Experience with truck mounted attenuators.
As a 1986 report done by the Authority's research engineer stated, "Numerous accidents in other states have been reported in which the saving of lives was attributed to the presence of impact attenuators. While their use does not reduce the number of accidents nor the potential, they have been shown to substantially minimize damage and lessen the chance for serious injury and death." Since that time, we have searched for hard data on the benefit/cost of using TMAs to satisfy our budget examiners and my own curiosity. Not being able to find such data from either using agencies or manufacturers, we set out in 1990 to compile such data. It is these data and more importantly the findings from analyzing them that are reported in this article.
From 1982 to 1992 our use of TMAs was primarily for protecting slow moving operations like line striping. It was not until two years ago that we acquired enough TMAs to make them available to most of our lane closure operations. However, the use of TMAs are at the work crew supervisor's discretion. Currently, all our TMAs are models manufactured by Energy Absorption Systems Inc.
In 1990, Professor Jack Humphreys of the University of Tennessee, Transportation Center published a paper entitled "Guidelines for the Use of Truck-Mounted Attenuators in Work Zones." Professor Humphreys noted, among other things, the hazardous nature of construction and maintenance work zones and that the accidental intrusion of a vehicle into the work area may present a significant hazard to workers and occupants of the intruding vehicle. It is common practice to use a work zone vehicle (such as a dump truck) to shield workers from an errant vehicle. Unfortunately, impact with a barrier vehicle can result in injury to occupants of the impacting vehicle. He went on to state that to overcome the severe nature of the hazard presented by a barrier or shadow vehicle, TMAs of various designs have been developed and are now in use across the United States. His paper noted that development of the first mobile crash cushion system attached to work vehicles was the Texas Crash Cushion Trailer, developed and tested in 1972. He further noted that there seemed to be little factual basis for the existing application policies for the use of TMAs and that his paper was not a research project. Such an effort would have involved the collection and analysis of data which are not readily available. The information in this paper responds to that challenge and covers some 40 accidents on the New York State Thruway over a 52-month period (February 1990 to June 1994) where a customer's vehicle collided with the rear of a maintenance truck. Eleven of the accidents were with trucks equipped with a TMA and 29 were with trucks not equipped with TMAs. While these data may fall short of being "statistically" significant, they are nonetheless both interesting and relevant.
We currently have TMA mounting brackets on our dump, hopper, and rack trucks. It takes two people about 15 minutes to hook up a TMA. We are looking into using pins rather than bolts to reduce this time somewhat. It should be noted that the TMA frame mounted on a dump truck reduces the utility of that truck. It takes about an hour to remove the frame from the truck if you need it to haul material. Depending on truck make and model, it takes a mechanic one to two days to install the frame in the first place. The last TMAs we purchased cost $7,600.
Examining the Data
Damage to vehicles involved is reported in Table 1. Significant findings are:
* Use of a TMA eliminated all damage to maintenance trucks.
* As would be expected, damage to a maintenance truck not equipped with a TMA is considerably more extensive when hit by a tractor trailer unit than by an automobile.
* All vehicles impacting a TMA sustained some level of damage.
* Damage to a trailer tractor unit is three times more extensive when hitting a maintenance truck without a TMA than hitting one equipped with a TMA.
* Damage to an automobile is only slightly less when impacting a TMA than hitting a maintenance truck without a TMA.
Table 1 -- Damage to Vehicles Involved in Maintenance Truck Rear-End Accidents
Without TMA With TMA Impacted Vehicle (Thruway Truck): Damage 33% 0% Hit by Tractor Trailer: Average Days Out of Service 104.5 0 Average Repair Cost $10,401 $6,454 Hit by Automobile: Average Days Out of Service 22.7 0 Average Repair Cost $871 $4,740 Impacting Vehicle: Damage 100% 100% Average Repair Cost (Est.) Tractor Trailer $24,600 8,333 Automobile $7,917 $7,163
Of the 11 accidents involving TMAs, only one was with the Alpha 2001 MD. All other units were Hex-Foam models. From this limited experience, it appears that the Alpha 2001 MD model cost about $2,000 less per collision to repair, which is a significant attribute of that model.
Table 2 reports the percent of accidents with injuries and fatalities.
* None of the collisions resulted in any maintenance employee fatalities.
* No collisions with a TMA resulted in any fatalities.
* The injury rate to maintenance employees was the same whether or not the maintenance vehicle held a TMA.
* Injury rate to tractor trailer occupants was high when the maintenance vehicle was equipped with a TMA.
* Injury rate to automobile occupants was higher with the maintenance vehicles not equipped with a TMA.
* Fatality rates were considerably higher to tractor trailer occupants than automobile occupants when coliding with a maintenance truck not equipped with a TMA.
Table 2 -- Percent of Accidents with injuries and Fatalities Thruway Truck Impacting Vehicle Injuries Injuries Only Fatalities Only Fatalities Without TMA: Tractor Trailer 20% 0% 20% 40% Automobiles 12.5% 0% 46% 12.5% Total: 14% 0% 41% 17% With TKAA: Tractor Trailer 0% 0% 33.3% 0% Automobiles 12.5% 0% 25% 0% Total: 9% 0% 27% 0%
A review of seat belt usage in accidents where an injury or fatality occurred reveals that use of seat belts was higher by maintenance employees than by tractor trailer or automobile occupants; tractor trailer occupant's seat belt usage was only 60 percent; automobile occupant's seat belt usage was 74 percent; in all automobile fatalities, occupants were using seat beats; and usage in all fatalities was well above the national average.
Accidents by months of year: Overall accident frequency was uniformly distributed between the winter period and nonwinter period; accident frequencies were higher in January, February, and October; and accident frequencies in July and August were low.
Accidents by day of week: Accident frequency was highest on Mondays and Thursdays; and no accidents occurred on Wednesdays.
Accidents by time of day: Accident frequencies increased immediately after normal meal hours (see Figure 1).
Weather and road conditions: Inclement weather and poor road conditions were not a significant factor in rear-end collisions. (Figure 2).
Traffic control setups: Maintenance vehicles in moveable traffic control setups were the most susceptible to rear-end collisions; slower moving snow and ice control vehicles were involved in 25 percent of the rear-end collisions; based on the last two observations, a maintenance vehicle on the pavement is perceived by motorists to be moving at the same speed they are moving; and the location of the maintenance vehicle on the pavement had no significance in the occurrence of rearend collisions. (Figure 3).
The Cost of Accidents
All direct Thruway Authority costs were obtained from our normal business records. The costs attributed to injuries and fatalities for both Authority employees and the colliding vehicle occupants were selected from tables published by the New York State Department of Transportation Traffic Engineering and Safety Division entitled Table B -- Average Accident Costs.
The costs to repair the vehicle that collided with our maintenance trucks were developed by estimating the extent of damage from photographs taken by our traffic supervisors at the accident scene. The assistance of an automotive body mechanic from our Albany Division in the damage estimating process is acknowledged and greatly appreciated.
In reviewing our accident cost data, presented in Table 3, a couple of numbers are particularly interesting:
* The cost incurred by the Authority equipment when a maintenance vehicle is hit by an automobile calculated out slightly higher for those hitting a TMA than hitting a truck without a TMA. This appears to be because the TMA is soft while the truck is "armored."
* The total cost of an accident to society was about the same for an automobile and a tractor trailer when impacting a TMA.
* The cost to society of accidents where no TMA was in use are considerably higher than those where a TMA was impacted.
* The total cost to society of a tractor trailer hitting a maintenance truck without a TMA was four times greater than a similar automobile collision. Could this be attributed to the seat belt usage compliance rates noted earlier?
Table 3-Average Accident Costs Number of Thruway Accidents Authority Total Automobiles: Without TMA 24 $14,453 $483,570 With TMA 8 $17,382 $46,820 Tractor Trailers: Without TMA 5 $32,794 $2,039,494 With TMA 3 $6,530 $44,563
Putting all this information and data together, what does it all come down to?
Using the data we compiled and New York State Department of Transportations "Willingness-to-Pay" approach method, we calculated the "Benefit/Cost Ratio" for a public agency using a truck mounted attenuator as 90. This is an outstanding benefit ratio which makes expenditures for this equipment extremely attractive. (Figure 4).
In summation, our data suggests that:
* Use of TMAs eliminates all damage to maintenance trucks.
* The Energy Absorption Systems Alpha 2001 MD units appear to be less expensive to repair than the older Hex-Foam models.
* No collision with a TMA resulted in a fatality.
* Fatality rates are considerably higher to tractor trailer occupants than to automobile occupants.
* Seat belt usage by tractor trailer occupants was only 60 percent.
* Moveable traffic control setups were the most susceptible to rear-end collisions.
* A maintenance vehicle on the pavement is perceived by motorists to be moving at the same speed they are moving.
* The cost to society of accidents where no TMAs were in use are considerably higher than those where TMAs were impacted. The benefit/cost ratio for using a TMA is 90.
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|Title Annotation:||New York State Thruway Authority|
|Author:||MacKay, Raymond G.; Powers, John L.|
|Date:||Oct 1, 1995|
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