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Vehicle Mass, Size and Safety

Federal Office of Road Safety - Contract Report 133

Full report in .pdf format [2.9MB]


A review of the international literature was undertaken to examine the relationship between a vehicle's mass or size and its influence on occupant protection. A number of specific objectives were addressed on this relationship and its consequence for vehicle down-sizing in Australia. There was considerable evidence that occupants in larger cars that crash have superior protection to those in smaller cars. However, the precise relationship between size, mass and safety was complex and not totally clear from this review. While mass appeared to be more relevant in multi-vehicle crashes, size seemed to be important in rollovers and single vehicle collisions generally. The question of whether mass or size has greatest influence on safety is relevant if future car construction emphasises lighter composite materials. Safety features act to offset mass effects with a suggestion that they have greatest importance for occupants of smaller cars. Vehicle downsizing was apparent during the seventies, the vehicle fleet mix has remained relatively stable since then. During this time, down-sizing seemed to have been driven more by changes in vehicle ownership and two-car families than world-wide oil shortages or economics in this country. Current and proposed design rules do not appear to have much influence on down-sizing. Economic analysis could throw additional fight on the costs and benefits of changes to the Australian fleet.

Executive Summary

An extensive review of the international road safety literature was undertaken by the Monash University Accident Research Centre for the Federal Office of Road Safety to examine the relationship between vehicle mass or size and occupant safety. More than 7O references on this topic were uncovered, essentially from the United States of America, but also from Sweden, Germany, the United Kingdom, France and Canada. One or two Australian references were also found.

The review set out to clarify issues of importance for Australian vehicles, to consider whether it is size or mass that is the dominant factor, to reflect on the safety consequences of changes to the vehicle fleet, and examine the Design rule implications for small and large vehicles. It was not intended to make specific recommendations but rather to raise relevant issues for discussion and identify areas that might require further research effort.

Mass and Size Effects on Occupant Safety

The literature on the crashworthiness relationship between vehicle mass, size and safety is rather ambiguous. There was general consensus by most authors that bigger cars were inherently more safe than smaller ones in a collision. However, trying to define this relationship more precisely from the literature is problematic, in part, because of the number of confounding influences and definitional differences.

It was concluded in general terms that mass is probably a more important safety feature than size for most car-to-car collisions, although there was a suggestion that size may predominate more in rollover crashes and single-vehicle accidents generally.

There was large variability in the mass and size effects reported in the literature. One report claimed that differences of only 45kg (100 pounds) can have a marked influence on fatality rates in multi-car crashes. Given the wide spread of effects reported, though, it would be extremely difficult to quantify precisely the consequences of down-sizing in terms of reduced occupant safety.

Very few of the reports claiming size effects were also consistent in terms of the amount of this effect. This may be a function of the different ways vehicle size was measured in these studies (overall dimensions, wheelbase, cabin size, etc). Given that the space inside the cabin will have a marked influence on the likelihood and severity of injury, it might be worthwhile in future examining size effects in terms of occupant space.

The level of restraint has been shown to have a marked influence on the mass (size) and safety relationship. One study reported that an unbelted driver in a 2000kg car had the same amount of protection as a belted driver in a 1140kg car. It was further claimed that drivers of small cars gained more from being restrained than those of larger ones. It is too early yet to confirm if there is any disproportionate benefit of driver airbags by vehicle size.

The relationship between vehicle mass and size and likelihood of collision is very much confounded by driver effects. Any effect of vehicle size on risk taking behaviour is at best only speculative at this time.

Down-Sizing and the Australian Fleet

Down-sizing in the Australian fleet seems to have occurred mainly during the seventies and early eighties. While it is often claimed that down-sizing occurred because of the oil crisis at that time, one local vehicle manufacturer argued that increased purchasing by women and increases in the rate of second vehicles at that time was the main motivation for down-sizing in this country.

Annual fleet and sales statistics show that the mix of small and large vehicles has been relatively stable throughout most of the 1980's and 1990's.

It seems that vehicle design rules in this country have had little if any effect on down-sizing in the past. New crash performance regulations recently introduced in Australia (ADR69) may have some marginal (upward) influence on car mass in future, although this influence will at best only be minimal and likely to apply generally across the whole vehicle beet.

Likely Changes in Future Vehicle Size or Mass

There are two major developments world-wide which may have consequences on fleet downsizing in the years ahead.

First, there were reports of a growing interest in the use of fight-weight materials such as aluminium and plastics in car construction. While the shell body of a car contributes less than half the total weight, nevertheless any substantial reduction in mass by the use of lighter materials could have some consequence on the safety of its occupants in multi-car crashes.

Second, the trend towards the use of finite element analysis (FEA) in car design to reduce mass while improving structural stiffness has the potential to influence the degree of safety for the vehicle's occupants.

Both these trends need to be closely monitored to ensure that occupant safety is optimised and to highlight the need for future regulations aimed at improving vehicle safety in this country.

Areas For Further Research

In the absence of a shift towards smaller vehicles in this country, it is difficult to point to specific areas requiring further research in down-sizing in Australia.

A cost-benefit study of the likely effects of down-sizing (and up-sizing) would be difficult at this time, requiring many assumptions and different scenarios of likely fleet changes and the effects on the community. Nevertheless, such an analysis could highlight what are the critical issues and the likely consequences of fleet size changes in the future.

Assessing in more detail the motivation for down-sizing both within the community and among local and overseas manufacturers would be helpful in demonstrating the need and directions for further research.