Post-collision fires in road vehicles

Statistical data from postcrash fires

The compiled data from USA 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 1/3 of the deaths being directly caused by fire. This is in accordance with existing Swedish data. According to the FARS (Fatality Analysis Reporting System) 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.

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.

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 US. On the contrary, fire events in fatal traffic accidents show increasing trends, in particular from 2012 onwards. Roughly 1/3 of these fatalities occurred in vehicles where 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. (2013) reported that 5% of fatalities in passenger cars on Swedish roads (1998-2008) occur in vehicles with a reported fire event, 1/3 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.

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. 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 20022014) 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.

An analysis of the major crash modes in FARS shows that frontal crashes are overrepresented: 70% of all fires had a frontal initial / main point of impact. Rear, right and left have similar frequencies as shown in Figure 1. The fire rate of each crash mode is highest for the frontal impacts, and decreases following rear, right and left crashes. Interestingly, right and left impacts have different fire rates.

Rollovers seem to increase fire rates. From the total accidents with rollovers, 3.8% of the cases sustained a fire, whereas accidents with no rollover have a fire rate of 2.8%. Rollovers could lead to higher fire frequency than other crash modes and states that the most frequent crash modes for major fires and fatalities are frontal and rollover.

The engine compartment is found to be the place where most fires originate in both databases, with the fuel tank following in the second position. The passenger area is last. In particular, fires ignited by the fuel tank lead more often to major fires than minor fires. Even though most fires start in the engine area, running gear and wheel area, the percent of fatalities vs fire events is much higher for fuel tank and fuel line originated fires. About half of the engine compartment fires spread to the passenger area (major fire), and that there is generally more time to escape engine compartment fires than fuel tank fires, which could be a factor explaining their smaller relative fatality rates.

Aggravating factors such as the strength of the impact or entrapment are important factors to take into account. Fire probability increases with increasing energy absorption. 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.

The fire threat should be addressed carefully. Real situation tests, which are not the standard tests performed when it comes to fire safety, have shown that the fire spreads faster when the engine has been running compared to when the engine is cold, that the kind of accident and severity affects the fire spread. In addition, the threat of toxic fumes could be greater than the risk from the flames, the ventilation potentially increasing smoke intrusion to the passenger compartment. These fumes typically contain HCl, HCN, SO2, aldehydes, isocyanates, VOCs (eg. Benzene), PAHs and PCDDs/PCDFs, all of them known toxic substances and a threat both to passengers and firefighters.

Polymers and fire in vehicles

Many types of polymers are used in more than 1,000 different parts of all shapes and sizes in an average car, these correspond to approximately 9 % of the car weight. It has been estimated that the polymers contents of an automobile is 115kg and petrol plus oils is 50kg, i.e. a total of combustibles of 165kg. Data from recycling of used automobiles show that automotive shredder residue could in average contain 100kg plastics per vehicle. The amount of combustible material present in a standard automobile is thus in the range 150–200kg. This is a rather large amount, which shows that plastics are a potential risk in car fires.

The results of the small scale tests showed that many of the materials from the automobile contained nitrogen and that these were capable of producing hydrogen cyanide (HCN) and other nitrogen containing compounds. The materials identified to be the greatest potential source of harmful compounds was that present in the upholstered seats in the automobile. Other material that produced even higher amounts of HCl included that in the door panel and electrical wirings.

As crashes are expected to become more survivable with advanced technology, fire events might become even more relevant. Database analysis gives an interesting general picture of the situation, but unfortunately the difficulty of analysing these fire events more precisely, the lack of reporting and thus high proportion of unknowns are obstacles to an in-depth analysis.

From SP Report 2016:55, Post-collision fires in road vehicles, a pre-study, by Raúl Ochoterena, Fabienne Roux, Anna Sandinge, Christoffer Nylander, Mats Lindkvist, Ulf Björnstig, David Sturk and Mikael Skrifvars.