TTMA-100 Trailer Mounted Impact Attenuator
Frequently Asked Questions
Click a question to see the answer.
Q: Does the TTMA-100 meet NCHRP Report 350 requirements?
The TTMA-100 has successfully passed all NCHRP Report 350 recommended crash tests, including both required and optional tests. FHWA approval letters CC- 90 and CC-90a provide official recognition that the TTMA-100 has successfully passed NCHRP Report 350 required and optional crash tests, respectively. Note that the TTMA-100 is the only TMA system that has successfully passed all required and optional crash tests when attached to a blocked truck to simulate an infinitely heavy tow vehicle.
Q: Is the TTMA-100 capable of meeting the safety performance evaluation guidelines in the update to NCHRP Report 350?
The TTMA-100 has been designed with sufficient reserve capacity to meet the increased test severity required by the new testing guidelines. Although the actual safety performance under the new criteria has yet to be proven, it is believed that the TTMA-100 will have sufficient capacity to meet the new testing guidelines when attached to an infinitely heavy support vehicle.
Q: Can the TTMA-100 safely attenuate impacts with a loaded tractor-trailer truck?
Truck mounted attenuators (TMA's) are designed to safely attenuate impacts with passenger vehicles and, as such, do not have sufficient energy dissipation capacity to fully attenuate high-speed, heavy-truck impacts. However, some real-world crashes appear to show that TMA's can mitigate the severity of some heavy-truck impacts with shadow vehicles. The TTMA-100 is the only truck mounted attenuation system to have passed all required and optional crash tests with a blocked support truck. Hence, it is safe to assume that the TTMA-100 has greater energy dissipation capacity than any other existing TMA system. While it is not designed for such impacts, the TTMA-100 does provide greater protection for drivers of shadow vehicles during heavy-truck impacts than any other existing TMA system.
A simplified impact analysis of an 80,000-lb truck impacting a shadow vehicle traveling at 15 mph was conducted to evaluate possible impact situations. The analysis showed that the TTMA-100 would have the energy dissipation capacity to reduce the truck-to-truck impact speed to 20 mph or less for the following crash conditions.
|Shadow Vehicle Weight, lb
|80,000-lb Truck Impact Speed, mph
Q: Has the TTMA-100 been crash tested with an arrow board and approved by FHWA? What happens to the arrow board upon impact?
The TTMA-100 has not been crash tested with an arrow board and STI has not sought FHWA approval. The support for an arrow board or variable message sign (VMS) is subjected to impact loads only when the TTMA-100 and the support truck are accelerated forward during a crash. All crash tests of the TTMA-100 were conducted with the support truck blocked to simulate an infinitely heavy vehicle. Hence, the TTMA-100 was not subjected to high accelerations that would cause problems for an arrow board or VMS. However, using the highest sustained accelerations measured during the pickup truck end-on test (test 3-51), design loads for the TTMA-100's sign mounting system were determined. This maximum acceleration was converted to an equivalent force that was then applied to the lightest allowable support truck and trailer combination, a 10,000-lb support truck with a 1,400-lb trailer. The trailer acceleration resulting from this level of applied force was then used as a design load when analyzing the stresses in the sign mounting system.
It is important to note that this procedure for estimating sign support loading is very conservative. Forces applied to an impacting vehicle are greatly reduced when the support truck is allowed to roll ahead. Therefore, when attached to a lighter support truck, actual accelerations on the trailer and support truck combination are much lower than those used in the sign support design process and the loading of the sign support system would be much lower. In other words, the sign mounting system was designed for the worst scenario.
Q: Can the TTMA-100 accommodate impacts on the side of the trailer?
All truck mounted attenuators (TMA's) are designed to safely accommodate vehicles striking from the rear of the support truck. No TMA system can accommodate even a modest impact on the side of the cushion. The TTMA-100 is no exception.
Q: Do the remnants of the burst tubes post a hazard to adjacent traffic?
The bursting process splits the bursting tube into four flat straps of metal which are curled outward by the flared portion of the mandrel. As shown in the crash tests, these remnants from the top, bottom and inside of the bursting tube curl upward, downward or inside of the trailer and do not pose any hazard to adjacent traffic. Only the remnant from the outside edge of the bursting tube could potentially interact with adjacent traffic as it is curls outward. However, the risk posed by this remnant is considered minimum for a number of reasons. First, the remnant does not go out too far from the trailer since it curls up during the bursting process. Furthermore, the remnant stays low, no more than a couple of feet above ground. Second the area covered by the remnant is directly in the path of the impacting vehicle. The impacting vehicle will be traversing the area and running over / through the remnant during the impact event. Finally, the remnants being a flat strips of metal, do not have any structural strength, thus minimizing the probability of the remnant penetrating into the occupant compartment of an adjacent vehicle. This is not to say that the remnants do not pose any hazard, but rather the probability of the remnant penetrating any adjacent vehicle is extremely low and improbable.
Support Truck Weight
Q: What support truck weight limits are associated with the TTMA-100?
The TTMA-100 is recommended for use with any support truck weighing 10,000 lb or more. The TTMA-100 is the only TMA design that has utilized a support truck blocked against forward motion to simulate an infinitely heavy vehicle in all of the NCHRP Report 350 required and optional crash tests. All other trailer and conventional TMA systems have utilized a 20,000-lb truck in at least some of their crash tests. Therefore, the TTMA-100 is the only system that can be safely used on support trucks weighing more than 20,000 lb. Note that FHWA does not recommend the use of any TMA system with any support truck weighing less than 10,000 lb due to concern over the potential for high roll-ahead distance that could pose a hazard to workers and others.
Q: Why does the support truck weight affect the performance of a TMA?
When an errant vehicle strikes the back of a TMA, it begins to push the support truck forward. Lighter support trucks are pushed further forward and at a higher speed than 5 heavier support trucks. Support truck roll ahead produces two beneficial effects. Firstly, any distance that the truck rolls ahead before the impacting vehicle is brought to rest effectively extends the length of the cushion. Extending the effective length of the cushion significantly increases its energy dissipation capacity. Secondly, as the support truck rolls ahead, it picks up speed. Because an impacting vehicle need only to be slowed to the rolling speed of the support truck, roll ahead also reduces the total amount of energy that must be dissipated during the impact. Thus, lighter support trucks greatly increase the capacity of the attenuator and reduce the total amount of energy that must be absorbed by the cushion. No TMA system other than the TTMA-100 has been able to pass all NCHRP Report 350 crash tests with the support truck blocked against roll ahead. Since some of the recommended tests with all other TMA's were conducted with a 20,000-lb support truck, only the TTMA-100 is truly safe when attached to a truck weighing more than 20,000 lb.
Note that the increased roll ahead with lighter support trucks can endanger construction workers or others near the crash site. For example, a parked 10,000-lb support truck would be expected to roll ahead about 70 ft when struck by a large SUV traveling at highway speeds. A similar impact with a rolling support truck moving at 15 mph could push it more than 160 ft forward. Provided such large roll-ahead distances do not endanger construction workers or others near the crash site, support trucks as light as 10,000 lb are acceptable for use with the TTMA-100. Note that, due to high roll-ahead distances and the potential risk to support vehicle operators, the FHWA does not recommend any TMA system be used on a support truck weighing less than 10,000 lb.
Q: Why did NCHRP Report 350 recommend testing with a 20,000-lb support truck?
Prior to NCHRP Report 350, there were no objective criteria for evaluating the safety performance of truck mounted attenuators. Most agencies conducted tests with support trucks in the 10,000 to 20,000 lb range. To establish uniform testing criteria that could be used to evaluate the safety performance of competing designs, the authors of the NCHRP Report 350 recommended the weight of 20,000 lbs for the support truck. This created potential liability problems for TMA users and restricted advancement of the state-ofthe- practice in TMA design. Agencies that needed to mount TMA's on support trucks heavier than 20,000 lbs found that none of the available designs had been tested with a truly heavy support truck. Further, because the guidelines required only a 20,000-lb support truck be used for full-scale crash testing, manufacturers had no motivation to develop designs for heavier support vehicles.
Q: Will the update to NCHRP Report 350 recommend testing with a 20,000-lb support truck?
The update to NCHRP Report 350 is nearing completion and it will mandate the two currently optional tests as well as adding one optional test with a mid-sized automobile. The new guidelines will require that manufacturers define the maximum and minimum allowable support truck weight for each TMA design. Three of the four required crash tests, small car and pickup truck end-on and pickup truck offset tests (tests 50, 51 and 52, respectively) will need to be to be conducted with the maximum allowable support truck weight. The remaining required test, pickup truck angled test (test 53) will be conducted with the minimum allowable support truck weight to evaluate the roll-ahead distance.
Pintle Hook Attachment
Q: How does the rotation of the TTMA-100 affect its safety performance?
The TTMA-100's ability to rotate represents a major improvement in TMA safety technology. While competing trailer attenuators utilize rotational restraints to prevent the attenuator from rotating, the TTMA-100 allows the trailer to rotate in order to keep the system aligned with the impacting vehicle to maximize energy dissipation. It is this innovative approach to energy management that has allowed the TTMA-100 to become the only TMA system to successfully pass NCHRP Report 350 optional offset and angled tests when attached to a support truck blocked against forward motion.
The primary concern about allowing a trailer to rotate is that the impacting vehicle would push the trailer out of its path and directly strike the rear of the support truck. However, as shown in the Figure 1, an impacting vehicle must slide along the trailer's impact face in order to disengage from the trailer.
See a more extended and complete answer
(PDF File, 295 KB, opens in separate tab or window)
Q: Can the TTMA-100 safely accommodate angular impacts?
As described above, the TTMA-100's impact plate is designed to capture the front of an impacting vehicle. The connection between the front of a vehicle and the rear of the trailer pulls impacting vehicles into the trailer. Further, the connection allows the trailer to rotate to align itself with the impact in order to maximize energy dissipation. In this way, the TTMA-100 provides the maximum energy dissipation, regardless of the angle of impact.
Q: Does the trailer's rotation push an impacting vehicle into traffic?
When the TTMA-100 captures the front of an impacting vehicle, it actually begins to pull the vehicle into the cushion. Further, by aligning itself with the impact, the TTMA-100 is able to maximize energy dissipation and avoid pushing the vehicle into adjacent traffic lanes. Comparison of vehicle rest locations after the pickup truck offset and angled tests (tests 3-52 and 3-53, respectively) with other TMA systems shows that the TTMA-100 has no greater tendency to push vehicles into adjacent traffic lanes than other trailer TMA designs. Further, the TTMA-100 was tested with a blocked support truck while all other systems were tested with relatively light, 20,000-lb support vehicles. A light support truck rolls ahead as much as 16 ft during an impact to significantly increase the effectiveness of an attenuation system. Hence, the TTMA-100 performed just as well as the other TMA and trailer systems, even though it was tested under much more stringent conditions. Therefore, it must be concluded that the TTMA-100 is less prone to pushing impacting vehicles into adjacent traffic lanes than its competitors.
Q: Can the trailer remain attached to the support truck with a simple pintle hook attachment?
A low-cost pintle hook attachment may become detached from a support truck during some high-energy impacts. However, when used with the recommended heavy duty safety chains, the TTMA-100 will remain attached to its support truck, even under severe impact conditions. Hence, the trailer will remain attached to the rear of the support truck and not pose a problem to construction workers or adjacent traffic.
Q: Can the TTMA-100 be used on snow plows?
The TTMA-100 can be attached directly to any vehicle with a standard pintle hitch and trailer wiring harness. There is absolutely no other modifications required. Further, the trailer is constructed with heavy gage steel and is galvanized to prevent corrosion. This combination of a simple attachment, durable construction and corrosion resistance make the TTMA-100 the first practical solution for safety protection of snow plows.
Q: Can the TTMA-100 be attached directly to work trucks?
With a simple pintle hook hitch and a standard trailer wiring harness, any work truck, including striping or sweeping trucks, can be safety treated with the TTMA-100. The 10 TTMA-100 can eliminate the need for shadow vehicles in most moving work zone operations.
Q: What is required to attach a TTMA-100 to a sanding or salting truck?
The TTMA-100 can be attached directly to some of the newer sanding, salting, and brining trucks without any modification. Special adaptations are currently under development that will allow the TTMA-100 to be attached to more traditional sanding trucks with rotating sanding equipment.
Q: Can I carry any additional work product, such as safety cones, on the TTMA-100?
It is not recommended for the users to carry any additional products on the TTMA- 100 without explicit approval by the manufacturer. First, there is the concern that the placement of the additional work products may interfere with the proper operation of the energy absorption system. Secondly, these products could potentially become projectiles in the event of a crash and pose a hazard to the impacting vehicle and/or nearby workers and adjacent traffic.
Q: Is the tow speed of the TTMA-100 limited in any way?
The TTMA-100 is safe to be towed at freeway speeds and does not significantly affect the handling characteristics of the tow vehicle. The TTMA-100's rugged construction makes it durable enough to be towed at freeway speeds. There is never any reason to stow the trailer.
Q: Has the TTMA-100 been designed for fatigue resistance?
The basic construction of the Trailer TMA eliminates the fatigue prone components of a conventional TMA system. Further the TTMA-100 utilizes heavy gage tubular construction to provide additional fatigue resistance.
Q: Can the trailer tongue of the TTMA-100 be made longer?
Increasing the length of the trailer tongue should not have a significant effect on the overall impact performance of the TTMA-100. Besides increasing the overall length of the TTMA-100, extending the trailer tongue should have virtually no effect on the trailer's impact performance. However, it should be borne in mind that the tongue weight would increase with tongue length, especially if the added tongue length was used for mounting a sign or storing materials. If the user could tolerate the expected increase in vertical hitch loads, extending the tongue should not require additional crash testing. However, if the trailer's axle had to be moved in order to keep the tongue weight down to an acceptable level, there would likely be a need for crash testing to verify the safety performance of the revised axle location.
Q: Can an arrow board or a variable message sign be used with the TTMA-100?
Yes, an optional arrow board support mount is available for use with the TTMA-100 to accommodate an arrow board or a variable message sign. The arrow board support mount is specifically designed to handle the maximum impact loads experienced by the TTMA-100 and the support truck during crash tests under NCHRP 350 test conditions. During an impact, the TTMA and the support truck are accelerated forward. The concern is that the support mount would collapse and allow the arrow board to crash into the windshield of the impacting vehicle. By designing for the maximum impact force experienced during a crash, the support mount will remain attached to the Trailer TMA and will not collapse, thus eliminating the concern that the arrow board or variable message sign would fall and contact the impact vehicle.
Q: Can I make my own arrow board mount for the TTMA-100?
The manufacturer's arrow board support mount is specifically designed to handle the maximum impact loads when the TTMA-100 and the support truck are accelerated forward during a crash to ensure that the support mount would not collapse, thus allowing the arrow board to crash into the windshield of the impacting vehicle. It is not recommended for the users to make their own support mounts unless the design has been structurally analyzed to ensure that it can sustain the impact load without failure. Furthermore, the manufacturer cannot assume any responsibility for the impact performance of the support mount, or if the support mount adversely affects the impact performance of TTMA-100.
Q: In light of the low profile of the trailer, is conspicuity of the TTMA-100 a problem under adverse weather and pavement conditions, such as snowing?
Conspicuity is an important consideration in the safe operation of the TTMA-100, especially under adverse weather conditions. The impact head is covered with chevron panels to improve its target value and conspicuity and reflective tapes are attached on the sides of the trailer. The lighting arrangement on the impact head include: two LED S/T/T lights, one on each end; three red marker lights; and the license plate light, located close to the top of the impact head. The lighting system of the TTMA-100 exceeds the lighting requirements for trailers set forth by the National Highway Traffic Safety Administration (NHTSA).
However, under adverse weather conditions such as blowing snow, the conspicuity of the trailer TMA will undoubtedly diminish, just like any other vehicle on the highway. In situations where conspicuity is of primary concern such as snow plowing, an arrow board is recommended to improve the conspicuity of the trailer TMA. Optional strobe lights can also be added to the trailer or arrow board supports to further enhance conspicuity.
Damage and Repair
Q: What is the expected extent of damage to the TTMA-100 in a typical crash and what will it take to repair the damaged trailer?
Crash test results indicated that there is no damage to the TTMA-100 in a 5-mph impact by a mid-sized automobile. For low-speed nuisance hits, it is expected that the damage to the trailer would be limited to the shear pins holding the mandrels to the energy absorbing tubes. It is a relatively easy process to replace the shear pins and users can actually perform the repair in the field.
For more severe impacts, the best available information indicates that more than 85 percent of the crashes would result in damage to only the first set of energy absorbing tubes. This 12 percentage is likely to be conservative since smaller and lighter cars would not reach the splice connecting the first set of energy absorbing tubes to the front tubes of the A-frame, even for a high-speed impact with a stopped support truck. For a moving operation, even impacts by heavy passenger vehicles would not reach the splice since the tow vehicle is moving forward slowly. While exact statistics are not available, conversations with a large TMA leasing company in New Jersey and New York indicated that 75 percent of their crashes are 30 mph or less. In crashes where the energy absorbing tubes are damaged, the repairs should be conducted at a certified facility to assure that the Trailer TMA would function properly.
Q: If the front tubes in the A-frame are damaged in a crash, can it be repaired or is it better to just get a new TTMA-100 at this point?
In the rare event that the severity of the impact is such that the front tubes in the A-frame are damaged, there are not many parts that are reusable. It would be easier and cheaper to buy a new trailer instead of repairing the damaged trailer.
Q: How much does it cost to repair the TTMA-100 after a crash?
For low-speed nuisance hits, the damage would likely be limited to the shear or restraining bolts holding the mandrels to the energy absorbing tubes. Repair would involve replacing the shear bolts and the plastic guide plates for a parts cost of $80.
For moderately severe crashes that result in damage to the energy absorbing tubes, the repairs should be conducted at a certified facility to assure that the Trailer TMA would function properly. The repair cost will vary depending on the exact components damaged. For example, the cost of the parts for replacing the energy absorbing tubes, the axle assembly and the light bar would come to less than $6,000 plus labor and shipping costs.
Q: Does the TTMA-100 have any detachable components?
The TTMA-100 is designed to retain all of its components during impact and the tube bursting system is designed to deflect the sides of the bursted tube out of the path of the impacting vehicle. Further, the tube side walls remain firmly attached to the trailer and do not become flying projectiles during high-speed impacts. Numerous tests have been conducted with the tube bursting technology used in both construction and permanent roadside safety hardware and the tube bursting components have never been shown to pose a risk of penetrating the occupant compartment of an impacting vehicle. It should be noted that a significant amount of vehicle debris, such as plastic grill components, lens covers, etc., are generated during full-scale crash tests and some videos of TTMA-100 testing shows these light weight components traveling away from the point of impact. This debris from the impacting vehicle is not unique to the TTMA-100 and, for the most part, it is blocked by the shadow vehicle. Hence, the TTMA-100 does not produce flying debris that would pose a danger to occupants of an impacting vehicle or nearby road workers.
Q: What happens if the TTMA-100 jack-knifes and what is the expected damage to the trailer?
In a jack-knife situation, the front end of one of the energy absorbing tubes in the A-frame would contact the back of the truck. The front ends of the tubes are very stiff and damage tolerant. Also, even if the front end of the tube is bent, it would not adversely affect the impact performance of the trailer. Experience during crash testing indicates that the weak link in the chain is likely to be the bolts holding the pintle hook to the tow vehicle or the lunette ring to the trailer. These bolts would only break in a very high speed jackknife, i.e. the towed vehicle goes out of control at high speed. Replacing these bolts is both simple and inexpensive.
Q: How much force is needed to pull out the mandrels? For example, is it likely for the mandrels to pull out if I run through a ditch?
The restraining bolts holding the mandrels to the energy absorbing tubes were not damaged in the 5-mph crash test. Thus, it is unlikely for the bolts to break in normal operations. However, after traversing rough terrain, it would be a good idea to check the restraining bolts.
Q: Do I need to register the TTMA-100 as a trailer with the state DOT?
The rules and regulations vary from state to state. For example, Texas DOT does not require registration for trailers weighing less than 26,000 lb. Please consult your state DOT for the applicable rules and regulations.
Q: What type of maintenance is required for the TTMA-100?
Even though the TTMA-100 is fully galvanized and designed for rugged use, it still requires some minimal periodic inspection and maintenance. Please refer to the "Maintenance Guidelines" section of the User's Manual.
Q: What are the weights associated with the TTMA-100 and optional equipment?
The weight of the TTMA-100 without any optional equipment is about 1,450 lb, with a tongue weight of 190 lb. A trailer equipped with an arrow board would weigh less than 2,000 lb with a tongue weight under 500 lb.
Q: Is there a warranty for the TTMA-100?
There is a one-year warranty against all defects in materials and workmanship.
Q: What is the probability that an errant vehicle would actually hit a TMA when it runs off the side of the road?
The probability that an errant vehicle would actually hit a TMA when it runs off the side of the road is difficult to define since it is a function of many factors, such as highway type, operating speed, angle of departure, distance from the edge of the roadway, etc. Basically, the probability of a Trailer TMA being hit by an errant vehicle is no different from that of any other roadside furniture under similar conditions.