Journal of Transportation Technologies, 2012, 2, 158-164
http://dx.doi.org/10.4236/jtts.2012.22017 Published Online April 2012 (http://www.SciRP.org/journal/jtts)
Comparison of In-Use Operating Costs of Hybrid-Electric
and Conventional School Buses
Shauna Hallmark, Robert Sperry
Center for Transportation Research and Education, Iowa State University, Ames, USA
Received November 15, 2011; revised January 10, 2012; accepted February 15, 2012
ABSTRACT
Hybrid-electric school buses became available in the US through a national consortium designed to bring hybrid-elec-
tric school buses to market by creating enough demand among school districts to encourage manufacturers to invest in
development of the technology. A number of school districts in the US joined the HESB project to purchase plug-in
hybrid-electric school buses. Sixteen hybrid-electric school buses were purchased and piloted in 11 states. Two of the
hybrid-electric school buses were purchased by the Nevada and Sigourney school districts in the state of Iowa, US.
In-use fuel economy and electricity operating costs were monitored for the two Iowa hybrid school buses and two con-
trol buses (one in each district). Fuel consumption and other operational metrics were calculated and compared for each
school district. The hybrid buses were deployed in January 2008 and data were recorded through May 2010. Valuation
of the data indicated that the Nevada HESB had 29.6% better fuel economy than the control bus and the Sigourney
HESB had 39.2% better fuel economy than the control bus. Electrical costs per mile were also calculated for the two
hybrid-electric school buses. Total operating costs per mile were calculated based on fuel use per mile for all buses and
electrical costs for the hybrid-electric school buses. The cost to operate the hybrid bus in Nevada was 37 cents/mile
while the control bus cost 42 cents/mile, making the hybrid bus 13% less expensive to operate. The hybrid bus in Si-
gourney was 27 cents/mile while the control bus was 34 cents/mile, making the hybrid bus 21% less expensive to oper-
ate. All values are in US dollars.
Keywords: Hybrid Vehicles; Fuel Consumption; School Bus; Energy
1. Introduction
Around 26 million school children are transported to
school annually in the United States on school buses re-
sulting in 5.8 billion miles of travel per year [1]. Al-
though transporting school children on school buses is
significantly more fuel efficient than transportation in
private vehicles, school buses still consume around 822
million gallons of diesel fuel annually. The cost of pro-
viding school transportation can be overwhelming for US
school districts as fuel costs rise and school budgets are
reduced due to eco nomic downturn. In response to rising
transportation costs, some school districts are cutting
services which increase the number of private vehicle
trips which can increase overall fuel use considerably.
The American School Bus Council [1] estimates that if
children who customarily ride the school bus took those
school trips in private vehicles, the school trips would
consume an additional 2.3 billion gallons of fuel. This
would result in even more consumption of non-renew-
able resources and continued reliance on foreign energy
sources. As a result, even though it may be challenging
for agencies, such as resource strapped school districts,
to invest energy and resources, it is important that agen-
cies take the lead in finding sustainable solutions. It is
also particularly important that school districts take the
lead in demonstrating sustainable practices to school
children.
One sustainable solution to school transportation prob-
lems is use of hybrid school buses, which have the po-
tential to reduce emissions and overall life-cycle costs
compared to conventional diesel school buses. Prior to
2006, the hybrid technology was only available in transit
buses and passenger vehicles with both having a track
record of fuel economy and emissions benefits. Hybrid
technology has recently become available in the school
bus market.
In order to create enough demand to bring hybrid
school buses to market, a national consortium was
formed in the US. The Hybrid-Electric School Bus
(HESB) Project was organized by Advanced Energy. The
project was designed to bring hybrid school buses to
market by creating enough demand among school dis-
tricts to encourage a manufacturer to invest in the devel-
C
opyright © 2012 SciRes. JTTs

S. HALLMARK ET AL. 159
opment of the technology [2]. The expected benefits of
the hybrid-electric school buses are fuel efficiency, re-
duced fuel costs, and decreased emissions.
A number of school districts joined the HESB project
and purchased plug-in hybrid-electric school buses. Six-
teen hybrid-electric school buses were purchased and
piloted in 11 states. Two of the hybrid-electric school
buses were purchased by Iowa school districts.
1.1. Performance of Hybrid School Buses
Although hybrid technology was relatively new in school
buses, a few studies are available which have assessed
the fuel economy and emissions impact of hybrid school
buses. It should be noted that most of the studies cited in
the following sections were assessments of buses that
participated in the HESB project.
Enova tested a plug-in hybrid school bus using a chas-
sis dynamometer to evaluate fuel economy under con-
trolled test conditions. The company used the West Vir-
ginia University Suburban Cycle (WVUSC) and Tor-
rance California Test Cycle (TCTC) [3]. With the
WVUSC, the research ers found that the hybrid bus had a
fuel economy of 12.70 mpg, while a conventional bus
used for comparison had a fuel economy of 7.10 mph; a
79% improvement. For the TCTC, the researchers found
a 57% improvement in fuel economy for the hybrid bus,
with 12.80 mpg for the hybrid and 8.10 mpg for the con-
ventional bus.
Hybrid-electric school buses in Vermont were tested
using on-road emissions equipment [4]. It was estimated
that the hybrid-electric school bus consumed 28.7% less
fuel than a standard diesel school bus. Additionally, the
researcher found that NOx emissions were 49% lower for
the hybrid-electric school bus than for the conventional
school bus, CO emissions were 72% lower, and HC
emissions were 49% lower.
Data were collected on a conventional charge-sus-
taining (hybrid) bus, a standard diesel control bus, and a
charge-depleting (plug-in hybrid) bus for the New York
Power Authority (NYPA) [3]. Fuel consumption for
three test routes was measured (urban, suburban, and
rural) and they found that the plug-in hybrid school bus
had a fuel economy 51% to 131% higher than the other
buses, depending on the test route. However, this was
only observed while the HESB operated in charge-de-
pleting mode. Once it began operating in charge-sus-
taining mode, it performed similar to the conventional
hybrid and diesel control buses.
In another study, a plug- in hybrid schoo l bus w as co m-
pared to a control bus by The University of Texas. The
study authors found that fuel economy for the conven-
tional bus was 6.9 mpg, while fuel economy was 9.0 mpg
for the HESB (an increase of 30%). The researchers also
reported that the control bus emitted approximatel y 6 3 kg
of CO2 per day while the HESB emitted 51 kg. When the
researchers factored in pollution for electricity regenera-
tion to recharge batteries, this represents a reduction of
23.5% [3].
Advanced Energy [3] has also been collecting in-use
fuel economy data from each school district involved in
the HSEB project across the US. Advanced Energy’s
researchers evaluated the fuel economy comparing hy-
brid-electric school buses and regular school buses.
Overall, the hybrid buses have done much better than the
control buses. Three school districts reported decreases
in fuel economy, 5%, 6%, and 12% respectively. The
remaining school districts have reported improved fuel
economy for the hybrid-electric school buses over con-
trol buses. Two districts reported improvements between
6% and 8%, three reported improvements from 11% to
14%, one had a 23% savings, one had a 47% improve-
ment, and one school district saw a fuel economy im-
provem ent of 57%.
Choi and Frey et al. [5] conducted a study which com-
pared energy use for a plug-in parallel-hybrid diesel-
electric school bus (PHSB) to a conventional school bus
using five real-world driving cycles. They found that the
direct diesel fuel economy was 3.2% to 9.5% better for
the PHSB compared to the conventional bus.
The National Renewable Energy Laboratory [6] ana-
lyzed real-world school bus drive cycle data for a
first-generation PHEV, a more recent PHEV, and a con-
ventional school bus. Depending on the drive cycle,
PHEV fuel savings ranged from 30% to 50% while the
buses were in charge depleting mode. While in charge
sustaining mode the PHEV showed only small fuel sav-
ings compared to the conventional bus.
1.2. Hybrid Performance in the Transit Market
Although only a few studies have been conducted to as-
sess the fuel economy or emissions for hybrid school
buses, transit buses have used hybrid technology for
some time and have overall shown significant fuel
economy and emissions improvements over conventional
buses.
Chassis dynamometer tests were conducted for 10
low-floor hybrid buses and 14 conventional high-floor
diesel transit buses run by New York City Transit [7].
Buses were evaluated over three driving cycles including
the Central Business District (CBD), New York bus cy-
cle, and the Manhattan cycle. The operating costs, effi-
ciency, emissions, and overall performance were also
compared while both types of buses were operating on
similar routes. They found that fuel economy was 48%
higher for the hybrid buses.
A study by Battelle [8] tested emissions using a dy-

S. HALLMARK ET AL. 160
namometer for one diesel hybrid-electric bus and two
regular diesel buses (with and without catalyzed diesel
particulate filters [DPF]). The researchers reported that
fuel economy for the hybrid bus was 54% higher than the
two regular diesel buses. In another study, two buses
were tested using a dynamometer at the National Re-
newable Energy Laboratory’s (NREL’s) Refuel facility
in Golden, Colorado [9]. One bus was a conventional
diesel and the other was a hybrid bus and both were
tested over several drive cycles including Manhattan,
Orange County Transit A, CBD, and King County Metro.
Results are shown in Table 1 for each drive cycle. The
table shows percent difference in fuel economy among
the buses (fuel economy was reported as miles per gal-
lon). Fuel economy was 30.3% to 74.6% higher for the
hybrid bus compared to the conv entional buses.
In another study, Clark et al. [10] evaluated six transit
buses with traditional diesel engines, two powered by
spark-ignited compressed natural gas (CNG), and one
hybrid transit bus in Mexico City using a mobile heavy-
duty emissions testing lab. Buses were tested over a drive
cycle representative of Mexico City transit bus operation,
which was developed using GPS data from in-use transit
buses. Depending on how fuel economy was evaluated,
the hybrid bus ranked 4th and 1st in fuel economy.
1.3. Objective
Two school districts in Iowa, (US) participated in the
Hybrid Electric School Bus Project (Nevada and Sigour-
ney) and each purchased one hybrid-electric school bus.
The Center for Transportation Research and Education
(CTRE) at Iowa State University (ISU) monitored and
evaluated bus performance for the two Iowa HESBs from
2008 to 2010. The project was funded by the Iowa En-
ergy Center (IEC). The team used information provided
by the school district to calculate fuel economy and elec-
tricity use for the two hybrid-electric school buses and
two traditional diesel school buses which were used as
controls.
The objectives of the research were to monitor fuel
and electricity use and assess the on-road operating costs
of the hybrid buses compared to conventional school
buses.
Tab le 1. Percentage chan ge comp ared to conventional buses .
Cycle Fuel economy
Manhattan +74.6%
OCTA +50.6%
CBD +48.3%
KCM +30.3%
1.4. Iowa Buses
Each of the two school districts purchased one hy-
brid-electric school bus with funding assistance from the
Iowa Energy Center (IEC). Each school district was also
asked to select a control bus which had similar charac-
teristics to the hybrid-electric bus (size, year manufac-
tured, etc.) and operated on similar routes. The hy-
brid-electric school buses were manufactured by IC Bus.
The school bus body is the same as that of a standard
school bus and bus specifications include the following
[2]:
 6.4 L 210 hp diesel
 Hybrid system with plug-in capability
 Post-transmission parallel drive
 80 kW electric motor
 35 kWh Li-ion phosphate battery pack
Both of the Iowa HESB buses are 2009 International
65-passenger buses with an International MaxForce (V8)
6.4 L engine with an Allison automatic 2000 transmis-
sion. The hybrid function can be turned off so that the
bus runs on the internal combustion engine (ICE) only.
The Nevada control school bus is a 2009 Bluebird
65-passenger bus with a Cummins ISB07 engine (220
HP, Allison automatic GEN4, 2500 PTS, TC 221). The
Sigourney control bus is a 2005 Bluebird, 65-passenger
with a Caterpillar C-7 (V6) engine (210 HP) with an Al-
lison automatic 2000 transmission.
The Nevada School District is located in Nevada, Iowa,
about 10 miles west of Ames, Iowa. The district has
about 13 operational buses and transports around 828
school children per day. The district uses a 20% biodiesel
blend. The blend amount varies, but the same fuel was
used in both the h ybrid and contro l buses. The Sigourney
School District is located about 90 miles southeast o f Des
Moines, Iowa. The district has about 11 buses and trans-
ports around 320 school children per day. Both locations
are considered to be rural co mmunities.
2. Methodology to Evaluate Operating Costs
Both school districts began recording information for the
hybrid buses in February of 2008. The Nevada school
district also began reporting for the control bus in Febru-
ary 2008 while the Sigourney school district did not se-
lect a control bus until October 2008. Data were recorded
for all buses through May 2010. Each time the buses
were fueled, school districts reported the date, odometer,
gallons of fuel added, fuel cost, and any maintenance or
other issues.
Initially, neither Iowa school district had a separate
electric meter for the hybrid bus. When a dedicated meter
became available, electricity use was recorded. However,
less electricity use information was available than for
fuel use. Additionally, although the districts did a good

S. HALLMARK ET AL. 161
job keeping track of major problems with the buses, they
did not regularly report minor maintenance, such as add-
ing oil. As a result, it was not possible to evaluate total
maintenance per mile.
The data collected for each period were compared
against school district notes to determine whether there
were any problems with the data that needed to be ad-
dressed. In several cases, the battery charging system on
the hybrid buses did not hold a charge. This occurred for
nearly six months for Nevada and seven months for Si-
gourney. In other cases, the schools indicated that data
were uncertain. (For instance, one bus was in the shop
for several days, and the school district did not know
whether additional fuel had been added). Data for which
the hybrid bus was not functioning properly or errors
were noted were removed from the data set. All costs
reported in the following sections are in US dollars.
2.1. Fuel Cost
Because temperature and weather conditions impact bus
fuel economy, data were first disaggregated and initially
evaluated by season. However since there was not suffi-
cient data to determine whether fuel economy between
the hybrid and controls buses was statistically different
by season, data were combined and annual average fuel
economy calculated.
Fuel economy for each time period was calculated us-
ing the following Equation:
t
tt
m
FE
g
 (1)
where:
t
F
E
m = fuel economy for period t (mpg)
t = miles driven in period t based on odometer read-
ing
t
g
= gallons of fuel used in peri od t
Average fuel economy was calculated using Equation
(2):
avg t
F
EFE

(2)
where:
avg
F
E = average fuel economy
In Nevada, the conventional bus had a fuel economy
of 6.35 mpg while the hybrid bus had an overall fuel
economy of 8.23 mpg for an increase of 29.6%. In Si-
gourney, the control bus’ fuel economy was 6.42 mpg
while the hybrid bus had an overall fuel economy of 8.94
mpg which is 39.2% higher. Results of a Wilcoxon
signed rank test indicated the differences were statisti-
cally significant at the 5% level of confidence for both
school districts. A more in-depth discussion of how fuel
economy was calculated and evaluated is summarized in
a correspondin g paper [11].
Fuel costs were averaged over the analysis period to
simplify estimation of costs. The average cost for a gal-
lon of diesel for Nevada over the analysis period was
$2.71, and the cost for Sigourney was $2.17. The average
fuel cost per mile was calculated using the following
equation:
fuel
mile avg
C
FC
F
E
 (3)
where:
mile
F
C
C = fuel cost per mile
fuel
The cost for Nevada was $0.33/mile for the HESB and
$0.43/mile for the control bus. In Sigourn ey, the co st was
$0.24/mile for the HESB and $0.34/mile for the control
bus.
= average cost of fuel per gallon
2.2. Electrical Cost
Average electricity use for the hybrid-electric buses was
reported for times when the hybrid was functioning and
when electricity readings were available. As indicated,
the hybrid buses did not have a dedicated electricity me-
ter available when the buses were initially placed in op-
eration. As a result, data were summarized for periods
when the meters were functioning and it was assumed
that electricity use was constant across the study period.
Electric use per mile was calculated using Equation
(4):
t
mt
K
WH
KWH m  (4)
where:
KWHm = kilowatt hours p er mile
KWHt = kilowatt use for period t
mt = miles driven in period t based on odometer read-
ing
Cost per mile for electricity use was determined using
the following equ ation:
mm
ECKWH Ckwh

 (5)
where:
ECm = average cost of electricity per mile
Ckwh = average cost per kilowa t t hour
MidAmerican Energy [12], Iowa’s largest utility, re-
ported that the average cost per kilowatt-hour was $0.06
for retail organizations. This was the value used to cal-
culate electricity cost per mile since the school bus ga-
rages could not obtain th e actual cost paid by the districts
for electricity. Electrical cost was 5.0 cents/mile for the
Nevada bus and 4.0 cents/mile for the Sigourney bus.
Operating cost per mile for the control buses was simply
the fuel cost per mile. Operating cost per mile for the
hybrid bus was calculated by adding fuel cost per mile

S. HALLMARK ET AL. 162
and electricity cost per mile.
3. Other Costs
Several long term costs which are important factors in
assessing whether investment in hybrid buses makes
sense for a school district could not be included in the
analysis. Initial feasibility studies and marketing of hy-
brid buses indicate that the hybrid technology would re-
sult in less engine wear and regenerative braking is esti-
mated to result in less brake wear [13-16]. However, the
battery pack s ad d ap prox imately 2000 po und s [1 3] wh ich
may increase wear on some parts such as shock absorbers
and tires. Replacement cost of the batteries will also add
to the lifecycle cost of the hybrid buses. IC Bus [13] in-
dicates that the lithium-ion batteries have a life of 5 - 7
years. The replacement costs, however, are to some ex-
tent unknown at this point. Battery technology is rapidly
evolving so the cost of b atteries in the future will depend
on the latest technology and costs when it is time to re-
place the batteries.
One of the original project goals was to track mainte-
nance costs for the hybrid and control buses. However,
the buses were only followed for 2 years which was not
sufficient time to identify differences in normal wear.
During the study, a number of maintenance issues spe-
cific to the hybrid buses occurred. The initial charging
connections and power cords furnished with the buses
proved to be inadequate for the function for which they
were needed. After both districts’ chargers burned (or
shorted) out, all were replaced with heavier duty cords
and more positive (locking) connections. In addition, the
batteries required about eight hours to achieve a full
charge, so the intermediate charge between the typical
morning and af ternoon routes was insufficient to provide
full electrical power in the afternoon. During periods of
maximum electrical use (and minimal engine use), the
idling engine did not circulate and heat enough water in
its cooling system to pro vide adequate h eat to the interio r
of the bus. This was unacceptable to the driver and the
riders, so the attempted so lution was an auxiliary electric
heater. However, during the study, any maintenance
costs specific to the hybrid system was covered under the
vehicle warranty so the schools did not incur any actual
costs due to the maintenance issues. Additionally, it was
felt that the initial maintenance problems with the h ybrid
buses were generational and will be overcome in future
models so these types of costs are not expected to occur
with more recent hybrid school buses.
Reduced emissions are another cost that could be in-
cluded in a lifecycle analysis. Studies have shown sig-
nificant reduction in criteria pollutants for hybrid school
buses compared to regular school buses. Students on hy-
brid school buses would have lower exposure to pollut-
ants, particularly particulate matter, resulting in a posi-
tive health benefit. However, quantifying costs associ-
ated with a reduction in pollutants was beyond the scope
of this study. Additionally the benefits of these reduced
costs are not accrued to the school distri ct s them sel ves.
4. Results
Operating costs by category are provided in Table 2. As
shown, the cost to operate the hybrid bus in Nevada was
38.0 cents/mile while the con trol bus cost 43.0 cents/mile,
making the hybrid bus 13% less expensive to operate.
The hybrid bus in Sigourney was 28.0 cents/mile while
the control bus was 34.0 cents/mile), making the hybrid
bus 21% less expensive to operate. As noted in Section 4
none of the maintenance costs directly related to the hy-
brid buses were paid by the school, so these costs were
not factored in. This would be typical of any new vehicle
while it was still under warranty. Additionally, as indi-
cated, differences in regular maintenance were not noted.
Regular maintenance includes items such as an oil
change, changing an air filter, etc.
The Nevada HESB was driven an average 987 miles
per month while the control bus was driven an average of
705 miles. The average mileage for the Nevada buses
was 846 miles per month. Assuming a school year of 9
months, the average cost for operating a HESB over an
average route would be 9 × 1362 × $0.236 = $2893 an-
nually while the cost to operate a traditional bus on a
similar route is 9 × 1362 × 0.267 = $3273. Total savings
are $380 annually.
The Sigourney HESB was driven an average of 1339
Table 2. Operational metrics for Iowa buses (costs are in USD).
Avg fuel economy in
km/l (mpg)
Avg fuel
cost in
$/liter ($/gal)
Avg fuel
cost per
km (cost/mile)
Avg kwh
per km
(per mile)
Avg cost
per kwh
Avg kwh
cost/km
(per mile)
Total avg
cost/km
(per mile)
Nevada HESB 3.50 (8.23) $0.72 ($2.71) $0.21 ($0.33) 0.53 (0.85)$0.06 $0.032 ($0.051) $0.24 ($0.38)
Nevada Control 2.70 (6.35) $0.72 ($2.71) $0.27 ($0.43) NA NA NA $0.27 ($0.43)
Sigourney HESB 3.80 (8.94) $0.72 ($2.71) $0.15 ($0.24) 0.37 (0.60)$0.06 $0.022 ($0.036) $0.17 ($0.28)
Sigourney Control 2.73 (6.42) $0.72 ($2.71) $0.21 ($0.34) NA NA NA $0.21 ($0 .34)

S. HALLMARK ET AL. 163
miles per month and the control bus was driven an aver-
age of 1010 miles per month for an average of 1175
miles per month. Based on this, the average operating
cost for a HESB would be 9 × 1625 × 0.174 = $2545
annually. The average operating cost for the control bus
on a similar route would be 9 × 1625 × $0.340 = $4972.
Total savings are $2427 annually.
As indicated, the annual savings are modest. At the
time of the study, a hybrid electric school bus was ap-
proximately $80,000 more than a conventional school
bus. Consequently, it would take some time for the cost
to be realized. However as discussed in Section 4, other
costs, such as the impact of exposure to pollutants, are
not included.
5. Summary
This paper summarizes the evaluation results of an in-use
fuel economy and operating cost evaluation for two
plug-in hybrid school buses deployed in two different
school districts in Iowa. Each school district selected a
control bus that ran a route similar to that of the hybrid
bus. Odometer readings, and fuel consumption were re-
corded for each bus. The buses were deployed in 2008
and data were collected through May 2010.
The buses were part of the Hybrid-Electric School Bus
Project organized by Advanced Energy which was de-
signed to bring hybrid-electric school buses to market by
creating enough demand among school districts to en-
courage a manufacturer to invest in the development of
the technology. A number of school districts in the US
joined the HESB project to purchase plug-in hybrid-elec-
tric school buses. Sixteen hybrid-electric school buses
were purchased and were piloted in 11 states. Tw o of the
hybrid-electric school buses were purchased by Iowa
school districts, Nevada and Sigourney.
In-use fuel economy and electricity operating costs
were monitored for the two Iowa hybrid school buses and
two control buses (one in each district). Fuel consump-
tion and other operational metrics were calculated and
compared for each school district.
Evaluation of the data indicated that the Nevada HESB
had 29.6% better fuel economy than the control bus and
the Sigourney HESB had 39.2% better fuel economy
than the control bus. Electrical costs per mile were also
calculated for the two hybrid-electric school buses. Total
operating costs per mile were calculated based on fuel
use per mile for all buses and electrical costs for the hy-
brid-electric school buses. The cost to operate the hybrid
bus in Nevada was 38.0 cents/mile while the control bus
cost 43.0 cents/mile, making the hybrid bus 13% less
expensive to operate. The hybrid bus in Sigourney was
28.0 cents/mile while the control bus was 34.0 cents/mile,
making the hybrid bus 21% less expensive to operate.
Both hybrid buses had fuel economy and operating
costs that were well below those for the control buses.
However, fuel economy was not as high as had been ex-
pected by the school districts based on in itial estimates of
12 mpg. Fuel economy may have been impacted in part
by maintenance problems experienced by both school
districts. The charging system for buses required several
fixes and resulted in both buses running only on the ICE
for several months during the analysis period. This may
have impacted fuel consumption results if the bus opera-
tor did not notice and report the problem as soon as it
occurred. Additionally problems with the charging sys-
tem resulted in the buses not being able to fully recharge
for the afternoo n run. As a result, fu el economy would be
lower than if the buses had been fully charged. Conse-
quently, the HESB may have actually had better fuel
economy than was reported.
Although the HESB experienced an unusual number of
maintenance problems, which were frustrating for the
school districts, the team believes that this problem is
generational and can be overcome in future models.
Transit buses have utilized both plug-in and conv entional
hybrid techno logies fo r some ti me, and it does not appear
that they have experienced the same challenges. As a
result, it is believed that the potential exists for manufac-
turers to overcome the initial problems.
The operatin g costs for the HESB were lo wer for both
school districts than for the conventional buses used as
controls. However, even with the savings that could be
achieved it will still be difficult for schoo l districts in the
US to recuperate the additional purchase price of the hy-
brid school buses. As a result, the difference in purchas-
ing prices for hybrid school buses may need to be subsi-
dized in order for the technology to be cost effective for
school districts.
6. Acknowledgements
The research team would like to thank the Iowa Energy
Center for funding this project. They would also like
thank the Nevada and Sigourney bus garages, drivers,
and supervisors for their assistance in collecting data. In
particular, the team would like to thank Mr. Dennis
Kroeger, who conducted much of the work and provided
much of the vision to bring hybrid school buses to Iowa.
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S. HALLMARK ET AL. 164
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