Fig. 1: The fire triangle related to buses and coaches.

Fig. 1: The fire triangle related to buses and coaches.

Bus transportation is regarded as one of the safest modes of public transportation. Millions of passengers ride safely every day to and from work, school and for pleasure. The manufacturers and operators of buses have gone to great lengths to establish and maintain this safety record. However, a fire resulting from a collision or failure of a component puts lives at risk and can have an enormous impact on operational costs, as well as customer confidence.

Aircraft, trains and passenger ships all have well-established standards, regulations, and certification processes to ensure the maintenance of high levels of safety as new materials and constructions are introduced. Buses on the other hand, mostly rely on the efforts of the OEM’s, operators and suppliers of fire systems to ensure safety with few strict regulations.

Bus fire safety encompasses many aspects of design, operation, maintenance, evacuation and even first responders. All aspects play a critical role in establishing effective standards and best practices but are typically the responsibility of different stakeholders in the development process. Buses pose many unique challenges in regards to fire risks. The varying modes of operation (city, highway, long distance), unique vehicle types (school, transit, coach, demand-response), and design changes to meet new emissions standards provide the context for fire safety; but each fire hazard, whether it is an engine compartment, battery compartment, wheel well or luggage compartment, poses unique challenges in the way of geometry, airflow, clutter, flame spread and evacuation. This can be summarized in a modification of the traditional fire triangle to the specific situation for buses and coaches as shown in Figure 1.  

The engine compartment, which is one of the more well-known fire risks, poses several challenges that effect the detection and suppression of fires. Ventilation, through fans and openings in the engine compartment, can produce high levels of airflow. This facilitates necessary cooling of the engine and compartment, but can also increase the intensity and spread of flames, which can have an enormous impact on fire detection times and suppression system effectiveness.  

The varying designs, geometry and clutter represent risks that must be analyzed. The first step in this process, and perhaps the most important one, is to conduct a thorough fire risk analysis, which should include a Failure Mode Effects Analysis (FMEA) during the design phase. The FMEA will identify the highest risks with regard to occurrence and severity and what methods are used to detect them. This will help ensure maintenance practices and training address the risks identified.

The first few months of operation and operation in all seasonal conditions are also critical. High temperatures created in the engine compartment can cause premature failure of components and potentially increase surface temperatures, which could ignite flammable fluids more readily than expected. The use of temperature strips in high risk areas to identify unexpected high temperature zones is a good “best practice.”

Needless to say, fire prevention is an ongoing process; and, even when dealt with vigilantly there is always some residual risk of an event as seen in Figure 2. Most would see this as unfortunate accident and we can sometimes believe that determining the origin and cause resolves the problem. Such incidents could also be viewed as one of the most important learning tools at the disposal of stakeholders, which can provide critical input into best practices for design, operation and human factors.
Research at the forefront
SP Fire Research is part of SP Technical Research Institute of Sweden and one of the largest fire research facilities in the world, with capabilities including the ability to conduct fire testing on full size buses. Staffed by researchers, engineers and industry experts, it offers a complete solution for evaluation, testing, and risk analysis of innovative technologies and solutions. Since 2004, SP has promoted global bus fire safety as a top priority. As part of this work, SP has been active in the development of standards and test methods specific to vehicle hazards. The aim of such test methods is to establish performance-based standards for the objective evaluation of safety performance of products and solutions. Products are tested for performance against realistic fire scenarios and environmental aspects specific to the hazard. More information concerning SP Fire Research can be found at

Fig. 3: Fire in a gas bus conducted as part of a reconstruction for the Swedish Accident Investigation Board.

Fig. 3: Fire in a gas bus conducted as part of a reconstruction for the Swedish Accident Investigation Board.

Most recently SP has been involved in the investigation of an incident in the south of Sweden, where two gas buses collided and started to burn. The work was commissioned by the Swedish Accident Investigation Authority (SAIA). The reason for fire was that gas ignited oil in the engine compartment. As part of the investigation, a full-scale reconstruction of the bus fire was carried out to establish the cause of ignition and answer the question of why the fire suppression system did not manage to extinguish the fire, see Figure 3. The investigation led to a number of conclusions and recommendations from the SAIA to the Swedish Civil Contingencies and the Swedish Transport

Agency indicating a pressing need to:

  • Develop procedures for first responders how to approach CNG bus fires.
  • Establish requirements for fixed fire suppression systems in engine compartments.
  • Require function control of fire suppression system in conjunction with the regular vehicle inspection.
  • Extended and customized professional driver training for bus drivers with exercises in fire safety and evacuation.

Bus Engine Fire Suppression Standards/Certification
SP Fire Research is leading the way for more effective fire suppression systems by establishing a certification process (SPCR183) and test method (SP test Method 4912) for testing  fire suppression systems against known fire threats and environmental conditions specific to bus engine environments. Successful systems are then issued a certificate and allowed to “P” mark their components, see Figure 4. More information can be obtained from

Fig.4: The P-mark is a voluntary certification/quality mark for the industry for verifying and securing a good level of quality for fire suppression systems in engine compartments of buses and coaches.

Fig.4: The P-mark is a voluntary certification/quality mark for the industry for verifying and securing a good level of quality for fire suppression systems in engine compartments of buses and coaches.

Several fire tests conducted on operational buses have confirmed the pass/fail criteria used in the certification demonstrates a high level of protection. More than 20 various types of systems (dry chemical, water-based, aerosols, gaseous) are pursuing the “P” mark. Several systems have already been approved or are in the final stages of approval.

Adoption of the standard is becoming more common around the globe, and U.S. transit operators are among the first to actively include the “P” mark in their procurement specifications.

The test method is comprised of the following parts:

  • Fire Testing: The fire test (Figure 5) simulates not only different types of fire hazards, but also includes airflow, clutter and other parameters that can have an impact on the system’s ability to suppress fires and provide protection against the possibility of re-ignition.
  • Component testing: Mechanical and thermal stress resistance, Mechanical shock, Corrosion resistance and Ingress Protection Rating of electrical equipment.
  • Risk assessment: A risk assessment by an experienced professional is required on new vehicle designs or when variations in design use condition and environment, could change the fire risk potential or system performance.  
  • Follow-up inspection: An annual inspection of the manufacturers’ production facility and quality control plan ensures compliance to the requirements of the certification rules.
  • Sampling of extinguishing agent: Sample of the extinguishing agent are tested against known standards and analyzed to ensure conformity.

Global increased awareness

Fig. 5: Fire testing in a test rig, simulating a bus engine compartment, at SP in accordance with SP method 4912.

Fig. 5: Fire testing in a test rig, simulating a bus engine compartment, at SP in accordance with SP method 4912.

All over the world there is an increased awareness related to fire safety. Government agencies, industry associations, OEM’s, operators and even special advocacy groups have formed committees and knowledge platforms to discuss the issues.

In the U.S., efforts are underway at both the federal and state level to improve bus fire safety. These efforts have included research and testing and the adoption of requirements for automatic fire detection and suppression systems.
All modes of bus transportation in the U.S. are using some form of automatic fire systems. Some make it mandatory for all vehicles; others focus on vehicles where evacuation issues are a concern.

The UNECE (United Nations Economic Commission for Europe) is considering a proposed change to Regulation 107, which could require the installation of fire suppression systems in the engine compartment of all single-deck, double-deck, rigid or articulated vehicles of category M2 or M3.

Future Goals

Fig. 6: Alternative fuels fires like ethanol can also to some extent be more challenging to extinguish.

Fig. 6: Alternative fuels fires like ethanol can also to some extent be more challenging to extinguish.

Fire safety in buses has been the focus of significant research in recent years, but much improvement still remains, in particular related to fire prevention and safe egress. The sharing of information and best practices can benefit all parties, but most importantly the safety of the passengers.

SP will be publishing its top most wanted fire-related issues of which a few are listed below. Some projects have already been funded and are in progress, other are awaiting funding opportunities:

Wheel well fires (containment, preventing fire through window)
The exposed environment and deep seated fire risk makes wheel well fires particularly difficult to detect and suppress. Tests have shown that it can take less than 5 minutes before toxic fumes and smoke enter the passenger compartment. Tire pressure/temperature monitoring systems represent one method for early detection; other methods need to be explored. Early testing of some coatings in the wheel well area have been shown to provide a significant increase in the evacuation time before toxic fumes enter the passenger compartment. Continued work in this area could provide a low-cost solution.

Bulkheads/fire partitions (smoke tightness)
The bulkheads between high-risk areas such as the engine compartment and the passenger compartment can allow toxic fumes and flame spread into the passenger compartment. New materials present new challenges as does the increased complexity of systems connecting from the engine compartment into the driver area. Methods to contain the fire in the engine compartment would mean better passenger safety and less damage from flame spread.

Detection systems
SP is currently working on a project funded by the Strategic Vehicle Research and Innovation Foundation in Sweden, an independent funding agency. The project has several co-financing partners, including vehicle manufacturers, insurance companies, end users, a transport agency and several suppliers of fire detection systems for vehicles. One part of the project aims to develop a new test method and propose a standard for fire detection and fire alarm systems in heavy vehicles.

Further, the project will investigate non-traditional methods such as predictive failure modes for high risk components, facilitating the rapid identification of pending failures to hoses and lines carrying flammable fluids or components that have greatly exceeded their operational parameters.

Use of flammable material in high-risk areas
The need for lighter, less costly materials is always present; but there are very few standards and regulations regarding the flammability and toxic fume production of such material. Further research is necessary to establish adequate performance requirements for the use of flammable material in high-risk areas in buses and coaches.  

Fig. 7: ASTA ZERO (Active Safety Test Area AB a state-of-the-art Proving Ground test facility in Sweden specifically designed for developments in active traffic safety.

Fig. 7: ASTA ZERO (Active Safety Test Area AB a state-of-the-art Proving Ground test facility in Sweden specifically designed for developments in active traffic safety.

Electrical fires
Electrical fires pose as many challenges as wheel well fires. Electrical arcing and shorts do not always trip protective devices, such as fuses. Some cables, such as those used from the battery to the starter and alternator, carry very high currents capable of producing enough heat to not only ignite nearby combustible materials, but also to cause breaches in hydraulic lines and metal covers.

To stop a potential electrical fire, it is necessary to remove the current to the effected cable. The adoption of best practices for routing, securing and protection can greatly reduce the risks. Early detection methods, such as current monitoring or better circuit protection should be explored.

Alternative Fuels
All over the world, new alternative fuels are emerging to replace fossil fuels, such as gasoline and diesel. Hybrids, natural gas, all-electric and even hydrogen buses are currently being used. The overall benefit is great, but with new fuels and technical solutions, the fire risks change. Additionally, first responders face new risks. Alternative fuel fires like ethanol can also, to some extent, be more challenging to extinguish. Ethanol is a water-miscible fuel and requires the use of alcohol-resistant foam concentrates — other foams are destroyed very quickly. Gentle application of the foam is also very important, see Figure 6.

Some countries have been using alternative fuels for more than 20 years, others are just beginning. One hindrance to the introduction of alternative fuels is the perception that they may result in explosions and increased risk. Dissemination of present knowledge and development of new data is key to the wide-scale introduction of alternative fuels.

Active Test area
SP, in cooperation with Chalmers University, has recently opened one of Europe’s largest test and research facilities for Active Safety – ASTA ZERO (Active Safety Test Area AB). ASTA ZERO is a state-of-the-art test facility specifically designed for developments in active traffic safety and includes unique environments to build any scenario to develop, test or certify new traffic safety solutions; making it possible to test literally all aspects of active safety in one place. More information about the arena can be found at      

Where do we go from here?
Responsibility to ensure safe travel on buses should be shared by all parties. Establishing standards and best practices will ultimately mean less legislation, or at least legislation that is backed by sound scientific knowledge and best practices. Very little data exists to specifically identify the quantity, type and cause of bus fires. In addition, the limited legislation, standards and best practices that are in place aren’t always readily available. In the near future, links to known organizations, committees and other sources of information will be published on the SP website along with conferences in which fire safety is on the agenda. We will continue to expand the envelope of knowledge in support of bus and coach safety through research, testing and certification.    

Joey Peoples is a Bus Fire Safety Manager,
SP Technical Research Institute of Sweden

Fredrik Rosén is Sales and Marketing Manager, SP Technical Research Institute of Sweden