Tractors and other mobile machinery in agricultural, forestry, construction and mining work, as well as materials handling, can give rise to serious hazards when the vehicles roll over sideways, tip over forwards or rear over backwards. The risks are heightened in the case of wheeled tractors with high centres of gravity. Other vehicles that present a hazard of rollover are crawler tractors, loaders, cranes, fruit-pickers, dozers, dumpers, scrapers and graders. These accidents usually happen too fast for drivers and passengers to get clear of the equipment, and they can become trapped under the vehicle. For example, tractors with high centres of gravity have considerable likelihood of rollover (and narrow tractors have even less stability than wide ones). A mercury engine cut-off switch to shut off power upon sensing lateral movement was introduced on tractors but was proven too slow to cope with the dynamic forces generated in the rollover movement (Springfeldt 1993). Therefore the safety device was abandoned.
The fact that such equipment often is used on sloping or uneven ground or on soft earth, and sometimes in close proximity to ditches, trenches or excavations, is an important contributing cause to rollover. If auxiliary equipment is attached high up on a tractor, the probability of rearing over backwards in climbing a slope (or tipping over forwards when descending) increases. Furthermore, a tractor can roll over because of the loss of control due to the pressure exerted by tractor-drawn equipment (e.g., when the carriage moves downwards on a slope and the attached equipment is not braked and over-runs the tractor). Special hazards arise when tractors are used as tow vehicles, particularly if the tow hook on the tractor is placed on a higher level than the wheel axle.
Notice of the rollover problem was taken on the national level in certain countries where many fatal rollovers occurred. In Sweden and New Zealand, development and testing of rollover protective structures (ROPS) on tractors (figure 1) already were in progress in the 1950s, but this work was followed up by regulations only on the part of the Swedish authorities; these regulations were effective from the year 1959 (Springfeldt 1993).
Figure 1. Usual types of ROPS on tractors
Proposed regulations prescribing ROPS for tractors were met by resistance in the agricultural sector in several countries. Strong opposition was mounted against plans requiring employers to install ROPS on existing tractors, and even against the proposal that only new tractors be equipped by the manufacturers with ROPS. Eventually many countries successfully mandated ROPS for new tractors, and later on some countries were able to require ROPS be retrofitted on old tractors as well. International standards concerning tractors and earth-moving machinery, including testing standards for ROPS, contributed to more reliable designs. Tractors were designed and manufactured with lower centres of gravity and lower-placed tow hooks. Four-wheel drive has reduced the risk of rollover. But the proportion of tractors with ROPS in countries with many old tractors and without mandates for retrofitting of ROPS is still rather low.
Rollover accidents, particularly those involving tractors, have been studied by researchers in many countries. However, there are no centralized international statistics with respect to the number of accidents caused by the types of mobile machinery reviewed in this article. Available statistics at the national level nevertheless show that the number is high, especially in agriculture. According to a Scottish report of tractor rollover accidents in the period 1968–1976, 85% of the tractors involved had equipment attached at the time of the accident, and of these, half had trailed equipment and half had mounted equipment. Two-thirds of the tractor rollover accidents in the Scottish report occurred on slopes (Springfeldt 1993). It was later proved that the number of accidents would be reduced after the introduction of training for driving on slopes as well as the application of an instrument for measuring slope steepness combined with an indicator of safe slope limits.
In other investigations, New Zealand researchers observed that half of their fatal rollover accidents occurred on flat ground or on slight slopes, and only one-tenth occurred on steep slopes. On flat ground tractor drivers may be less attentive to rollover hazards, and they can misjudge the risk posed by ditches and uneven ground. Of the rollover fatalities in tractors in New Zealand in the period 1949–1980, 80% occurred in wheel tractors, and 20% with crawler tractors (Springfeldt 1993). Studies in Sweden and New Zealand showed that about 80% of the tractor rollover fatalities occurred when tractors rolled over sideways. Half of the tractors involved in the New Zealand fatalities had rolled 180°.
Studies of the correlation between rollover fatalities in West Germany and the model year of farm tractors (Springfeldt 1993) showed that 1 of 10,000 old, unprotected tractors manufactured before 1957 was involved in a rollover fatality. Of tractors with prescribed ROPS, manufactured in 1970 and later, 1 of 25,000 tractors was involved in a rollover fatality. Of fatal tractor rollovers in West Germany in the period 1980–1985, two-thirds of the victims were thrown from their protected area and then run over or hit by the tractor (Springfeldt 1993). Of nonfatal rollovers, one-quarter of the drivers were thrown from the driver’s seat but not run over. It is evident that the fatality risk increases if the driver is thrown out of the protected area (similar to automobile accidents). Most of the tractors involved had a two-pillar bow (figure 1 C) that does not prevent the driver from being thrown out. In a few cases the ROPS had been subject to breakage or strong deformation.
The relative frequencies of injuries per 100,000 tractors in different periods in some countries and the reduction of the fatality rate was calculated by Springfeldt (1993). The effectiveness of ROPS in diminishing injury in tractor rollover accidents has been proven in Sweden, where the number of fatalities per 100,000 tractors was reduced from approximately 17 to 0.3 over the period of three decades (1960–1990) (figure 2). At the end of the period it was estimated that about 98% of the tractors were fitted with ROPS, mainly in the form of a crushproof cab (figure 1 A). In Norway, fatalities were reduced from about 24 to 4 per 100,000 tractors during a similar period. However, worse results were achieved in Finland and New Zealand.
Figure 2. Injuries by rollovers per 100,000 tractors in Sweden between 1957 and 1990
Prevention of Injuries by Rollovers
The risk of rollover is greatest in the case of tractors; however, in agricultural and forest work there is little that can be done to prevent tractors from rolling over. By mounting ROPS on tractors and those types of earth-moving machinery with potential rollover hazards, the risk of personal injuries can be reduced, provided that the drivers remain on their seats during rollover events (Springfeldt 1993). The frequency of rollover fatalities depends largely on the proportion of protected machines in use and the types of ROPS used. A bow (figure 1 C) gives much less protection than a cab or a frame (Springfeldt 1993). The most effective structure is a crushproof cab, which allows the driver to stay inside, protected, during a rollover. (Another reason for choosing a cab is that it affords weather protection.) The most effective means of keeping the driver within the protection of the ROPS during a rollover is a seat-belt, provided that the driver uses the belt while operating the equipment. In some countries, there are information plates at the driver’s seat advising that the steering wheel be gripped in a rollover event. An additional safety measure is to design the driver’s cab or interior environment and the ROPS so as to prevent exposure to hazards such as sharp edges or protuberances.
In all countries, rollovers of mobile machinery, mainly tractors, are causing serious injures. There are, however, considerable differences among countries concerning technical specifications relating to machinery design, as well as administrative procedures for examinations, testing, inspections and marketing. The international diversity that characterizes safety efforts in this connection may be explained by considerations such as the following:
- whether there exist mandatory requirements for ROPS (in the form of regulations or legislation), or recommendations only, or no rules at all
- the need for rules for new machinery and rules applicable to older equipment
- the availability of inspection carried out by authorities and the existence of social pressure and cultural climate favourable to observance of safety rules; in many countries, the obedience to safety guidelines is not checked by inspection in agricultural work
- pressure from trade unions; however, it should be borne in mind that workers’ organizations have less influence on working conditions in agriculture than in other sectors, because there are many family farms in agriculture
- the type of ROPS used in the country
- information and understanding of the risks to which tractor drivers are exposed; practical problems often stand in the way of reaching farmers and forest workers for the purposes of information and education
- the geography of the country, especially where agricultural, forestry and road work is carried out.
The nature of rules governing requirements for ROPS and the degree of implementation of the rules in a country, has a strong influence on rollover accidents, especially fatal ones. With this in mind, the development of safer machinery has been abetted by directives, codes and standards issued by international and national organizations. Additionally, many countries have adopted rigorous prescriptions for ROPS which have resulted in a great reduction of rollover injuries.
European Economic Community
Beginning in 1974 the European Economic Community (EEC) issued directives concerning type-approval of wheeled agricultural and forestry tractors, and in 1977 issued further, special directives concerning ROPS, including their attachment to tractors (Springfeldt 1993; EEC 1974, 1977, 1979, 1982, 1987). The directives prescribe a procedure for type-approval and certification by manufacture of tractors, and ROPS must be reviewed by an EEC Type Approval Examination. The directives have won acceptance by all the member countries.
Some EEC directives concerning ROPS on tractors were repealed as of 31 December 1995 and replaced by the general machinery directive which applies to those sorts of machinery presenting hazards due to their mobility (EEC 1991). Wheeled tractors, as well as some earth-moving machinery with a capacity exceeding 15 kW (namely crawlers and wheel loaders, backhoe loaders, crawler tractors, scrapers, graders and articulated dumpers) must be fitted with a ROPS. In case of a rollover, the ROPS must offer the driver and operators an adequate deflection-limiting volume (i.e., space allowing movement of occupants’ bodies before contacting interior elements during an accident). It is the responsibility of the manufacturers or their authorized representatives to perform appropriate tests.
Organization for Economic Cooperation and Development
In 1973 and 1987 the Organization for Economic Cooperation and Development (OECD) approved standard codes for testing of tractors (Springfeldt 1993; OECD 1987). They give results of tests of tractors and describe the testing equipment and test conditions. The codes require testing of many machinery parts and functions, for instance the strength of ROPS. The OECD Tractor Codes describe a static and a dynamic method of testing ROPS on certain types of tractors. A ROPS may be designed solely to protect the driver in the event of tractor rollover. It must be retested for each model of tractor to which the ROPS is to be fitted. The Codes also require that it be possible to mount a weather protection for the driver onto the structure, of a more or less temporary nature. The Tractor Codes have been accepted by all OECD member bodies from 1988, but in practice the United States and Japan also accept ROPS that do not comply with the code requirements if safety belts are provided (Springfeldt 1993).
International Labour Organization
In 1965, the International Labour Organization (ILO) in its manual, Safety and Health in Agricultural Work, required that a cab or a frame of sufficient strength be adequately fixed to tractors in order to provide satisfactory protection for the driver and passengers inside the cab in case of tractor rollover (Springfeldt 1993; ILO 1965). According to ILO Codes of Practice, agricultural and forestry tractors should be provided with ROPS to protect the operator and any passenger in case of rollover, falling objects or displaced loads (ILO 1976).
The fitting of ROPS should not adversely affect
- access between the ground and driver’s position
- access to the tractor’s main controls
- the manoeuvrability of the tractor in cramped surroundings
- the attachment or use of any equipment that may be connected to the tractor
- the control and adjustment of associated equipment.
International and national standards
In 1981 the International Organization for Standardization (ISO) issued a standard for tractors and machinery for agriculture and forestry (ISO 1981). The standard describes a static test method for ROPS and sets forth acceptance conditions. The standard has been approved by the member bodies in 22 countries; however, Canada and the United States have expressed disapproval of the document on technical grounds. A Standard and Recommended Practice issued in 1974 by the Society of Automotive Engineers (SAE) in North America contains performance requirements for ROPS on wheeled agricultural tractors and industrial tractors used in construction, rubber-tired scrapers, front-end loaders, dozers, crawler loaders, and motor graders (SAE 1974 and 1975). The contents of the standard have been adopted as regulations in the United States and in the Canadian provinces of Alberta and British Columbia.
Rules and Compliance
OECD Codes and International Standards concern the design and construction of ROPS as well as the control of their strength, but lack the authority to require that this sort of protection be put into practice (OECD 1987; ISO 1981). The European Economic Community also proposed that tractors and earth-moving machinery be equipped with protection (EEC 1974-1987). The aim of the EEC directives is to achieve uniformity among national entities concerning the safety of new machinery at the manufacturing stage. The member countries are obliged to follow the directives and issue corresponding prescriptions. Starting in 1996, the member countries of the EEC intend to issue regulations requiring that new tractors and earth-moving machinery be fitted with ROPS.
In 1959, Sweden became the first country to require ROPS for new tractors (Springfeldt 1993). Corresponding requirements came into effect in Denmark and Finland ten years later. Later on, in the 1970s and 1980s, mandatory requirements for ROPS on new tractors became effective in Great Britain, West Germany, New Zealand, the United States, Spain, Norway, Switzerland and other countries. In all these countries except the United States, the rules were extended to old tractors some years later, but these rules were not always mandatory. In Sweden, all tractors must be equipped with a protective cab, a rule that in Great Britain applies only to all tractors used by agricultural workers (Springfeldt 1993). In Denmark, Norway and Finland, all tractors must be provided with at least a frame, while in the United States and the Australian states, bows are accepted. In the United States tractors must have seat-belts.
In the United States, materials-handling machinery that was manufactured before 1972 and is used in construction work must be equipped with ROPS which meet minimum performance standards (US Bureau of National Affairs 1975). The machines covered by the requirement include some scrapers, front-end loaders, dozers, crawler tractors, loaders, and motor graders. Retrofitting was carried out of ROPS on machines manufactured about three years earlier.
In countries with mandatory requirements for ROPS for new tractors and retrofitting of ROPS on old tractors, there has been a decrease of rollover injuries, especially fatal ones. It is evident that a crushproof cab is the most effective type of ROPS. A bow gives poor protection in case of rollover. Many countries have prescribed effective ROPS at least on new tractors and as of 1996 on earth-moving machines. In spite of this fact some authorities seem to accept types of ROPS that do not comply with such requirements as have been promulgated by the OECD and the ISO. It is expected that a more general harmonization of the rules governing ROPS will be accomplished gradually all over the world, including the developing countries.