Panel data analysis is used within a fixed effect model to examine the relationship between vehicle safety ratings and fuel efficiency of  45 new vehicle models sold in the between 2002 and 2007. While conventional wisdom and most early literature suggest that lighter, more fuel efficient vehicles are less safe to their occupants, the tests show a positive relationship between vehicle safety ratings and fuel efficiencies not only within and across most size classes but also for vehicles produced by both the US and Asian automakers.We also explore the design initiatives by manufacturers to compensate for the reductions in weight/size of fuel-efficient vehicles.This article will be published in Transportation Research Part D 15 (2010) 112-116

Chialin Chen

Yu Ren   

Xiamen University Institutional Repository

Author's personal copyNotes and commentsExploring the relationship between vehicle safety and fuel efficiencyin automotive designChialin Chen a,*, Yu Ren ba Queen’s School of Business, Queen’s University, Kingston, Ontario, Canada K7L 3N6b Wang Yanan Institute for Studies in Economics, Xiamen University, Xiamen, Fujian 361005, Chinaa r t i c l e i n f oKeywords:Vehicle safetyFuel economyAutomobile designa b s t r a c tPanel data analysis is used within a fixed effect model to examine the relationship betweenvehicle safety ratings and fuel efficiency of 45 new vehicle models sold in the US between2002 and 2007. While conventional wisdom and most early literature suggest that lighter,more fuel efficient vehicles are less safe to their occupants, the tests show a positive rela-tionship between vehicle safety ratings and fuel efficiencies not only within and acrossmost size classes but also for vehicles produced by both the US and Asian automakers.We also explore the design initiatives by manufacturers to compensate for the reductionsin weight/size of fuel-efficient vehicles. 2009 Elsevier Ltd. All rights reserved.1. IntroductionThe effect of fuel economy on automobile safety, which has been a controversial issue since the energy crises of the 1970s,now receives growing attention due to the significantly tougher Corporate Average Fuel Economy (CAFE) standards proposed bythe US government. While the proponents of the new standards hail the potential benefits of energy and environmental con-servation, the opponents argue that vehicle safety will be compromised with the tougher fuel economy standards. Studies donein the 1980s consistently report that higher fuel economy performances achieved by automakers through vehicle downsizingwould significantly increase occupant fatality risk (Evans and Wasielewski, 1987). However, some recent studies show that thecorrelation between fleet fuel economy improvement and traffic fatalities is not necessarily positive (Ahmad and Greene, 2005;Noland, 2004, 2005). Suggested reasons for the weakening relationship between vehicle safety and fuel efficiency include driv-ers of light-weight vehicles off-setting the possibility of serious injury by less aggressive driving (Yun, 2002) and changes in thecharacteristics of other vehicles embracing stiffness and heights of the underlying structure (Toy and Hammitt, 2003).We study the effect of automakers’ multi-attribute design decisions on the relationship between vehicle safety and fuel effi-ciency based on the data of new vehicle models sold in the US using safety rating data published by the US National HighwayTraffic Safety Administration (NHTSA), 2002–2007 and fuel efficiency data from the US Environmental Protection Agency (EPA)covering, 2002–2007. Since the focus is on automakers’ design decisions, we use the safety rating data as a direct indicator of avehicle’s safety performance (Zachariadis, 2008). The time period allows us to compare possible changes in the relationship be-tween vehicle safety and fuel efficiency with those before 2002 when most prior analysis was completed.2. Data and methodologyThe safety rating data is based on the crash test results of the New Car Assessment Program (NCAP) conducted by NHTSA.They comprise the government’s 5-star results used to assist consumers in making purchasing decisions. Each year the1361-9209/$ - see front matter  2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.trd.2009.10.003* Corresponding author.E-mail address: cchen@business.queensu.ca (C. Chen).Transportation Research Part D 15 (2010) 112–116Contents lists available at ScienceDirectTransportation Research Part Djournal homepage: www.elsevier .com/ locate / t rdAuthor's personal copyagency chooses those new vehicles that are predicts to have high sale volume, those that have been redesigned with struc-tural changes, and those with improved safety equipment to perform three different types of crash tests: frontal, side impact,and rollover tests. The frontal crash test (the equivalent of a single-car crash or a two-car crash between vehicles of similarsize/weight) can only be compared with vehicles in the same class and within a 250 lbs weight range. The side impact testresults can be compared with all vehicles, but since NHTSA did not start rollover tests until 2001, the data do not cover theentire analysis period. Therefore, the analysis focuses on the frontal and side impact tests only.1The fuel efficiency data is based on the National Vehicle and Fuel Emissions Laboratory tests performed by EPA, with effi-ciency measured in miles per gallon weighted as an average of a 55% city and 45% highway driving cycle. Since NHTSA onlyconducts crash tests on selected models every years, initially the models with consistent frontal and side impact tests datafor 2002–2007 are isolated, and their safety rating data are matched with their fuel efficiency data. Any model with missingsafety rating data, as well as any model that was introduced after 2002 or discontinued before 2007 is excluded in the anal-ysis. Additionally, since NHTSA does not specify the exact specification of each model subjected to safety tests, for those withmultiple options, the average fuel efficiency of all the different options of the same model is used in the calculations. As aresult, 45 models are identified with consistent safety rating and fuel efficiency data, embracing eight compact, 11 mid-size,Table 1List of vehicle models.Makers Vehicle models Size classesAudi A4 Compact carFord Focus Compact carHonda Civic Compact carKia Rio Compact carMitsubishi Lancer Compact carToyota Corolla Compact carVolkswagen Jetta Compact carVolvo S60 Compact carAcura TL Mid-size carChevrolet Malibu Mid-size carChevrolet Monte Carlo Mid-size carChrysler Sebring Mid-size carHonda Accord Mid-size carHyundai Elantra Mid-size carMitsubishi Galant Mid-size carNissan Altima Mid-size carNissan Maxima Mid-size carPontiac Grand Prix Mid-size carToyota Camry Mid-size carToyota Prius Mid-size carChevrolet Impala Large carFord Crown Victoria Large carFord Taurus Large carHyundai Sonata Large carLincoln Town car Large carMercury Grand Marquis Large carToyota Avalon Large carAcura MDX Sport utility vehicleChevrolet Trailblazer Sport utility vehicleChrysler PT cruiser Sport utility vehicleFord Explorer Sport utility vehicleGMC Envoy Sport utility vehicleHonda CR-V Sport utility vehicleJeep Grand Cherokee Sport utility vehicleJeep Liberty/Cherokee Sport utility vehicleMercury Mountaineer Sport utility vehicleNissan Frontier Sport utility vehicleSubaru Forester Sport utility vehicleSuzuki Grand Vitara Sport utility vehicleToyota Highlander Sport utility vehicleToyota RAV4 Sport utility vehicleChrysler Town and country MinivanDodge Caravan MinivanHonda Odyssey MinivanKia Sedona MinivanToyota Sienna Minivan1 Each frontal or side impact safety rating is based on the average of the frontal driver and passenger ratings or the side driver and passenger ratings for avehicle model.C. Chen, Y. Ren / Transportation Research Part D 15 (2010) 112–116 113Author's personal copyand seven large cars, together with five minivans, and 14 SUVs (Table 1). By geographic region of automakers, 20 of the mod-els were produced by US manufacturers, 22 by Asian, and three by European manufacturers.2To establish a relationship, a panel data analysis is used with the fixed effect model (Wooldridge, 2001):yit ¼ bxit þ v i þ eit; ð1Þwhere yit and xit are the safety rating and fuel efficiency of vehicle model i in year t, and vi is any time-constant unobservedvariable that affects the design of vehicle model i. b and eit are the regression coefficient and error term.3 Averaging the equa-tion over time for each vehicle model i leads toyi ¼ bxi þ v i þ ei: ð2ÞSubtracting (2) from (1) for each year t, we haveyit  yi ¼ bðxit  xiÞ þ eit  ei: ð3ÞParameters are estimated by pooled ordinary least squares. Since the time-constant variable has disappeared in the equa-tion, there is no need to assume that vi is uncorrelated with the independent variable as in standard regression estimation.We therefore focus on the time-dependent effect between vehicle safety and fuel efficiency (under short-term equilibriumconditions) while eliminating the effects of other factors which are known to influence both vehicle safety and fuel effi-ciency, such as mass and size.43. ResultsTable 2 presents the test result for the 45 models based on the frontal, side impact, and average safety ratings (calculatedby averaging the two individual ratings). The test results all show a significant positive relationship between vehicle safetyratings and fuel efficiencies. Since frontal safety ratings can only be compared to other vehicles in the same size class, thepositive relationship between them and fuel efficiency is only applicable to cases involving single-car crashes or two-carcrashes between vehicles of similar weights/sizes.5 However, the test results based on the side crash ratings, which can becompared across all size classes, indicate that there exists a positive relationship between vehicle safety ratings and fuel effi-ciencies over the period.We also group all the vehicle models by size class, that not only allows us to examine the relationship within classes, butalso helps alleviate potential problems associated with the results of frontal tests only be comparable within the same class.6Table 3 presents the test results for compact cars, mid-size cars, large cars, SUVs, and minivans. The significant positive rela-tionship between vehicle safety ratings and fuel efficiencies remains within most of the size classes except for result basedon the frontal tests for compact cars and all the test results for minivans. While some of these statistically insignificant resultsmay be due to limited variation in the data and small sample sizes, the positive relationships between safety ratings and fuelefficiencies observed in other classes support the findings based on all the vehicle models.To see if the origin of vehicles is relevant, vehicles produced by US and Asian manufacturers are examined separately (Ta-ble 4). (European manufacturers are not included because of the small sample size.) The results based on the frontal, sideimpact, and average safety ratings again show significantly positive relationships between safety ratings and fuel efficienciesfor vehicle models produced by both groups of automakers. The positive coefficients of the three safety ratings of US vehiclesare all higher than those of Asian vehicles suggesting more effort spent by US automakers in improving simultaneously vehi-cle safety and fuel efficiency than their Asian counterparts.Table 2Frontal, side impact, and average test results (all vehicle models).All vehicles (270 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.149**Side impact safety rating vs. fuel efficiency 0.192**Frontal safety rating vs. fuel efficiency 0.170***Denote statistical significance at 5% level.** Denote statistical significance at 1% level.2 We only focus on vehicles powered by the conventional internal combustion engines, and exclude hybrid electric vehicles (HEVs) and vehicles powered bynon-gasoline fuels.3 The random effect model is not used due to the possible correlation between fuel efficiency and mass or size, i.e., Cov(xit, vi) – 0.4 Since we use the fixed effect model to analyze the data, any time-invariant effects are eliminated. The estimation is thus conducted without a constant(Hsiao, 2003).5 According to the 2007 Traffic Safety Facts published by NHTSA (available at http://www.nhtsa.gov), occupant fatalities in single-car crashes accounted for45% of all motor vehicle fatalities in 2007 while occupant fatalities in multiple-car crashes accounted for 42% of all fatalities, and the remaining 13% were non-occupant fatalities.6 The test results based on the frontal safety ratings should still be used with caution because the differences in weight between some of the vehicle modelswithin the same class exceed the 250-pound limit for comparison specified by NHTSA.114 C. Chen, Y. Ren / Transportation Research Part D 15 (2010) 112–116Author's personal copy4. Concluding remarksWhile earlier analysis often suggests lighter, more fuel-efficient vehicles are less safe for their occupants, the relationshipsbetween vehicle safety ratings and fuel efficiencies seem to have been mostly positive from 2002 to 2007. This is in line withsome of more recent reports based on pre-2002 data, which show either a positive relationship (Ahmad and Greene, 2005) orno relationship between vehicle fuel efficiency and traffic fatalities (Ross and Wenzel, 2002; Noland, 2004, 2005). Althoughthe results based on frontal ratings cannot be directly extended to those cases of multiple-car crashes that involve vehicles ofsignificantly different weights/sizes, they do show that fuel-efficient vehicles can also be as safe as, if not safer than, their lowgas-mileage counterparts in accidents involving single-car crashes and side impact collisions.AcknowledgementsThis research was supported in part by Natural Science and Engineering Research Council Canada (Grant: RGPIN/249675)and the Starting Research Fund of Xiamen University. The authors would also like to thank Jacqueline Dixon and Jingjing Xiefor their assistance in collecting and compiling data for this study.ReferencesAhmad, S., Greene, D.L., 2005. Effect of fuel economy on automobile safety. Transportation Research Record 1941, 1–7.Evans, L., Wasielewski, P., 1987. Serious or fatal driver injury rate versus car mass in head-on crashes between cars of similar mass. Accident Analysis andPrevention 19, 119–131.Hsiao, C., 2003. Analysis of Panel Data. Cambridge University Press, Cambridge.Noland, R.B., 2004. Motor vehicle fuel efficiency and traffic fatalities. Energy Journal 25, 1–22.Noland, R.B., 2005. Fuel economy and traffic fatalities: multivariate analysis of international data. Energy Policy 33, 2183–2190.Table 3Frontal, side impact, and average test results by size classes.Size class: subcompact/compact (48 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.016Side impact safety rating vs. fuel efficiency 0.190**Frontal safety rating vs. fuel efficiency 0.103*Size class: mid-size (66 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.227**Side impact safety rating vs. fuel efficiency 0.242**Frontal safety rating vs. fuel efficiency 0.234**Size class: large cars (42 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.396**Side impact safety rating vs. fuel efficiency 0.424**Frontal safety rating vs. fuel efficiency 0.410**Size class: SUVs/pickup trucks (84 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.419**Side impact safety rating vs. fuel efficiency 0.101**Frontal safety rating vs. fuel efficiency 0.260**Size class: minivans (30 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.002Side impact safety rating vs. fuel efficiency 0.024Frontal safety rating vs. fuel efficiency 0.013* Denote statistical significance at 5% level.** Denote statistical significance at 1% level.Table 4Frontal, side impact, and average test results for US and Asian manufacturers.US vehicles (120 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.244**Side impact safety rating vs. fuel efficiency 0.229**Frontal safety rating vs. fuel efficiency 0.236**Asian vehicles (132 observations) CoefficientFrontal safety rating vs. fuel efficiency 0.166**Side impact safety rating vs. fuel efficiency 0.181**Frontal safety rating vs. fuel efficiency 0.174***Denote statistical significance at 5% level.** Denote statistical significance at 1% level.C. Chen, Y. Ren / Transportation Research Part D 15 (2010) 112–116 115Author's personal copyRoss, M., Wenzel, T., 2002. An Analysis of Traffic Deaths by Vehicle Type and Model. American Council for and Energy-Efficient Economy (Report NumberT021), Washington, DC.Toy, E.L., Hammitt, J.K., 2003. Safety impacts of SUVs, vans, and pickup trucks in two-vehicle crashes. Risk Analysis 23, 641–650.US Environmental Protection Agency, 2002–2007. Gas Mileage Guide. US Environmental Protection Agency (<http://www.fueleconomy.com>).US National Highway Traffic Safety Administration, 2002–2007. Crash Test Ratings. US National Highway Traffic Safety Administration (<http://www.nhtsa.dot.gov>).Wooldridge, J., 2001. Econometric Analysis of Cross Section and Panel Data. MIT Press, Cambridge, MA.Yun, J.M., 2002. Offsetting behavior effects of the corporate average fuel economy standards. Economic Inquiry 40, 260–270.Zachariadis, T., 2008. The effect of improved safety on fuel economy of European cars. Transportation Research Part D 13, 133–139.116 C. Chen, Y. Ren / Transportation Research Part D 15 (2010) 112–116

Exploring the Relationship between Vehicle Safety and Fuel Efficiency in Automotive Design

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Exploring the Relationship between Vehicle Safety and Fuel Efficiency in Automotive Design

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