Bogged down in the four-wheel drive debate?This topic is sponsored by NRMA – ACT Road Safety Trust. Four-wheel drives are becoming more common on the nation's roads, but there are questions about their impact on road safety and the environment. Key textThey used to be big, ugly and dirty, sound like a tractor and have the suspension of a stump-jump plough. But times have changed. Four-wheel drive vehicles are domesticated now. Increasingly sleek and shiny, many of them are more at home in the city than the outback, more a family wagon than a farm animal. There have never been more on Australian roads: every fourth new car sold today is a four-wheel drive. Some people say they are a roadblock to road safety and to reducing fuel consumption and pollution. Do they have a case? What is a four-wheel drive?
A four-wheel drive (called a sports utility vehicle, or SUV, in the United States) is a vehicle which can deliver power to all four wheels instead of the usual two. Four-wheel drives are most useful when traction is low and the tyres are likely to slip; for example, when a dirt road is muddy or particularly rough, or in sand or snow. Four-wheel drives perform better in such conditions simply because four tyres have more traction than two. Safety of four-wheel drives One of the reasons that the influx of four-wheel drives into the urban environment is causing concern is their generally larger size. People who have zipped around city streets for years in small or medium-sized cars are suddenly feeling intimidated by what have been dubbed 'urban assault vehicles'. So, when assessing the safety of these large four-wheel drives, we need to consider both crashworthiness – the relative ability of a vehicle to protect its occupants from severe injury in the event of a crash – and aggressivity, which is the risk that a vehicle poses to other road users in a collision. A third factor should also be taken into account: the stability of the vehicle. Four-wheel drives are more likely than passenger cars to roll over because they have a higher centre of gravity relative to their wheelbase. An analysis of 1998 crash data by the Australian Transport Safety Bureau provides evidence to support this view. It showed that a significantly higher proportion of four-wheel drives involved in fatal crashes in that year rolled over compared to passenger cars (35 per cent versus 13 per cent). The same analysis also found that the proportion of four-wheel drive vehicles that rolled over without a previous collision was more than three times the proportion for passenger cars (21 per cent and 6 per cent respectively). These crashes mostly involved single vehicles that had driven off a straight or curved road and rolled over; the Bureau considered it unlikely that the increased incidence of rollovers could be explained fully by differences in the terrain and roads used by four-wheel drives compared to other vehicles. The study of vehicle crashworthiness and aggressivity described below does not take into account these one-vehicle accidents, with the probable effect that four-wheel drives score better than they otherwise would. Crashworthiness In any collision between two objects, the bigger object has a natural advantage. Risk is related to change in velocity: the shorter the time in which a vehicle changes its speed from, say, 80 kilometres per hour to zero, the more difficulty it will have in absorbing the crash energy without damage. Change in velocity, in turn, is determined partly by the relative mass of the colliding objects. Imagine a big player running into a smaller player on the football field: there's a good chance the smaller player will be flattened and the bigger player might not even fall over. Since four-wheel drives have, on average, a greater mass than standard passenger vehicles they are likely to come off best in a collision. Mass isn't always a help, though. Many four-wheel drives are not particularly well-designed or outfitted for safety (although this is changing as they become more popular as passenger vehicles). Their chassis (frames) are relatively stiff, so more of the force of impact is transferred to the cabin, where the occupants are seated. Moreover, the safety of occupants is compromised by the increased likelihood of rollover.
Nevertheless, an empirical study conducted recently by the Monash University Accident Research Centre (MUARC) found that many four-wheel drive models ranked higher than many smaller cars for crashworthiness. Researchers collated actual crash data from Victoria, New South Wales, Queensland and Western Australia on the extent of injuries suffered by more than a million drivers involved in tow-away crashes in the 1990s, and used them to assign crashworthiness ratings to 213 car models. The average rating (number of serious injuries per 100 drivers involved in accidents) for four-wheel drives was 3.19, which was better than the overall average of 3.93. It was also better than the score for medium-sized cars (3.62), commercial vehicles (3.74), small cars (4.28) and passenger vans (4.44), but worse than the rating for luxury two-wheel drive cars (2.81) and similar to that of some large cars (3.11). The study was subject to several qualifications. For example, the ratings were calculated based on what happened to the driver only; the fate of passengers was ignored. In addition, ratings for some models had quite wide confidence limits, which means that the number of data points was small or the spread of values wide (or both). Nevertheless, of the 51 models for which crashworthiness was statistically better than average, ten were four-wheel drives. In contrast, of the 44 models estimated to have significantly inferior crashworthiness, only two were four-wheel drives (26 were small cars). The crashworthiness of four-wheel drives is likely to improve further in the future as car manufacturers devote more energy to this rapidly growing market. Aggressivity Vehicular safety is, however, a two-way street. A car might be highly crashworthy, offering good protection to its occupants, but its aggressivity rating may also be high and therefore dangerous to occupants of other vehicles and also to unprotected road users – pedestrians, cyclists and motorcyclists. This would be no great surprise: one of the predictors of good crashworthiness, superior mass, is also a predictor of high aggressivity (other predictors include vehicular wheelbase length and bonnet height and length). Moreover, four-wheel drives are more likely to be fitted with bullbars, which can be dangerous to pedestrians, cyclists and occupants of other vehicles. In the MUARC study described above, researchers used their dataset to predict the aggressivity rating (defined as the rate of serious injury inflicted per 100 drivers of other vehicles involved in collisions) for eight vehicle market groups – four-wheel drive, commercial, large, luxury, medium, passenger vans, small and sports. They found that four-wheel drives had the highest average aggressivity rating: 3.21 versus the overall average of 2.37. Small cars had the lowest rating (1.79). There was a spread of values within the four-wheel drive category, from as high as 5.49 to as low as 1.39. MUARC has also developed a measure of aggressivity to unprotected road users. It is based on the severity of injury to pedestrians, cyclists and motorcyclists when they are hit by a vehicle. Using this measure, four-wheel drives again had the highest aggressivity rating of the eight vehicle market groups. These results provide support for both sides of the argument. Four-wheel drive enthusiasts are right to argue that many models offer them good protection, certainly better than is provided by most small cars, while opponents are also correct when they say the increase in four-wheel drives poses an added danger to unprotected road users and also to people in other cars. Fuel consumption Environmentalists fear that if four-wheel drives become more popular, Australian roads will soon be overrun by gas-guzzlers, which not only slurp down huge quantities of a limited natural resource but make an unjustifiably high contribution to air pollution, particularly greenhouse gas emissions.
According to the Fuel Consumption Guide published by the Australian Greenhouse Office, most four-wheel drives score poorly on fuel economy. For example, the Ford Explorer, a four-wheel drive, five-door wagon, drinks 16 litres of fuel for every 100 kilometres of city roads it travels and 11 litres per 100 kilometres on the highway. In contrast, the Holden Zafira, a two-wheel drive, five-door people-mover, consumes 9 litres per 100 kilometres in the city and 6.8 litres on the highway. The Explorer's engine is considerably larger than that of the Zafira, but the Zafira can carry three more people. Four-wheel drives are particularly thirsty in the city. Many models (such as the Ford Escape, Ford Explorer, Holden Jackeroo, Grand Cherokee jeep, Mazda Tribute and BMW SAV) suck down more than 12 litres per 100 kilometres in standard city tests, and some as much as 18 litres. In contrast, large two-wheel drive cars such as the Mitsubishi Magna and the Holden Commodore use 10 litres or less. This sort of difference adds up. Families that opt for a Magna over a Cherokee, for example, would burn 700 fewer litres over the course of a year in which they drove 10,000 kilometres along city streets. They would save a substantial sum of money and emit 1600 kilograms less carbon dioxide, a major greenhouse gas. Australian road traffic is responsible for about 13 per cent of the nation's greenhouse gas emissions, a percentage that is rising rapidly. If Australia is to meet its greenhouse gas emissions target (no more than 108 per cent of 1990 levels over the period 2008 to 2012) we will need to be careful about which cars we buy and how we use them. A substantial increase in our use of large four-wheel drives for frequent city driving will make this target much harder to reach. The Australian government often voices its commitment to limiting the increase of greenhouse gas emissions. Perversely, though, it continues to encourage the sale of four-wheel drives, nearly all of which are imported, by imposing an import tariff of only 5 per cent – compared to 15 per cent for other passenger cars. This difference in tariff acts as a subsidy for people who purchase a four-wheel drive. Making the right choice The debate about four-wheel drives can be emotive and accusatory. Some people believe it is their inalienable right to own a four-wheel drive. Others are incensed by what they see as the disregard four-wheel drive owners – who, they say, almost never go bush anyway – have for the safety of other road users or for the health of the planet. It doesn't have to be so fractious. Some four-wheel drives are quite fuel-efficient: the Landrover Freelander, for example, burns a comparatively modest 11 litres per 100 kilometres in the city and 6.8 litres on the highway; some four-wheel drives are less aggressive than other four-wheel drives. If you need a four-wheel drive for its additional traction, why not opt for one with these sorts of attributes? If the main concern is safety, why not explore the wide range of two-wheel drives that are more crashworthy (ie, safer) than four-wheel drives and are also less aggressive and more fuel-efficient? The information is increasingly available, so it should just be a matter of weighing the safety of friends and family, the cost, and the environmental impact. In this sort of sum, two plus two won't always equal four. Related Nova topics: Death-defying designs for car safety Fatal impact – the physics of speeding cars The shocking truth about road trauma
Activities
Further readingAustralasian Science August 2000, page 7 Toorak tractor killers (by Stephen Luntz) Describes research at the Monash University Accident Research Centre that determines the 'aggressivity' of vehicles.
New Scientist A collection of articles on cars and motoring is available.
13 Decmeber 2003, page 23 Pedestrians at risk from sports utility vehicles (by Paul Marks) Describes American research that found a pedestrian hit by a large SUV is more than twice as likely to die as someone hit by a car.
8 March 2003, pages 12-13 Crunch time for the SUV (by Anil Ananthaswamy) Discusses safety and economic issues relating to sport utility vehicles.
23 June 2001, page 23 Hit and stun (by Max Glaskin) Describes a pop-up car bonnet that could protect pedestrians from fatal head injuries.
11 April 1998, page 52 Return of the gas guzzler (by Ian Lowe) Examines an Australian report on fuel consumption.
Scientific American August 2001, page 2 Another cup of CAFE, please (by The Editors) Discusses why raising Corporate Average Fuel Economy standards would be a good way to improve fuel economy in the United States.
Useful sitesHow four-wheel drive works (How Stuff Works, USA)Uses diagrams to help explain the basics of a four-wheel drive system. Includes definitions.
Royal Automobile Club Victoria, Australia
Australian Broadcasting Corporation (transcripts)
Monash University Accident Research Centre, Australia
Clean vehicles – building a better SUV (Union of concerned Scientists, USA) Looks at how existing technologies can be used to offer consumers a four-wheel drive that is safer, cleaner and more cost effective.
Vehicle compatibility: Analysis of fatal crashes (Australian Transport Safety Bureau)
A study which looks at the relative risk of injury and death of occupants in passenger vehicles of different sizes.
Road safety and 4WDs (Parliamentary Library, Australia)
Summarises some 4WD road-safety issues, driver attitudes and the national road safety policy.
Vehicle properties determining aggressivity (2001 Road Safety Conference, Australia) Describes a study that used multiple regression analysis to determine which physical features of a vehicle are most likely to contribute to vehicle aggressivity.
Glossarycentre of gravity. The point around which a body's weight is equally balanced in all directions. The total weight of the object is concentrated at this point. coefficient of friction. The ratio of the force that is necessary to move an object and the weight of the object. It is a measure of the amount of friction that exists between two materials as one slides over the other. The coefficient of friction is zero if there is no friction, and it is infinite if no motion is possible. For more information see How Brakes Work: Friction (How Stuff Works, USA). greenhouse gas. A gas that is transparent to incoming solar radiation and absorbs some of the longer wavelength infrared radiation (heat) that the Earth radiates back. The result is that some of the heat given off by the planet accumulates, making the surface and the lower atmosphere warmer. For more information see The greenhouse effect (CSIRO Atmospheric Research, Australia). mass. The amount of matter in an object. traction. The amount of forward thrust that a wheel can provide before it slips. It is the product of the weight bearing down on the wheel (generally 25 per cent of the vehicle weight on a level road) and the coefficient of friction, which depends on the nature of the tyre and the surface of the road. Traction helps determine the steepest road a vehicle can climb. weight. The downward force of gravity on an object.
External sites are not endorsed by the Australian Academy of Science. Page updated April 2006. The Australian Foundation for Science is also a supporter of Nova.
This topic is sponsored by NRMA – ACT Road Safety Trust.
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