Shock-absorbing Air Bag System to Reduce Railway Pedestrian Fatalities

First version: 10/14/2009. Last update: May 22, 2010



Collision physics for a front-of-locomotive air bag appear feasible. The next investigative task is to develop a "sketch design" addressing various issues such as "hinging upwards." If a feasible sketch design arises, then a follow-on investigation should estimate cost and ascertain whether it is practical to prototype the design. Should the air bag system eventually prove viable, it will take considerable time to bring about a wide-scale implementation.


The Problem:

  • In the US in 2006, there were 500 fatal collisions between trains and pedestrians. Of these 500, about 360 were suicides (see article below). Additionally there were about 300 fatal collisions between trains and cars/trucks. Railway pedestrian collisions are also an international problem.

  • A series of high schoolers in Palo Alto have committed suicide via Caltrain commuter rail near Charleston and East Meadow intersections. Psychologically speaking, these are “dramatic” suicides where less-dramatic suicide methods would not be substituted. In the first 11 months of 2009, there have been 14 Caltrain pedestrian fatalities, some accidental and some intentional. Exhorts Mark Simon, Caltrain Executive Officer for Public Affairs, "We wish you well in your efforts to provide a reliable and available means of preventing rail fatalities. Fatalities on our right of way are a continuing source of concern at this agency and I can attest that every death on our system ripples through this organization and impacts all of us. We will follow your progress with keen interest."

  • In addition to Caltrain, Amtrak Capitol Corridor intercity passenger rail experiences multiple fatalities from accidents/suicides per year. See this article about the spike in Bay Area fatalities:

  • There have been 9 Washington DC metro suicides in 11 months:

  • The UK Rail Safety and Standards Board estimates the total cost of suicides (trackside and at stations) to the UK rail industry in 2003 was more than 11M GBP, @ 61,000 GBP per suicide. This includes delay to trains, lost working time as a result of trauma suffered by staff and the equivalent value of trauma as a minor injury. (RSSB – 2005 Annual Safety Performance Report).  BBC, Nov 11, 2004, "Each year 200 people end their own lives on the UK's railways. ... it highlights the very real problem of suicides on the UK's rail network."

  • Last year, nearly 2,000 people committed suicide in Japan by jumping in front of a train. .

  • Germany experiences roughly 936 railway suicides per year. 

  • Roughly 23 combined Toronto TTC subway and Go Transit commuter rail suicides per year:

Press Coverage:

Promising Air Bag Concept:

Imagine that the truck is a locomotive and the car is a pedestrian. #23 is a 5-foot-long air bag system

  • US Patent #6106038: "System for Collision Damage Reduction:" 

  • The inventor believes that his 5-foot-long, front-facing exterior air bag system allows a driver to survive a 45-mph collision between the driver’s 3,000 lb car and an immovable wall. The inventor calculates based on 18 g’s of deceleration, although others point out that humans can withstand stronger deceleration forces. 

  • The inventor's patent document hints at a longer air bag system with higher crash capability. A collision between a 200 lb human and a 60 mph locomotive changes the physics requirements compared to the 45-mph car/wall collision. The inventor envisioned his invention being applied to trains and buses, as well as cars.

  • The inventor appears to have considered and addressed many design challenges. The design is much more complicated than a typical inside-the-vehicle air bag system. A special, extra-strong air bag fabric is used. The venting system is clever: the “collidant’s” velocity drops off exponentially with each additional foot of air bag compression. To ensure that the air bag doesn’t pop, blowout patches relieve the pressure. An inside-the-vehicle air bag fills and deflates rapidly. The inventor’s external air bag stays inflated until the collision occurs. A radar or sonar sensing system is used to detect an imminent collision and initiate inflation of the air bag. The inventor creates downward pressure to keep the air bag from hinging upward. A system of air bag compartments is used.

  • The inventor envisions a very low cost per vehicle, but experts are skeptical. A Federal Railway Administration staffer guessed at an “acceptable budget” of $10,000 per locomotive for an air bag safety system.

  • For a 60 mph locomotive colliding with a 200 lb pedestrian, a calculation of a 7.5 foot air bag is given below by David Maymudes. This is a "promising" mathematical conclusion - the abstract collision physics are feasible. The question now becomes, is there a feasible, practical, low-cost design? Design issues include: a) preventing the air bag from hinging upwards, b) selecting an air bag design/fabric that won’t pop when pressed to the train tracks, c) meeting the requirements of commuter rail operators, d) preventing pedestrians from getting their ankles caught, e) how to fund, develop, and test a prototype, etc.

  • Inventor: Peter Dreher.

Air Bag Concept: Implementation Comments

  • Principal Engineer, TRW Automotive: "I believe that this concept is possible. I believe that it would take quite a bit of development due to the volume of the 'bag' and the volatility of the propellants commonly used in air bag systems. We would need to perform a lot of experimentation but I overall I think it can be developed."

  • David Maymudes explains the collision physics math for a 20g deceleration solution for pedestrian/train collisions:

    • The solution should exert a near-constant decelerating force, which is more the case for things that are "crushable" rather than "springy."

    • From a physics standpoint, there's the question about how long it takes to safely accelerate somebody from 0 to 60mph; at 1g, it's about 3 seconds, so at 20 g it's 1/20th of that, about 0.15 seconds. During that time the train travels about 15 feet; assuming constant acceleration, the person's "average velocity" will be 30mph, since they're changing from 0-60, so during that same time they'll move 7.5 feet, so you need 7.5 feet or so of "crush distance" to allow them to move that much less than the train is moving.

    • After the "save," the pedestrian is safely cradled in a crushable substance, traveling at 60mph along with the train until the train slows. Some mechanism / property of the solution could work to keep the pedestrian safely in place until the train stops.

    • It will be difficult to make this work perfectly at all possible orientations of the body in question.


  • Jerry Roane's comments about Peter Dreher's design:

    • A human can stand much more than 18g's in most cases. 64 g's is more like a fully fatal g force.

    • For a train, I would imagine that a pre-inflated air bag would be easier and cheaper than inflating the air bag when a collision is imminent. Keep in mind that a gas generator is an ordinance device and can kill you if you get the gas pressure applied incorrectly.

    • Also the edges of the safety zone would be more of a design factor than a middle of the sweet spot hit, so the shape of the air bag may not be a simple wedge but it may take on a hammerhead shark kind of look to get all persons in front of a moving train to be safely caught by the device. I am sure you want to catch the person not shove them to the side where there is a great risk of fatally hitting something near the train tracks.

    • I also would not suggest that an air bag inflate just in time because of the cost and danger of the hot gas generator. On a train there is no cost penalty for having the protection mat already inflated or foam rubber-like materials already in the catching configuration.

    • I would also suggest that you do want an upward force as the deflection occurs because the last thing you need it to run up on the body and trap it down as you roll over the body. Getting enough up force but not enough to reach the windshield would be a design requirement.

    • Another comment of the train air bag patent drawing. The side metal bars would be dangerous if the object to be caught was not centered in the bag. This needs to do more good than harm so the metal side apparatus needs to be rethought.

    • I would add front wheels to this bag and make it longer so you are not pushing the limits of human endurance. Once you have spent the effort and money to have a soft catch you might as well make it a few feet longer to be easy on the body. Longer would make the up-force vector have a wider design window too.

    • For cost reasons I would recommend a pre-inflated structure or a plastic collapsible structure so the dangerous hot gas generator explosive is not required. It works on the car steering column because it is mandated but a lower cost solution is to skip the instant inflation requirement in favor of full sized ahead of time. I would also push for a much longer, wheeled car that is coupled to the train and rolls on the tracks to maintain its vertical axis alignment. There is nothing on the train tracks ahead or behind the train so a permanently deployed bag or collapsible structure can lead and end every train including the slow ones. A small embedded controller could take an active roll in grabbing the pedestrian rather than expecting a static device to keep the distraught person held safely.


  • Walter Brewer's comments and calculations: a) The air bag catcher has to be designed to hold the person/object in place while the train decelerates for several seconds. Otherwise it is back to the sweep aside case. Taking a somewhat less lawsuit-vulnerable 20 g. the capture to a 60 mph train requires about 6 feet. It increases as the square of the train speed. That assumes a neat control of the body deceleration to a constant 20 g. If the air bag volume, or equivalent, is large compared to the body that is probably doable. A linear build up of air bag force would require longer distance before the 20 g peak is reached.

For a constant 20 g acceleration; V squared = 2 x acceleration x distance. (32.2ft/sec squared = 1 g.)

Thus at 60 mph: 88 sq = 2 x 32.2 x 20 x distance. (88 ft/sec = 60 mph)

Distance is ~ 6 ft.

This is in respect to the train of course to bring the body up to train speed

For the time required: Distance = 1/2 x 32.2 x 20 x time squared. Thus: time sq = 6 divided by1/2 x 32.2 x 20. Time = 0.14 sec.

Train will have traveled 0.14 x 88 = 12 feet in that time.

  • Tom Rubin: a) I’m someone who has worked with railroads quite a bit, and a change of this type will be slow to bring about. At an absolute minimum, we’re talking years of research and development, then tests on test tracks, and then a pilot program on “real life” engines. Difficult and expensive – but, I think, well worth looking into. If this air bag system concept fails utterly, it will likely do so fairly quickly into the testing, so the cost might be no more than a few million to get to the point where there is either a reasonable chance of something that could work and have a significant positive impact on rail safety, or not. b) I agree as to the air bag technology and would like to see the technology developed further. Other than cost, there are not a lot of reasons not to deploy them (you do need to make sure that only the bags on the front of the lead car are deployed, and you need to make sure that they are not triggered unintentionally, particularly by maintenance personnel, but these should be doable).

  • Advanced Origami folding behavior may have applications in air bag engineering, especially for this challenging application:


A Rough Sketch of One Possible Implementation:

The locomotive is 15' tall. The inflated air bag is 15' long by 7' tall. The person is 6' tall:

In the processing of the collision in 0.15 seconds, the top portion of the air bag deflates faster than the bottom portion. Hence, for a standing person, the top of the body moves forward faster, resulting in the body position rotating to be nearly prone. Slight air bag hinging lifts the feet up off the ground:

(Source PPT for these rough sketch graphics:


Other Air Bag Concepts:

Ford has revealed plans for external air bag "future technology for protecting pedestrians and other vehicles during a collision.  An over-the-hood air bag offers protection to pedestrians, and occupants in other vehicles." (Car Design News, May 2001). See picture at right:

May 2009, Autopia, "Researchers at Cranfield University in England have developed an external air bag they say will significantly reduce pedestrian fatalities and injuries in the event of a crash."

  • See the two patents below.

  • The Mars Pathfinder external air bag system was designed to protect the 264kg Lander from a 60 mph collision with a planet. Development of the air bags required significant design and test work. The air bags are made out of a high strength fiber called Vectran. See:
  • See also United States Patent 6883631, External air bag occupant protection system. 

  • See also the article entitled "External air bag armor protects vehicles against blastwaves." "While the shrapnel created by a roadside bomb or improvised explosive device is lethal all on its own, the concussive force of a blast can seriously injure, incapacitate and kill the crew of a vehicle right through its armor. To combat the effect of blast waves, a company called Survival Consultants International has developed a wall of air bags triggered by a light sensor. Light, the company maintains, is the only thing faster than a concussive wave caused by an explosion. In a split second, the triggered sensors confer with an on-board processor on whether or not to deploy the bags, which in turn combat the force of a blast wave."
  • An “advanced transit inventors web forum” brainstormed some possible solutions:


US Patent #6293205: "Train collision system."


"A train collision system helps reduce the severity of impact between a train and a land vehicle or pedestrian. The system uses a flatbed rail car that is coupled to the front of a train. Several deformable barrels, each at least partially filled with an inert material, are attached to the top." Inventor: Paul A. Butler.


US Patent #6474489, "Collision Attenuator"

A train collision attenuator mounted on a leading end of a train for attenuating the force of impact between a moving train and a pedestrian or motor vehicle. The train collision attenuator includes an energy absorbing assembly and a mounting assembly. ... A selectively-inflatable, externally-mounted air bag including an upper pedestrian cushioning portion and a lower pedestrian support portion is also provided. Inventor: Thomas S. Payne (Union City, CA)



A compressed air "megagust" concept to reduce damage to the locomotive:

"I've worked with compressed air before and am convinced a jet air blast could blow most organic objects off to the sides of the rails before they made contact with the engine. Even large animals, like moose, while still afoot, could be turned off the roadbed by an assisting megagust." Fred Harrison Central Point, OR CORPpower/JSS/EORS


Preventing "Dramatic Suicides"

There is a well-known phenomenon known as a “suicide magnet” or "suicide cluster" – where a location and/or a methodology where one suicide occurs becomes a trigger for others with suicidal tendencies. The best known in the world is the Golden Gate Bridge, with over 1,200 known to date. Other notable “suicide bridges" include the George Washington Memorial Bridge in Seattle, the San Diego-Coronado Bridge, the Sunshine Skyway Bridge in Saint Petersberg, the Cold Spring Canyon Bridge in Santa Barbara County, the Van Stadens Bridge, near Port Elizabeth in South Africa, the Bloor Street viaduct in Toronto (500 people), and the Hornsey Lane Bridge in London. The Eiffel Tower and the Empire State Building were also very well known as suicide magnets prior to specific mechanical barriers being put in place. In Japan, there is the “Aokigahara hanging forest” bordering Mt. Fuji, which averages about 80 per year.

One frequent question is, if they want to commit suicide, won’t they just go someplace else? Well, perhaps, but there does appear to be reason to believe that attempting to put in a barrier at specific locations would have at least some effect – in any case, if the “suicide magnet” effect may be going on, it is reasonable for responsible individuals to at least make responsible efforts to stop such suicide efforts at the specific location where it is occurring. In Palo Alto, volunteer and professional security vigils are in place to deter incidents.

In another example, amidst numerous cliffs in Ireland, a single cliff became a spot for frequent suicides. A priest stopped the trend by holding vigil at that particular cliff. Local citizens stopped committing suicide and did not substitute other cliffs. [Thanks to Tom Rubin and Doug Solomon for this section.]

One conclusion from a study of folks who survived Golden Gate Bridge jumps. On the way down, they all concluded that they wanted to live. 


The Psychological Toll on Railway Operators [Tom Rubin]

"The train operator does not want to experience fatal collisions. Speaking as someone who was responsible for both safety and human resources at the Southern California Rapid Transit District in LA when we opened the Long Beach Blue Line, the most dangerous light rail line in the U.S., this was, and is, a huge problem. Unfortunately, the way the human mind works, it is very common for the last thing that a person about to be run into by a vehicle sees is the eyes of the operator of that vehicle, looking right at them.

I am told that this is not anything that one forgets, no matter how long it has been, and no matter how much one tries, and no matter how much counseling one gets.

In this situation, you never, NEVER, NEVER let the operator go back on duty. The way that the rules go, the operator must immediately get a test for substance abuse, which is a good excuse for getting them out of there as soon as the on-site investigation is initiated. You then attempt to get them to counseling as soon as possible. You generally try to get them to take some time off before getting back to operating a vehicle in transit service, if at all possible. Keep in mind that, under the bargaining unit agreement, if the operator wants to go back to work, unless there is a specific physical reason not to allow it, they can go back to work. For some, I am told, this is not a bad thing, but, at a minimum, supervisors are asked to keep an eye out – which is difficult to do, as operators work alone and generally have very little contact with anyone else, particularly rail operators, who work in an enclosed compartment with no passenger contact.

After having spent too many hours at too many crash sites, all I can say is, be careful out there and respect those railroad warnings; they are there for a reason."


See also the article "Death on the tracks: For Caltrain crews, trauma of suicides may never go away." From the article: "One of the things that really sticks with an engineer is the sound of striking somebody. It just is a very distinct, hollow sound, and it's got a metal ring to it. The other thing is this feeling of helplessness. When it's an obvious suicide and somebody is putting themselves out there, there is nothing you can do." 

One U.S. Rail Pedestrian Suicide per Day

Published Tuesday, November 24, 2009, by KCBS Radio AM 740.

National Suicide Study Examines Tragedies in Palo Alto, By Matt Bigler

More Americans are killing themselves on train tracks. That's the finding of a new national study on suicides. The study includes a close look at a recent suicide cluster in Palo Alto.

In the past six months, four students at Gunn High School have taken their own lives at or near the same Caltrain crossing in Palo Alto, what Karen Marshall with the American Association of Suicidology <> calls a suicide cluster. "A larger than expected number of suicides in a short period of time in the same place," she described. A study of suicide clusters in Palo Alto and Washington DC recommends restricting access to problem train crossings, with barriers or thorny bushes.

Local authorities, however, have been somewhat resistant to that, pointing the finger instead at reporting on suicides. "By showing videos of the passing train, the loud train steaming down its corridor, we make it more accessible in the minds of someone else who's feeling troubled or feeling depressed," suggested Dan Ryan with the Palo Alto Police Department.

The suicide study finds that every week, 7 Americans end their lives by jumping in front of trains.


Federal Research Proposal for Transit Cooperative Research Program (TCRP)

A federal research "problem statement" is being developed. For TCRP program details, please see:


Draft TCRP problem statement:



Additional Federal Research Funding Sources:

  • Assuming that an air bag engineer steps forward as a Principal Investigator, pursue the March 2010 federal TRB Innovations Deserving Exploratory Analysis (IDEA) funding round (both for Transit IDEA and Safety IDEA). Public entities should provide letters of support.

  • In addition, the TRB Studies and Special Programs Division conducts policy studies at the request of the U.S. Congress, the executive branch agencies, states, and other sponsors. A group of public entities can run an idea up the political hierarchy to secure such funding.

  • Conceivably, Stimulus II funding could be applied.


Miscellaneous Notes

  • There are more freight rail pedestrian fatalities than passenger rail fatalities because there are a lot more freight trains, although the passenger trains tend to be more concentrated where there are people, for obvious reasons.

  • 79 mph is the maximum operating speed for almost all rail service outside of the northeastern US corridor



  • Initial communication has been made with: Caltrain, Amtrak Capitol Corridor, City of Palo Alto, design firms (IDEO, Mindtribe, Makani Power [air bag expertise], Mission Motors, Humcycles), Stanford Design Program, UC Berkeley SafeTREC (Safe Transportation Research and Education Center), Pasadena Art Center College of Design (automotive design, industrial design, etc), National Transportation Research Board committees: Commuter Rail Committee and Rail Operations Safety Committee, DVExperts, Federal Railroad Administration Office of Safety, inventor Peter Dreher, TRW's Vehicle Safety Systems division, Bosch Palo Alto Research Lab (Bosch makes air bags), and UK Rail Safety and Standards Board (RSSB).