realistic
estimates of the impact of the policy on real traffic speeds yields a reduc-
tion of only 50 to 70 grams km
-1
.
For the other pollutants TEMAT predicts an increase in the fleet average
emissions of NO
x
and PM with 2-3% and 3-4% respectively (Table 1).
Emission factors of NO
x
derived from both models show a decrease for
most types of trucks (3.5 to 32 tonnes). In sharp contrast an increased
emission is simulated for the heaviest trucks (+0.2 – 0.5 g km
-1
) (Figure 1).
As a result the fleet averaged emission factor is also higher.
The results for PM are even more confusing (Figure 2). PM emission
factors decrease for the 3.5-7.5 and 16-32 tonnes weight classes and in-
crease for the 7.5-16 and 32-40 tonnes weight classes. All changes (in-
creases and decreases) become smaller in the future. Because of the domi-
nance of the largest trucks the fleet average emission factor also increases.
Table 2. Relative emissions for different speed reductions (VeTESS results)
CO
2
NO
X
PM
Scenario 1: 90 km h
-1
→ 80 km h
-1
IVECO Eurocargo 7500 kg 84% 71% 84%
IVECO Eurocargo 12,000 kg 86% 72% 100%
MAN 30,000 kg 91% 89% 103%
Scania 30,000 kg 90% 85% n.a.
Scenario 2: 100
km
h
-1
→ 90 km h
-1
IVECO Eurocargo 7500 kg 73% 85% 71%
IVECO Eurocargo 12,000 kg 80% 88% 67%
Table 3. Avoided external costs under scenario 1 (median, 95% CI).
2005 6 mio Euro (48 ; -34) 62% chance for positive outcome
2020 10 mio Euro (38 ; -8) 85% “
Deleted: Effect of speed
reduction on emissions of heavy
duty lorries
6 Int Panis L, De Vlieger I, Pelkmans L, Schrooten L
-0,8
-0,6
-0,4
-0,2
0
0,2
0,4
0,6
2005 2010 2020 2005 2010 2020
Theoretical Real
g NO
X
/km
32-40 tonne 16-32 tonne 7,5-16 tonne 3,5t-7,5 tonne Fleet average
Fig. 1. Scenario1, difference in the NO
x
fleet emission factors (TEMAT)
-0,015
-0,01
-0,005
0
0,005
0,01
0,015
0,02
2005 2010 2020 2005 2010 2020
Theoretical Real
g PM/km
32-40 tonne 16-32 tonne 7,5-16 tonne 3,5t-7,5 tonne Fleet average
Fig. 2. Absolute difference in the PM fleet emission factors for the 3.5-32 tonne
trucks, 90 km h
-1
compared to 80 km h
-1
(TEMAT)
A high R
2
was reported in MEET for this emission function, indicating
it was based on a small sample. The lack of consistency between the ef-
fects for the different classes indicates a large amount of uncertainty.
Detailed modelling of the engine and vehicle characteristics with VeT-
ESS provides us with an additional set of results (Table 2). Emissions of
Effect of speed reduction on emissions of heavy duty lorries 7
Formatted:
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NO
x
are expected to decrease for the selected vehicles while only one ve-
hicle showed a very small increase in PM emissions.
Faced by these partially conflicting and uncertain findings we looked for
tools to convert the result into one integrated conclusion. The observed
emission differences where converted to (avoided) external costs [8]. In
this way we could balance the positive effect for CO
2
against the impacts
for NO
x
and PM by expressing them in similar monetary units. All the as-
sumptions were converted to probability distributions and used in a Monte
Carlo analysis. The total benefit of the proposed policy was found to be
positive and likely to increase. Nevertheless the range of results was wide
and the possibility that a speed reduction could have negative environ-
mental impacts could not be ruled out (Table 3).
Table 4. Relative emissions for lighter truck categories (TEMAT results)
Maximum speeds Real speeds
100 -> 80 km h
-1
2005 2010 2020 2005 2010 2020
3.5-7.5 ton
nes
72% 72% 72% 73% 73% 73% CO2
7.5-
16 tonnes
73% 73% 73% 77% 77% 77%
3.5-7.5 ton
nes
69% 69% 69% 69% 69% 69%
NOx
7.5-
16 tonnes
76% 76% 76% 82% 82% 82%
3.5-7.5 ton
nes
91% 91% 91% 92% 93% 93% PM
7.5-
16 tonnes
103% 103% 102% 104% 104% 105%
Results for policy scenario 2
This scenario only looks at effects on trucks below 12 tonnes because they
are the only class that can drive faster than 90 km h
-1
. Policy options for
these vehicles include the installation of mandatory speed limiters like
those used on all heavier trucks at this moment (90 km h
-1
) or reducing
their speed even further to 80 km h
-1
similar to the proposed policy under
scenario 1 for heavier trucks.
In the results presented in Table 4 we see that there is a consistent and
significant decrease in the emissions of CO
2
and NO
x
(minus 13 to 28 %
and minus 12 to 31 % respectively). For PM there is a difference between
both weight classes. The PM emissions for the smallest category (3.5 -7.5
tonnes) would decrease by 3 to 9 % but trucks in the 7.5-16 tonnes class
could see their PM emissions increase by up to 5%.
Deleted: Effect of speed
reduction on emissions of heavy
duty lorries
8 Int Panis L, De Vlieger I, Pelkmans L, Schrooten L
Discussion
The finding that emissions of CO
2
decrease but emissions of NO
x
and PM
could increase is consistent with the results from other studies. Using other
models and different fleets [5,11] similar results were obtained. The COST
346 [12] working group decided that a further speed reduction (below
80km h
-1
) would not affect fuel consumption but would increase emissions
of NO
x
and PM. The large uncertainties are blamed on the fleet composi-
tion [5, 12] and on difficulties to derive a typical driving pattern [13].
The choice of gear features among the most prominent changes to the
driving pattern and is likely to be influenced by changes to the speed limit.
Although this may be more important in urban locations than on highways.
The VeTESS model was therefore used to study the effect of different gear
shifting strategies in connection with different speed limits (see [2] for de-
tails). Our conclusions were confirmed for any gear shifting strategy for
any speed reduction down to 80 km h
-1
although we found some variation
in the magnitude of the effect. Further speed reductions below 80 km h
-1
however resulted in much higher emissions for some (but not all) trucks al-
though the fuel consumption remained fairly stable. These results were
presented to and discussed with both individual manufacturers and the
ACEA expert group. It is clear that they design and build long distance
haulage trucks to minimize fuel consumption at the most prevailing speed
limits in Europe (80 km h
-1
on highways). The optimum is between 80 and
85 km h
-1
which confirms our findings. They would however not elaborate
on effects on the other emissions because only the standardized driving cy-
cles are used in the engine certification.
Under scenario 2 we found that all changes in emissions are larger when
the speed reduction is more important. On the other hand the effects are
expected to decrease significantly in the future with more advanced fleets.
The absolute difference then becomes very small and given the small share
of light trucks on highways, the overall effect is likely to be negligible.
Manufacturers confirm that fuel consumption is not perceived as an issue
in this case. Manoeuvrability and other qualities prevail.
Conclusions
1. All results consistently indicate that lower maximum speeds for
trucks on motorways result in lower emissions of CO
2
.
2. Results for NO
x
and PM are not consistent and uncertain but probably
too small to offset the clear benefits of the CO
2
reduction.
Effect of speed reduction on emissions of heavy duty lorries 9
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3. The chances that this policy has environmental benefits increases
over time (with future fleets that are technologically more advanced).
4. The magnitude of the overall environmental benefits is very uncertain
but averages between 6-10 mio € per year.
5. Scenario 2 has clear environmental benefits, but the magnitude of the
effects is much smaller than those of scenario 1.
This study was heavily debated in parliament and on national radio and
TV. Although it answered some questions and raised others, people found
it hard to ignore the conclusions. Paradoxically none of the parties referred
to the large uncertainties. The discussion faded away and later reports on
safety and economic effects drew less attention. The proposal was later
discussed by the Belgian government that recently decided not to change
the speed limits but to impose a ban on overtaking on all 2 lane motor-
ways.
References
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Panis L (2005) Sustainability assessment of technologies and modes in
the transport sector in Belgium, Belgian science policy, Brussels.
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reduction on emissions of heavy
duty lorries
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