BY CHRIS DALY
The National Institute for Occupational Safety and Health (NIOSH) regularly conducts firefighter fatality investigations to identify the root causes of fatal incidents and make recommendations for preventing similar occurrences in the future. Reviewing these firefighter fatality reports can provide emergency providers with valuable lessons that will help to prevent future injuries and fatalities.
Although speed and lack of proper training often are cited as the causes of many emergency vehicle crashes, other factors also contribute to these fatal incidents. This article focuses on hydroplaning and tire blowouts.
On June 16, 2003, a firefighter in Kentucky lost his life while driving his personal vehicle to a high-water emergency (NIOSH Report #2003-19). According to State Police reports, the firefighter was traveling eastbound on a two-lane paved road when he drove over a large area of standing water. This pool of water caused the front driver side of the vehicle to hydroplane off the roadway and strike a billboard (photo 1). The unrestrained victim was quickly extricated and transported to a nearby trauma center, where he succumbed to his injuries approximately one hour later.
(1) The damage that resulted when the victim hydroplaned off the road and struck a billboard. (Photo courtesy of NIOSH, Report #2003-19.)
A civilian witness to this crash stated that the victim had passed her vehicle as she was traveling at approximately 40 miles per hour (mph). Because there was no roadway evidence for the State Police investigator to measure, it was not possible to determine the firefighter’s rate of speed. It is safe to say, based on the witness’s statement, that the victim was traveling in excess of 40 mph. To understand why this crash occurred, we must understand why a vehicle hydroplanes. Hydroplaning is the result of the combination of the car’s traveling speed, the condition of the tires, and the condition of the roadway.
You can drive a vehicle down the road, steer, stop, and make evasive maneuvers because of the friction between the tires and the roadway. This friction is generated at the point where the face of the tire meets the roadway, known as the “contact patch.” In reality, this contact patch is only about the size of your hand. Imagine that the entire weight of your 60,000-pound fire truck is being supported by 10 tires, each of which is making contact with the roadway in an area the size of your hand. The condition of this contact patch determines whether you will be able to start, steer, or stop in a safe manner.
Bald tires have more traction on a dry road surface than a treaded tire because more “rubber” is meeting the road surface (i.e., a larger contact patch). Unfortunately, few of us work in regions that do not experience some type of inclement weather. That’s why there are treads on tires. Tire tread is designed to push water, slush, mud, and other road-obscuring debris out of the way, allowing the rubber tire face to come in contact with the road surface (photo 2). This rubber-meeting-the-road surface provides the traction necessary to safely drive down the highway.
(2) Tire tread is designed to channel water, slush, snow, and other road-obscuring debris away from the tire face so that the tire can come in contact with the road surface. (Photo by author.)
As your vehicle travels down the road on a rainy day, the tread on your tire is constantly pushing water out of the way, allowing the tire to come in contact with the road surface. If your vehicle is traveling too fast and you are not giving the tire tread enough time to push the water out of the way, problems will arise.
As you drive across a road that has a substantial amount of water on it, a wedge of water builds up in front of your tire. This wedge of water will cause your tire to rise up off the roadway and lose all contact with the road surface. When this occurs, you will no longer have control of your vehicle, and you will begin to hydroplane (photo 3).
(3) A tire hydroplaning on an area of standing water. (Image courtesy of SmartMotorist.com.)
Research has shown that if your vehicle should begin to hydroplane, it is important that you NOT apply your brakes. Instead, firmly grip the steering wheel, take your foot off the accelerator, and attempt to keep a safe heading by NOT turning the steering wheel. If all goes well, your vehicle will regain traction, and you will regain steering control. Unfortunately, there is little you can do once the vehicle begins to hydroplane. Therefore, you must remember to slow down in inclement weather and avoid a hydroplaning situation altogether.
The exact cause of hydroplaning is complex, but you must understand several points. First, hydroplaning is a direct function of speed, tire tread depth, tire pressure, and the depth of the water you are driving through. You should regularly check tire tread depth and tire pressure on every vehicle in your fleet (including your personal vehicle). Underinflated tires are more likely to hydroplane at a lower speed, because there is less pressure inside the tire to press down on the road and “push” the water out of the way. This fact is clearly evident in the formula used by crash investigators to determine an approximate speed at which a vehicle (equipped with sufficient tire tread) will hydroplane (Table 1).
It is evident that the lower the tire pressure, the lower the speed at which a vehicle will hydroplane. This formula assumes that the water you are driving through is not deeper than your tire tread. Should you find yourself driving through a pool of water that is deeper than your tire tread, your vehicle may hydroplane at an even lower speed. It is not uncommon to see vehicles hydroplaning between 30 to 40 mph. Also, despite the high tire pressures seen on commercial motor vehicles, studies have shown that even large trucks can hydroplane. You should consult your fleet maintenance personnel or local tire supplier to determine what tire pressure you should maintain on your vehicle.
Tire tread depth is also a critical factor when determining the point at which a vehicle will hydroplane. Tire tread constantly channels rain, snow, and other debris out of the way so that the rubber tire face can meet the road surface. Many states consider a tire to be “bald” if the tread depth is less than 2/32 of an inch. On commercial motor vehicles (vehicles more than 26,001 pounds-i.e., a fire truck), the front steering tires must not have treads less than 4/32 of an inch. In reality, studies have shown that tires will lose much of their effectiveness when tread depths reach less than 5/32 of an inch.2 If the tread depth is less than 5/32 of an inch, vehicles will hydroplane at a slower speed than vehicles with deeper treads. In addition, stopping distances can be increased and wet weather driving can become more dangerous when driving a vehicle with shallow tire treads.
Considering that most emergency vehicles operate under extremely inclement road conditions, it is imperative that tires not be allowed to wear down past a safe tread depth. Some municipalities may hold off replacing tires as a cost-saving measure, but a new set of tires will pay for itself over the long run through increased safety and reduced chances of vehicle crashes. Ask your “purse holders”: “Which costs more, a crash that results in a lawsuit or a new set of tires?”
The best way to handle a hydroplane situation is not to hydroplane in the first place. Regardless of whether you are driving your personal vehicle or a 70,000-pound ladder truck, wet weather requires extra caution. Avoid excess speeds that may lead to hydroplaning, and avoid driving through standing pools of water. Should you encounter a large area of water and you do not know how deep it is, don’t try to drive through. How many times have we been called to rescue a hapless civilian in a hurry to get home who does not heed this small rule of common sense? Our desire to be the first one on-scene must not cause us to make poor decisions such as driving through a pool of water with an unknown depth. Remember the old cliché: “We’re no good if we don’t get there.”
On August 19, 2001, an Oregon firefighter lost his life when the tanker he was driving experienced a catastrophic failure (blowout) of the front right tire (NIOSH Report #F2001-36). As a result, the vehicle left the roadway and struck a large boulder and a tree. The firefighter victim became heavily entrapped in the tanker; emergency medical personnel pronounced him dead at the scene (photo 4).
(4) After suffering a tire blowout, the tanker sustained major damage as it struck a boulder and a tree on the side of the road. (Photo courtesy of NIOSH, Report #2001-36.)
Investigation revealed that the victim was traveling on an interstate highway after picking the truck up from a repair shop. As he traveled down the highway, he attempted to move into the left lane and pass a tractor trailer. After passing the tractor trailer, he was attempting to change back into the right lane when the right front tire blew out. After the blowout, the vehicle traveled approximately 535 feet before striking the boulder and tree.
State Police investigators inspected the vehicle after the crash and found that the tire that failed was “a 1979 model and that the outer shell fragment of the tire revealed a brittle and obviously aged material.”3 In addition, “pieces of the steel cords showed signs of rust from years of moisture exposure due to openings in the tread.”(3) This fatality occurred in 2001, which makes the tire in question approximately 22 years old at the time of the crash (photo 5).
(5) The tire failed, causing the tanker to lose control. (Photo courtesy of NIOSH, Report #2001-36.)
On March 3, 2004, a Florida firefighter was killed when the right front tire of the brush truck he was driving blew out (NIOSH Report #F2004-15). The vehicle left the road, struck a culvert, flipped over, and came to rest upside down in approximately two feet of water (photo 6). The victim firefighter was trapped in the cab of the vehicle and subsequently drowned.
(6) After suffering a tire blowout, this brush truck left the roadway and flipped over in a ditch. (Photo courtesy of NIOSH, report #2004-1.)
Investigation of this crash revealed that the victim was traveling approximately 55 mph along a straight road when the blowout occurred. A crash reconstructionist who investigated the incident noted that the tire damage “resembled similar damage documentation patterns resulting from a previous impact that may have compromised the inner tire radial ply and liner.”4 In other words, the tire had sustained an impact on an earlier date that resulted in damage to the inside of the tire. As a result of this existing damage and other possible factors, a tire blowout resulted (photo 7). The reconstructionist noted that this damage is “nearly impossible to detect because the tires may still hold air and show no outward signs of deformation.”(3)
(7) The violent nature of the blowout is apparent in this photograph. (Photo courtesy of NIOSH, report #2004-15.)
A number of factors can lead to a tire blowout. Some believe that a tire blowout is the result of too much air in a tire; this is not the case. The two most common causes of tire blowout are tires that are underinflated or overloaded.
Consider that the structure of the tire does not support the weight of the vehicle-the air inside the tire supports the weight of the vehicle. This is the same as the concept pertaining to lifting air bags used for rescue.
An improperly inflated tire does not have enough air pressure inside to support the weight of the vehicle. When this occurs, the structure of the tire begins to support the weight of the vehicle. In this situation, the sidewall of the tire begins to bulge out as it takes over the job of supporting the vehicle’s weight. The lower the air pressure, the more the tire bulges.
If you picture a tire attached to a parked vehicle, you can plainly see how the top of the tire is rounded and the bottom of the tire bulges between the axle and the roadway. Now, imagine this tire driving down the road at 60 mph, rotating a few hundred times a minute, and constantly flexing between being rounded while at the top of the rotation and “squished” as it comes in contact with the roadway (photo 8). This constant flexing causes the sidewalls of the tire to heat up. Under normal circumstances, a properly inflated and loaded tire can handle the heat generated by this flexing of the sidewalls, and there will be no problem. However, if the tire is low on air and therefore not able to properly support the weight of the vehicle, the tire may overheat as it rotates around the axle. This excess heat is caused by the constant overflexing of the sidewalls. The more the sidewalls have to flex, the more heat that will build up. If the tire heats up too much, it may fail and cause a blowout.
(8) A tire mounted on an axle is asymmetrical. The top of the tire is rounded while the bottom of the tire is “squished.” As the tire rotates, the constant flexing of the sidewalls produces heat. (Photo by author.)
A similar situation may occur if the vehicle is overweight. In this case, the tire may have the proper air pressure inside for the weight the vehicle was designed to carry. However, if too much equipment or an oversized load is placed on the vehicle, the recommended air pressure no longer will be able to support the weight of the vehicle. As a result, the tire will begin to bulge at the bottom, just as it would if it were underinflated. Once again, the constant overflexing of the tire as the vehicle drives down the road may cause excess heat to build up and the tire to suddenly fail without warning. Combine an underinflated tire with an overloaded vehicle, and disaster is likely.
Bridgestone/Firestone conducted a study of emergency medical service vehicles to examine the inflation pressures of dual-tire assemblies. The results of this study were quite startling. For starters, 39 percent of the tires couldn’t be checked because there was no access to the valve stems.5 Of those tires that could be checked, two-thirds were found to be underinflated by at least 20 psi, or 25 percent capacity. (5) According to the tire industry, a tire that is 20 percent underinflated is considered to have been “run flat.” A tire that has been “run flat” may result in damage to the tire, which can result in an unexpected and catastrophic blowout. Tire pressures must be checked regularly to ensure that tires are properly inflated.
|Inside dual over 20 psi underinflated||33%|
|Outside dual over 20 psi underinflated||15%|
According to statistics provided by Michelin Tire, tire blowouts result in 23,000 collisions and 535 fatalities each year.6 Often, it is not the tire blowout itself that leads to the crash but the driver’s reaction to the blowout. It is imperative that drivers be trained on the proper methods for handling a tire blowout.
Imagine that you are driving down the road when you suddenly hear a tremendous “bang” that is followed by your vehicle’s shaking and pulling to one side. The natural reaction for most drivers is to slam on the brakes and immediately try to pull over to the side of the road. After experiencing a tire blowout, applying the brakes is one of the worst actions a driver can take. Instead, the driver should apply light acceleration to maintain the vehicle’s forward momentum and then grip the steering wheel tightly and regain control of the vehicle. Once the vehicle is under control, the driver should slowly decelerate and pull to the side of the road.
The University of Michigan Transportation Research Institute conducted in August 2000 a study of fatal truck crashes caused by tire blowouts.7 This analysis of blowout-related crashes revealed some interesting crash outcomes based on which tire blew out. The results of this study are summarized below:
“In general, the fatal truck crashes precipitated by a blowout can be divided into the following three scenarios:
- If the front left tire blows, the truck loses control to the left, veers into oncoming or adjacent traffic, and rolls immediately or after a collision with another vehicle. These crashes are primarily multiple-vehicle crashes; 15 of the 22 involved two or more vehicles. (7)
- If the right front tire blows, the truck loses control to the right, veers off the road and rolls or collides with roadside structures, or both. These are typically single-vehicle crashes. Ten of 13 right front tire blowouts were single-vehicle crashes. (7)
- If a drive or trailer axle blows, the truck typically, though not always, remains under control. [Note: In some cases, the crash is entirely unrelated to the flat tire. For example, in one case, the truck was rear-ended by an alcohol-impaired driver. The truck driver said he was driving normally and was on his way to get the flat repaired, remarking that “one tire being flat did not slow the truck down that much.”] (7)
This study addressed only crashes that resulted in fatalities. It is unknown how many other crashes occurred that did not result in a fatality.
Although routine vehicle inspections are common within our nation’s fire stations, few of these inspections include a comprehensive check of the tires. Many people assume that if a tire does not appear to be flat, there probably isn’t a problem. In reality, it is imperative that a vehicle’s tires be checked once a week. Even if there are no outward signs of damage, a tire can lose air in other ways. In tubeless tires, an improper seal between the tire and the rim can cause air to escape. Punctures caused by nails, screws, or other roadway hazards may lead to small leaks that can slowly rob a tire of its air. In addition, most people don’t realize that air can actually permeate its way through a rubber tire to the outside atmosphere. This permeation through the tire membrane can result in an average air loss of one to two psi per month. (5) Compound this air loss over several months, and a tire can suddenly be 20 percent underinflated.
Many people believe that the amount of air that is supposed to be in the tire is printed on the tire’s sidewall. In reality, that figure is the maximum air pressure that should be in the tire to support the maximum load, not the recommended tire pressure for the vehicle. Consider this fact: Model ABC tire may be on your vehicle as well as your neighbor’s vehicle that is parked next to yours. Although these two vehicles may have the same tires, one vehicle may be much smaller than the other vehicle. The reality is that Model ABC tires are used on many vehicles. For this reason, each vehicle manufacturer works with the manufacturer of Model ABC tires to determine the optimum recommended tire pressure for its vehicle. That’s why the sidewall of the tire tells the consumer the maximum air pressure allowed in Model ABC tires to carry the maximum load. In real life, most vehicles will not be carrying the maximum load the tire is designed to carry and, therefore, you will need to find out what the recommended tire pressure is for your vehicle (photos 9, 10).
(9) This tire information placard found on the trunk lid of a personal vehicle recommends a tire pressure of 30 psi for all four tires. This does not match the maximum tire pressure printed on the sidewall of the vehicle’s tires in photo 10.
(10) This is the maximum tire pressure and maximum load the tire can support. It does not match the recommended tire pressure shown in photo 9. Photos by author.
To determine the recommended tire pressure for your vehicle, check the service manuals or the tire load and inflation placard. This placard can be found on most cars and light trucks, usually on the driver door jamb or the trunk lid. For custom vehicles, such as fire trucks, you may have to contact the vehicle manufacturer for this information (photos 11, 12).
(11) The tire information placard found in the crew cab of this custom pumper provides vital information such as gross vehicle weight ratings for each axle and the load rating of the tires at 120 psi.
(12) The tire information placard on this custom quint also provides information such as the Tire-Limited Maximum Speed Limit and the standard tire load ratings. (Photos by author.)
The tire load and inflation information on this placard is based on the weight of the vehicle (including passengers and load), as well as on the size of the tire the vehicle is designed to use. The best way to ensure that you have the appropriate amount of air in your tires is to have the vehicle weighed. Once you have determined how much weight each tire is carrying, you can then refer to the tire load and inflation pressure chart provided by the tire manufacturer. This chart will tell you the recommended air pressure for the tire based on how much weight it is supporting. If for no other reason, it is important to have your vehicles weighed to ensure that you are not posing a safety and liability hazard by operating an overweight vehicle. As noted, overweight vehicles can also lead to tire blowouts (photo 7).
When checking tire pressures, “Cold Tire Pressure” refers to the fact that the tire hasn’t been driven on for the past three hours. If you have to drive the vehicle somewhere to put air in the tire, it is not recommended that you drive farther than one mile, because a rolling tire generates heat, and the heat will cause the air pressure inside the tire to rise and give a false reading when checking the tire pressure. Many fire departments have the on-site capability to inflate a tire at the fire station. If you must drive the vehicle somewhere to add air, check the cold air pressure before starting your trip. Record each tire pressure, and determine how much air you will have to add. Drive to your “fill site,” and add the appropriate amount of air based on your calculation-as an example, if your tire requires 35 psi and the cold tire pressure is 29 psi, you will have to add 6 psi. When you reach your fill site, the tire pressure may have risen to 32 psi. In that case, you have already calculated that you need 6 psi to bring the cold tire pressure to the appropriate reading, so fill the tire to 38 psi.
It is also important to make sure that dual-tire assemblies are matched appropriately. Both tires should be of the same make and size and should be kept at the same inflation pressure (photo 13). Dual tires should be within a tolerance of not more than one-quarter of an inch in diameter and three-quarters of an inch in circumference.8 If the tires on a dual assembly are different sizes, the larger tire will drag the smaller tire along. Bridgestone/Firestone studies found that a dual tire assembly that differed by just a 5-psi inflation pressure caused a 5/16 of an inch difference in circumference. (8) This minor difference in size would cause the smaller tire on the dual axle to be dragged 13 feet over just one mile. (8)
(13) Dual tires must be of the same make, model, and air pressure to keep the larger tire from dragging the smaller tire. (Photo by author.)
Checking tire tread is a simple task. Simply purchase a tire tread depth gauge at a nearby auto supply store (around $5); add this vital safety check to your routine vehicle inspection procedure. As tire treads begin to wear down, note that on your inspection sheet and notify your fleet manager immediately. As mentioned before, although most state and federal standards require at least 4/32 of an inch on the front tires and 2/32 of an inch on the rear tires, studies have shown that tires significantly lose their effectiveness when tread depths are more shallow than 5/32 of an inch.
Routine tire checks should also ensure that tire valve stem caps are in place. Most manufacturers recommend metal valve stem caps, which contain a rubber gasket that helps keep the air inside the tire. A missing valve stem cap can lead to an unnecessary loss of air pressure from inside the tire. Make sure to have a few extra caps on hand in the maintenance room.
Proper maintenance and inspection of emergency vehicle tires go far beyond a simple glance to see if the tire appears flat. Every emergency service should take extra time each week to ensure that their vehicle tires are in safe operating condition. When performing these routine vehicle inspections, check your personal vehicles as well. Many of us forget that if our vehicles aren’t maintained in a safe condition, we won’t make it to the firehouse in the first place. By ensuring that our equipment is safe and our drivers are trained, we can hope to reduce the number of crash-related fatalities each year.
1. “NPRM on Tire Pressure Monitoring System FMVSS No. 138,” U.S. Department of Transportation, Office of Regulatory Analysis and Evaluation Planning, Evaluation and Budget, Sept. 2004.
2. “How Safe Are Worn Tires?” Consumer Reports. [http://www.consumerreports.org/cro/cars/tires/dangers-of- worn-tires-204/overview/index.htm] Date accessed: Oct. 17, 2006.
3. NIOSH Fatality Assessment and Control Evaluation Investigative Report #F2001-36.
4. NIOSH Fatality Assessment and Control Evaluation Investigative Report #F2004-15.
5. “Ready to Roll: The Shocking Truth!” Bridgestone/Firestone, publication BF50919, July 2001.
6. “Police Tire Blowouts: Avoiding Deadly Consequences,” Captain Travis Yates, Policeone.com [http://www.policeone.com/policeone/frontend/parser.cfm?object=Columnists&tmpl=article&id=92723]. Accessed Oct. 17, 2006.
7. Bareket, Z., D.F. Blower, and C. MacAdam, “Blowout Resistant Tire Study for Commercial Highway Vehicles,” The University of Michigan Transportation Research Institute, Aug. 31, 2000.
8. “Preventing on the Road Tire Failures,” Tire Retread Information Bureau,” [http://www.retread.org/Inflation/index.cfm/ID/288.htm]. Date accessed: Nov. 29, 2006.
CHRIS DALY, a 17-year veteran of the fire service, is a member of the Goshen Fire Company in West Chester, Pennsylvania, and a full-time police officer specializing in the reconstruction of serious vehicle crashes. He developed the “Drive to Survive” training program (www.drivetosurvive.org) and lectures nationally on the prevention of emergency vehicle crashes. He has a master’s degree in safety from Johns Hopkins University.