Edited by Loren Faulkner
With most states making contractors responsible for impact repairs during construction, highway contractors should take a cost benefit approach in their crash barrier/attenuator selection.
Traffic volume is increasing at an alarming rate. With limited space for additional lanes, traffic flow is constantly being studied to keep the motoring public safe. Safety devices have become increasingly more important in protecting these motorists. The impact attenuator (crash barrier) is one of these safety devices that are used to protect the ends of barriers or fixed objects along the roadway. Crashing into an untreated end of a barrier or fixed object can result in serious injuries or fatalities due to the vehicle stopping abruptly.
|Caltrans crew in San Francisco installing severe duty/low maintenance (SD/LM) attenuator northbound on I-101 at the 080.|
Three Types of Attenuators
Significantly improving over the years, impact attenuators generally decelerate impacting vehicles gradually to a stop. There are three basic categories of impact attenuators; inertial barriers (sand barrels and water barriers), standard-duty impact attenuators, and severe duty/low maintenance attenuators (SD/LM).
Sand barrels have limited use, as they do not redirect vehicles. It is suggested that they not be used in median applications where a vehicle can penetrate and enter into oncoming traffic. While they have a low initial cost, sand barrels have significant weaknesses. Since they allow penetration by the vehicle, side impacts near the rear of the array may not protect the motorist from the fixed object.
These modules are sacrificial and must be replaced, which can be expensive in high-impact locations. Additionally, when impacted, sand and barrel fragments can be thrown into all traffic lanes, causing hazardous conditions. It can take repair crews up to five hours to clean up while being exposed to heavy traffic. Additionally, closing down many traffic lanes for cleanup causes an inconvenience to motorists.
Water barriers essentially have the same issues except they use water instead of sand. This water requires additives in colder states, which must be considered in their usage during winter months.
From a price perspective, standard-duty impact attenuators are priced mid-range but tend to be the most expensive to repair. On average, it costs $4,000 to $10,000 to repair these attenuators, while increasing crews' exposure time to traffic during repairs. Designers typically use these attenuators in low Average Daily Traffic (ADT) situations, where the frequency of impacts is low.
Over the years, attenuators have evolved, creating a whole new category of attenuators — SD/LM attenuators. Unlike sand barrels and standard-duty attenuators, low repair costs are incurred when repairing SD/LM attenuators, as they are simple to repair and require very few replacement parts. Initially, this category only included elastomeric attenuators.
These attenuators rebounded after an impact to a percentage of their original length. The concept was to reserve residual energy for another impact and to have very low maintenance field time; however, all attenuators require maintenance after an impact because they need to be inspected and reset to their original condition, in order to ensure performance as originally designed and installment for future impacts.
Also, the rebounding characteristics have a tendency to cause the vehicle to be thrown backwards and possibly into oncoming traffic. Recently, there was a speed-dependent attenuator that completed a two-year in-service evaluation for the FHWA with a published repair cost of $39 per impact. This attenuator can vary its stopping forces to allow a safe ride down for different speeds and masses. This is not an elastomeric attenuator, so there is no rebound. This attenuator also has no damage on side impacts, which can be an issue with other SD/LM attenuators.
Having worked for the California DOT since 1972, Jon Stidman has certainly seen his fair share of crash attenuators. As a maintenance supervisor for District 4 (San Francisco) he can name the different designs and models of attenuators, but ultimately only one model stands apart from the rest.
"Since working for the state of California, I have worked with many styles of crash attenuators. I am most impressed with the performance and reparability of SD/LM attenuators," said Stidman. "We installed one at the northbound on I-101 at the 080 junction, as this location frequently experiences impacts. The attenuator has been impacted three times since being installed last year, and all three repairs took less than 30 minutes to reset. The last time it took us 22 minutes from start to finish with an approximate $1 cost, which included shear pins for the repair. Using this type of attenuator is far more cost effective. Furthermore and most importantly, it shortens employees' exposure time to traffic."
Choosing just one of these designs is essential for DOTs keeping costs low and increasing efficiency. If states have numerous brands of attenuators on the roadway, it becomes a management nightmare. Training maintenance crews to perform expert repair on multiple attenuators is difficult at best. Parts can have long lead times and carrying inventory on multiple attenuators can be cost prohibitive.
Many states look for an attenuator that is cost beneficial, user friendly and has minimal repair parts to inventory. When they choose one attenuator, they have few options. One of those options is to file for a Finding in the Public Interest (FIPI) to sole source a product. States must present evidence to the FHWA that this will improve synchronization of maintenance, increase safety and reduce costs.
The other bidding method is to bid as few choices as possible, calling out the attenuators of choice. This method still causes inventory and training issues. FHWA regions view FIPIs differently, and it can be an easy or difficult task based on the region.
Today's best practice is to use a cost-benefit approach to determine the attenuator category for work zone and permanent applications. Work zones and roadways with high ADT (25,000 plus) tend to be high-impact areas, because traffic is being shifted due to road construction and high traffic volumes. The cost differences between standard-duty and SD/LM attenuators have been dramatically reduced, making for an easy justification for SD/LM attenuators.
(Editor's Note: Jeff Smith is vice president of Specialty Products, SCI Products Inc. www.workareaprotection.com )