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Figure 1. A tanker and a lorry caught fire after a rear-end collision near Kungälv, Sweden.2

Post-collision fires in road vehicles

A recent study on post-collision fires published by SP Technical Research Institute of Sweden (now RISE Research Institutes of Sweden) in collaboration with Umeå University and other partners1, in which data from crashes occurred between 2002 and 2014 were investigated, shows that the loss of human lives and body injuries as a consequence of post-crash fires either by smoke inhalation or due to burn injuries is not uncommon.

Different databases were analysed for the years 2002 to 2014 and relevant literature was summarised. In particular, the FARS, CDS and NFIRS databases were compiled for the United States and compared to the literature, data from Sweden was obtained through the literature and data from England was provided by the fire statistical services. Additional attempts to obtain data from Belgium and France unfortunately provided little usable information. The data from the United States and existing literature for both the US and Sweden could however allow a reasonable overview of the available data and general situation.

The compiled data from the United States suggests that up to 3 % of vehicles involved in fatal crashes have a related fire event, and that over 5 % of fatalities occur in cars having caught fire, up to one third of the deaths being directly caused by fire. This is in good accordance with existing Swedish data. According to the FARS data, fire events in fatal crashes and associated fatalities show an increasing trend, in particular in the last four years, even though car crashes become less frequent. Most car fires are due to frontal crashes and start in the engine compartment; however fuel tank fires tend to be more severe and deadly in comparison. Increased energy absorption leads to more severe fires. The toxicity of the fumes produced by the burning material is important. Databases contain valuable information, but the amount of missing data is an obstacle to the analysis, in particular when it comes to specifically post-crash fire data. Strict conclusions are therefore difficult to articulate, but the results are however interesting.

Due to increased traffic safety measures, the amount of vehicles and fatalities involved in fatal traffic accidents has been decreasing over the years in the United States. On the contrary, fire events in fatal traffic accidents show increasing trends, in particular from 2012 onwards as shown in Table 1. In 2004 Digges3 noted that during 2000, 2.9 % of fatal crashes had an associated fire event, which was virtually unchanged from the 1990s where fire events ranged between 2.6 and 2.9 %. However our analysis shows that the proportion of motor vehicle in transport (MVITs) catching fire in an accident has been of over 3 % from 2011 according to FARS data (up to 3.3 % in 2013). The proportion of fatalities taking place in these vehicles follows a similar increasing trend, being over 5% from 2012 onwards. Roughly one third of these fatalities occurred in vehicles were fire was noted as the most harmful event, but it is unfortunately not possible to conclude that these fatalities are directly caused by the fire. As a comparison, Viklund et al.4 reported that 5% of fatalities in passenger cars on Swedish roads (1998-2008) occur in vehicles with a reported fire event, one third of these fatalities being directly caused by fire. Data from England (2009-2015) ranges around 22 to 24 % of these fatalities being directly caused by the fire, which is slightly lower. This generally indicates that fires related to crashes in vehicles are still a significant problem.

The significance of post-crash fires can also be observed in the NFIRS database in Table 2. According to NFIRS data for the years 2005 to 2013, the main causes for car fires are mechanical failure (43 %) and electrical failure (21 %), collision accounting for only 4 % of all vehicle fires. However, when it comes to vehicles fire fatalities, collision is the leading factor, with 55 %.

Even though car fires due to collisions are relatively rare, they play an important role when it comes to fatalities and should be studied thoroughly so as to lower this fire risk.

Taking a closer look to the FARS database, all categories of vehicles do not have the same probabilities of catching fire. Passenger cars and light trucks have similar rates, ranging around an average of 3 %, the rates of passenger cars slowly increasing over the years, the one of the light trucks being more stable. Large trucks have a much higher rate, varying between 5 and 7 %. The amount of large trucks involved in these accidents are however much smaller than passenger cars and light trucks, which could be an explaining factor of the higher variability of the data over the years. It would be interesting to determine the leading mechanisms behind the higher fire rate for large trucks. Hazardous loads could play a role, as well as the extra fuel tanks built in these vehicles. Interesting to note is that the rate of fatalities occurring in burning vehicles is slightly higher for light truck vehicles (average of 5.2 % for 2002-2014) than for passenger cars (4.6 %), while it is much higher for heavy trucks (20.5 %). This is due to the fact that heavy truck passengers rarely die in fatal crashes compared to the occupants of the other vehicle involved, unless there is a fire entry. As heavy trucks were not the main point of interest of this study, this question was not analysed any further, but would be of interest for later research.

Aggravating factors such as the strength of the impact or entrapment are important factors to take into account. Using the CDS database, it is observable that fire probability increases with increasing energy absorption as shown in Figure 1. The higher this energy, the more frequent and severe the fire cases are. The severity of the injuries increases between accidents, accidents with a fire, accidents combining both fire and entrapment (data not shown).5

The large amount of fatalities following vehicle fires can be understood by the low fire performance of the materials used for vehicle manufacturing. Although there is a legislation specifying some minimum requirements with regard to fire performance of materials fitted in vehicles, the existing legislation is flaccid when compared to the fire safety regulations for the aeronautic, marine and rolling-stock sectors. For example, fire prevention requirements in buses,6,7 do not cover aspects such as the limitation of peak heat release rate, smoke yield and toxicity as done in regulations for trains, planes and ships.

Figure 2: Fire rates in crashes according to energy absorption from CDS data between 2005 and 2014, unweighted and weighted.

Figure 2: Fire rates in crashes according to energy absorption from CDS data between 2005 and 2014, unweighted and weighted.

Reducing fires in vehicles is strongly related to the survivability of the occupants against a collision event. Statistics show that occupants travelling on modern vehicles have higher probabilities of surviving to a severe impact than of those travelling on vehicles with older technologies. This is however expected, the active and passive safety systems increase the survivability rate but these do not reduce the risks of a fire as a post-collision event.

In order to reduce the number of injuries and human fatalities associated to post-crash fires in road vehicles, it is necessary to study the causes of these lethal fires. Of particular interest are the ignition sources, vehicle types, fire dynamics and toxicology mechanisms which directly contribute to the loss of human lives, for instance, the dynamics and toxicity of fires due to upholstery and materials in passenger compartments or toxic gases due to fires in electric vehicles. New materials and traction systems (e-vehicles) introduces new toxic substances when burning – but intoxication by some of them could be reduced by using available antidotes – which make knowledge of these substances necessary.

RISE Safety and Transport works intensively for improving the fire safety of road vehicles. RISE safety and Transport welcomes collaborative research efforts that will lead to safer means of transportation and believes that more research funding is needed for reducing the amount of deaths and injuries caused by post collision fires. Further, a more stringent legislation with regard to the materials fitted in vehicles will lead to safer vehicles.

For more information, go to www.sp.se/fireresearch

References

  1. Ochoterena R., et al., Post-collision fires in road vehicles, a pre study, SP Rapport 2016:55, 2016.
  2. Photo by Linus Olsson from Olycksundersökning Tankbilsolycka E6 2011-10-05, 2011.
  3. Digges, K. H., & Stephenson, R., A Research Program in Crash-Induced Fire Safety. SAE International (2004-01-0475).
  4. Viklund Å., Björnstig J., Larsson M., and Björnstig U., Car Crash Fatalities Associated With Fire in Sweden, Traffic and Injury Prevention 14, 823–827, 2013.
  5. Digges, K., & Stephenson, R., Fireworthiness: a final report on the technology base. In ESV Conference, (2009).
  6. Directive 95/28/EC of the European Parliament and of the Council of 24 October 1995 relating to the burning behaviour of materials used in the interior construction of certain categories of motor vehicles, 1995.
  7. UNECE Regulation No. 118: Uniform technical prescription concerning the burning behaviour and/or, the capability to repel fuel or lubricant or materials used in the construction of certain categories of motor vehicles, UNECE, Switzerland, 2012.
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Raúl Ochoterena is employed as a senior researcher at the Division of Safety and Transport of RISE. His research field involves combustion, aerosols and optics.