By Wayne Jones, P.E.
We have all heard the old conundrum: If a tree falls in the woods and there is no one there, does it still make noise? The modern version of this may well be: If tires roll along a pavement in the desert, will there be anyone there to hear it? While the answer to the original version has been debated endlessly, the answer to the current version of the question is easy. “Yes.”
Thanks to a joint research effort between the Volpe National Transportation Systems Center and the California Department of Transportation (Caltrans),someone is listening.
Five years ago, Caltrans built five test sections in the high desert north of Los Angeles (LA) on SR-138, approximately 22 miles west of Lancaster, California, and began a long term tire/pavement noise monitoring effort.
According to Judith L. Rochat, Ph.D., Physical Scientist, with the U.S.DOT Volpe National Transportation Systems Center, the first goal of their study was to determine how much tire/pavement noise reduction can be achieved when comparing relatively thin open-graded friction courses(OGFC) to Caltrans’ normal dense graded hot mix asphalt (HMA) surface mixes. The second goal of the study, and a much harder question, is: How long does the acoustic attenuation provided by OGFCs typically last?
Where Are We?
To understand why the results of such a long term study are needed by Caltrans, we must first understand the traffic noise issue. Current Federal Highway Administration (FHWA) policy requires an environmental impact statement whenever a new highway is planned or a highway’s capacity is scheduled to be increased. Part of the environmental study that DOTs are required to perform is a noise impact study. The main tool that state DOTs use is the FHWA’s Traffic Noise Model®(TNM).
The TNM software, developed and maintained by the Volpe Center, predicts future noise impacts in the vicinity of highways and on the adjacent properties, after the project has been completed and put into service. If predicted noise levels exceed maximum allowable criteria (and each state’s criteria are different),then all reasonable and feasible noise abatement methods must be considered. Current FHWA policy recognizes earth berms and barrier walls as the only types of physical barriers acceptable as abatement methods. Zoning laws and land use planning are the non-physical tools in the abatement tool box. However, such regulations do very little for existing situations.
As a result, Caltrans has become the largest builder of noise walls among the 50 state DOTs. Through 2004, the last year for which statistics are available, Caltrans has built over 480 linear miles of noise barrier wall costing, in 2004 dollars, in excess of $590 million.
But noise barriers alone are not the panacea for all situations. For example, noise walls are ineffective where there is rolling country or multistory buildings along the highway.
Anywhere there is a line of sight between the flow of traffic and the listener, noise walls do very little, if anything, to reduce the amount of noise reaching the listener (i.e., if you can see the traffic, you can hear the traffic noise). In this situation, reducing the amount of noise being generated may be the only practical solution.
The TNM calculates an average from the large database of noise readings on a wide variety of pavement surfaces to form an “average pavement surface” that the predictions are based upon. Over the years, acoustic consultants using the TNM have found that on certain pavement surfaces, the TNM software tended to over-predict the actual amount of noise generated once the pavement was placed in service. Which raises the question: Could a group of quieter pavement surfaces be identified and cataloged into the model and then be used as an additional form of noise abatement? It stands to reason that if less noise is generated, millions of scarce highway improvement dollars could be saved by building noise barriers of shorter length, lower height, or in some instances, eliminated all together.
Where Are We Going?
Before pavement surface can be used as an input to the TNM, several legal and policy issues need to be addressed. If the TNM predicts a wall is necessary, and a state DOT builds and maintains that wall, the assumption is that the state and FHWA have met their legal requirements by providing the wall and that the wall will continue to function as a noise abatement device well into the foreseeable future.
When exposed to traffic and the elements, pavement surfaces change over time, and generally get louder. If a quieter pavement surface is used as a noise mitigation technique, how long will that mitigation last and what assurances are there that the pavement surface will be replaced before it exceeds acceptable noise readings?
Before the pavement’s surface can be used as an abatement tool, three questions need to be answered in a legally binding way:
- How much noise reduction can be achieved by specifying a quieter pavement surface?
- How long will the reduction last?
- What are the available options to restore the pavement to its original acoustical performance?
How Do We Get There?
In an effort to jump-start the TNM recalibration process, the FHWA’s Office of Pavement Technology, working with the scientists at the Volpe Center, has recently started the process of analyzing the database already contained in the TNM. This desktop review of the existing database will look at the quality of the data already included in the inventory and prioritize what pavement types and noise measurements are needed to fill any gaps.
Another effort to accelerate the recalibration process is to look at existing ongoing studies. In recent years, there have been a large number of noise readings taken from existing pavements of all ages. For example, to date, the National Center for Asphalt Pavement Technology has taken well over 300 such readings in 18 different states on HMA as well as PCC pavements of all ages and conditions. While these “snapshots” provide a good comparison in terms of ranking a pavement’s surface from loudest to quietest, there is very little, if any, data on noise readings over time.
This brings us back to the high desert north of LA, where Ms. Rochat and her colleagues traveled with three vans carrying over 50 cases of acoustic equipment, to take the last in a series of noise measurements on the Caltrans test sections.
The “Thin Lift Study,” as the SR-138study came to be known, is one of the longest ongoing studies in the country. Only the 8-year Caltrans study on I-80 in the Davis, California, area is longer. The Thin Lift Study started out as a very ambitious effort with over 20 sites and 15 different asphalt pavement surfaces. Overtime, the noise issue became predominant and the flat open terrain of the high desert provided an excellent site to do long-term tire/pavement noise monitoring.
Five years ago, the existing pavement was in such bad shape that a structural overlay was necessary so the pavement would hold up under the demands of traffic.SR-138 turns out to be a convenient east/west connector for traffic running between I-5 on the west and the Edwards Air Force Base area on the east. This puts more truck traffic on the roadway than a typical two-lane desert highway. Having placed the structural overlay the entire length of the project, each test section then had the same foundation, and the only differences would be in the surfaces placed on the five test sections.
Rochat explained that after the structural overlay was installed, a baseline noise reading was taken on all five sections to get a good reference point from which to compare each section. Next, the five different surfaces were placed. Section 1 was a dense graded HMA control section. Section 2 was a 75mm “thick lift” of OGFC; Section 3 was a 30mm “thin lift” Asphalt 19Spring 2007of OGFC. Section 4 was a 30 mm rubber modified OGFC—similar to what the Arizona DOT has used successfully in the Phoenix Metropolitan area. Finally, Section 5 was a 30mm bonded wearing course.
Beginning four months after the construction of the test sections was complete, and every six months since, standard passby noise readings, in accordance with FHWA’s guidelines, have been taken at the test sections.
While the final report on the Thin Lift Study is not expected to be finished until late this summer, Rochat was able to make some generalities. Section 5, the bonded wearing course turned out to be the noisiest surface, providing very little noise reduction over the control section. Subsequently, it was dropped from further testing. Section2, the thick OGFC, proved to be the quietest surface in the group.
Ms. Rochat expressed her confidence that Caltrans would continue to monitor the pavement until the test sections are replaced for pavement performance reasons. This would allow a comparison between the longevity of acoustic performance and overall pavement performance.
Before the question of using the pavement surface as a noise mitigation option is answered, years of research, data collection and analysis must be undertaken and completed to recalibrate the TNM. All of this must be done to make absolutely certain that the predictions coming out of the model represent real world situations.
|Wayne Jones is the Asphalt Institute’s Field Engineer in Columbus, Ohio.|