Overview of Noise and Rationale for Rubberized Asphalt Noise Studies

Noise pollution is the presence of intrusive and unwanted sounds that can seriously affect physical and psychological health. Some examples of the effects from noise pollution include the loss of hearing, anxiety, sleeplessness, aggression increase in heart rate, and stress. Noise is measured by decibels (dBA) which are a logarithmic function of the ratio of the sound pressure squared over the reference pressure squared. Appendix "A" provides definitions of acoustic terminology used in this report. Levels of noise can range from very faint to painful and dangerous. For example, human breathing has a dBA of 10 which is considered very faint, office activities have an average dBA of 50, which is considered moderate, and a jet engine at 75ft has a dBA of 140 which is considered painful or dangerous. Because noise has potentially harmful effects, local, state, and federal agencies established noise thresholds beyond which traffic noise abatement must be considered.

Specific noise policies and standards which affect decisions regarding noise mitigation in Sacramento County are provided in Appendix B. It is evident from the various noise standards shown in Appendix B which apply to both development and roadway construction projects in Sacramento County, that this topic is given considerable attention in the environmental review process. The comprehensive County noise criteria has set standards that are often exceeded due to the ever increasing traffic noise levels that cannot be mitigated in traditional ways.

In light of this routine occurrence, the investigation into alternative noise abatement options, other than barriers, was considered to be warranted by Sacramento County. The initial studies of rubberized asphalt were commissioned by the County in 1993. Subsequent testing has been commissioned by the County twice since the initial tests were conducted in 1993. The following sections provide an overview of how traffic noise is generated, followed by the detailed rubberized asphalt test procedures and results of those tests.

How Traffic Noise is Generated and the Implications for Rubberized Asphalt

Traffic noise is generated primarily by the interaction of the tires and pavement, by the internal combustion engine of the vehicle, and by the engine exhaust. For automobiles, the vast majority of the noise is generated by the interaction of the tires and pavement due to quieter engines and exhausts on modern vehicles. As a result, the effective noise source height for automobiles is considered to be zero (0) feet above the pavement, or right where the tire meets the road.

For medium duty trucks (2 axle trucks), there is a slightly larger contribution of noise from the engine compartment and exhaust pipe, so the effective noise source height is considered to be an average of those sources at two (2) feet above the pavement. For heavy trucks, not only is there a greater contribution of noise from the engine and exhaust, the exhaust stack opening is typically 11 feet or so above the pavement. Therefore, the effective noise source height for heavy trucks (3 axles or more), is considered to be eight (8) feet above the pavement, or the weighted average heights of the tires, engine and exhaust stacks.

This information pertaining to the noise generation of the various vehicle types is relevant in that rubberized asphalt is believed to obtain most if its’ noise-reducing properties from a combination of the porosity and ductility of the rubberized roadway surface. As a result, tire noise is reduced, but engine and exhaust noise is not appreciably affected by the rubberized surface. Therefore, a roadway containing primarily automobile traffic would be expected to exhibit greater decreases in traffic noise following paving with rubberized asphalt that would a roadway that has a high percentage of heavy trucks.

Traffic Noise Prediction Model

A discussion of the method by which traffic noise is predicted is appropriately included in this report in that normalization of the traffic conditions present during the various noise measurement surveys was accomplished using the Federal Highway Administration Highway Traffic Noise Prediction Model (FHWA-RD-77-108). This normalization was required to isolate the effectiveness of the rubberized paving from the other variables affecting traffic noise generation which were present during the noise tests.

The FHWA Model is the traffic noise prediction model used by Sacramento County for traffic noise assessment. Several adaptations of the model have been developed, including Stamina and Sound 32, but these models are all fundamentally based on FHWA-RD-77-108.

The Federal Highway Administration is currently working on a new traffic noise prediction model which will theoretically replace the existing model, called the Traffic Noise Model (TNM). The TNM will reportedly make adjustments to traffic noise predictions based on roadway surface, but it is not known whether rubberized asphalt will be included in those surfaces. According to FHWA officials, the new TNM has been released and is in use by various State Departments of Transportation (DOT's). It is likely that the new TNM will be required in situations where state or federal funding is involved, but it remains to be seen whether the complexity of the new model will be required for all traffic noise modeling efforts. At the time of this writing, the new TNM has not been adopted for use on California roadways by Caltrans.

Traffic Noise Prediction Model Calibration

The FHWA Model provides reasonably accurate traffic noise predictions under "ideal" roadway conditions. Ideal conditions are generally considered to be long straight roadway segments with uniform vehicle speeds, a flat roadway surface, good pavement conditions, a statistically large volume of traffic, and a unimpeded view of the roadway from the receiver location. However, ideal conditions are more the exception than the rule. As a result, it is often necessary to calibrate the FHWA Model through site-specific traffic noise level measurements and concurrent traffic counts.

The calibration process is performed by conducting concurrent traffic noise level measurements and vehicle counts, and comparing the measured level with that predicted by the Model for the given traffic conditions. This calibration procedure can be used to normalize the model output for varying traffic volumes, speeds, and truck compositions present during the noise measurement samples. Once these factors have been normalized, and the other variables affecting measured traffic noise levels (measurement equipment, distances, measurement technique, etc.) held constant, the differences between measured traffic noise levels before and after the paving with rubberized asphalt can be attributed to the roadway surface. This calibration procedure is the basis for the assessment of the noise-reducing properties of rubberized asphalt reported in this report.

Traffic Noise Prediction Model Inputs

Inputs to the FHWA Model include the number of vehicles per hour, the percentages of medium (2 axle) and heavy ( 3 or more axles) trucks, the average vehicle speeds, the distance between the traffic and receiver, and the characteristics of the intervening ground located between the roadway and the receiver (hard vs. soft site). During the calibration procedure described above, each of these factors was accounted for.

Specific Rubberized Asphalt Test Procedure

The fundamental methodology employed to determine the effectiveness of rubberized asphalt in reducing traffic noise levels in Sacramento County was to take the difference between normalized traffic noise levels measured before and after paving of certain County roadways with rubberized and conventional asphalt overlays. As stated previously, there were several factors which influenced traffic noise generation which needed to be carefully considered in the analysis. Those factors, which include test roadway geometries, noise level measurement equipment location and configuration, atmospheric conditions, and traffic volume, speed, and heavy truck usage, are discussed below.

Test Roadways Evaluated in the Sacramento County Studies: The roadways selected by Sacramento County for assessment of the noise reducing properties of rubberized asphalt were Alta Arden Expressway between Howe and Watt Avenues, and Antelope Road between Auburn Boulevard and Old Auburn Road.

The paving of Alta Arden Expressway was completed in October of 1993, and was not associated with any other widening or reconstruction of that roadway. Therefore, the effects of rubberized asphalt in reducing traffic noise levels on this roadway could be studied without complications which arise from additional travel lanes, roadway realignment, or substantial changes in speeds which could result from such modifications.

The paving of Antelope Road with rubberized asphalt was completed following a roadway widening project on this roadway around April of 1995. As a result, the roadway geometry varied considerably between the pre- and post-paving noise level measurement periods. An effort was made to conduct the noise level measurements at the same distance from centerline before and after the paving. However, due to the widening, the near travel lane moved closer to the noise measurement sites, and speeds increased due to reduced congestion on this roadway. It is not specifically known to what degree the change in roadway geometry and speeds affected the noise measurement results. It is likely, however, that the post-paving noise levels were marginally higher than had the widening not occurred.

The paving of the Bond Road control segment with conventional (non-rubberized) asphalt occurred as part of a roadway widening project in August of 1995. As a result of the roadway realignment, the roadway geometry varied considerably between the pre- and post-paving noise level measurement periods. An effort was made to conduct the noise level measurements at the same distance from centerline before and after the paving. However, due to the widening, the near travel lane moved closer to the noise measurement sites, and speeds increased due to reduced congestion on this roadway as well. It is not specifically known to what degree the change in roadway geometry and speeds affected the noise measurement results. It is likely, however, that the post-paving noise levels were marginally higher than had the widening not occurred, as was the case for Antelope Road.

Elapsed Time Between Measurements: In the Alta Arden assessment, the traffic noise measurement survey was conducted one month prior to the paving with rubberized asphalt. The survey was repeated one month after paving, 16 months after paving, and six (6) years after paving with rubberized asphalt.

In the Antelope Road assessment, a period of 16 months elapsed between the "before" and "after" noise measurements. The asphalt overlay was installed approximately 10 months into this period, around April of 1995. Therefore, the "before" measurements were conducted approximately 10 months prior to the paving, and the "after" measurements were about 6 months after the paving with rubberized asphalt. The measurement survey was subsequently repeated in September of 1999, approximately 4 _ years after the paving with rubberized asphalt.

In the Bond Road assessment, the traffic noise measurement survey was conducted one month prior to the paving with conventional asphalt. The survey was repeated one month after paving, and again four (4) years after paving with conventional asphalt

Asphalt Compaction: Compaction of the asphalt overlay reduces the porosity of the road surface, which is believed to account for some of the noise reduction properties of the rubberized asphalt pavement. According to Sacramento County Public Works Agency, Transportation Division staff, the compaction of the paving material is essentially complete within one year of the paving. Therefore, the varying periods of time which elapsed between the paving of the test roadways and the follow-up measurements provides insight into the effects of compaction on the noise-reducing properties of rubberized asphalt. The specific findings regarding compaction follow in a later section of this analysis.

Noise Measurement Duration, Equipment Locations and Configurations: The noise level measurement surveys initially consisted of continuous measurements over a minimum period of 24-hours, and short-term (15-minute) measurements at various locations along each of the three test subject roadways.

The continuous noise level measurements were conducted to evaluate the differences in noise levels over 24-hour periods before and after the paving. A benefit of the continuous noise level measurements was that a statistically large sample of noise level data was obtained by which the effects of the rubberized asphalt could be generally evaluated. However, it was not practical to monitor and account for all of the factors which affected the measured noise levels over the continuous sampling periods. Therefore, the findings based on the continuous sampling are considered approximate and relevant only to the measurement periods which were not separated by extensive periods of time (i.e. periods during which traffic volumes and compositions would be expected to be relatively similar).

The short-term noise level measurements were conducted at various distances from the roadway centerlines. The continuous and short-term traffic noise measurements were conducted at a microphone height of 5 feet above ground. These measurements provided a statistically smaller sample of data by which to evaluate the effects of rubberized asphalt than did the results of the continuous monitoring, but traffic counts conducted during the short-term samples allow normalization of the measurement data as discussed previously in this report. The short-term sampling periods also allow for monitoring of all factors which affect the traffic noise measurement results. Therefore, the normalized results of the short-term samples are believed to provide a more reliable indication of noise reduction attained by the use of rubberized and conventional asphalt paving materials on the test subject roadways.

Larson Davis Laboratories (LDL) Model 870, 700 and 820 integrating sound level meters were used for the continuous and short-term noise level measurements. The meters were calibrated before use with LDL acoustical calibrators to ensure the accuracy of the measurements. The equipment used meets all applicable specifications of the American National Standards Institute for precision sound level measurement systems. The equipment configurations were identical for all of the before and after measurements, with the meters set to the A-weighting network and slow response.

Atmospheric Conditions: Weather conditions were considered to be effectively similar for the before and after short-term traffic noise level measurements at each location. However, due to the close proximity of the noise level measurement microphones to the roadway centerlines, variations in weather conditions between the before and after noise level measurement periods are not believed to have significantly affected the measurement results. In all cases, the measurements were conducted on dry pavement.

Traffic Volume, Speed and Heavy Truck Usage: The continuous and short-term noise level measurements were conducted during typical weekday periods. Given the relatively long period between the initial and final noise measurement periods (4 to 6 years), the traffic volumes are believed to have varied significantly. Therefore, continuous noise level measurements were not used during the 1999 measurement surveys as use of such data could lead to erroneous conclusions regarding the noise-reducing properties of rubberized asphalt.

Traffic counts conducted during the short-term samples indicated that heavy truck traffic accounted for a very low percentage of the total traffic on each of the test subject roadways during those measurement periods. This finding is important in that heavy trucks generate considerably more engine and exhaust noise than automobiles, as stated previously. As a result of the low number of heavy trucks, the traffic noise was generated primarily by the interaction of tires and pavement, which is the component of the traffic noise intended to be isolated in this study.

Average vehicle speeds were observed to be marginally after paving at the test subject roadway locations where an additional lane was added, and fairly similar at the locations where the roadway geometry was not significantly altered. This assumption is based on observations and speedometer checks.

Specific Sacramento County Rubberized Asphalt Test Results

The normalized and averaged results of the various traffic noise surveys conducted on the three test subject roadways are presented in Table 4. The Table 4 data is presented in the form of changes in traffic noise levels relative to pre-paving conditions.

Evaluation of the Table 4 data indicates that, immediately after paving the test roadways with rubberized and conventional asphalt, traffic noise decreased along all three roadways. However, once a sufficient amount of time had elapsed for the various roadways to be fully compacted, the roadways paved with rubberized asphalt still exhibited good traffic noise reduction, whereas the noise reduction of the conventional asphalt overlay was lost.

As stated previously, the Antelope Road test procedure was complicated in that the pre and post-paving tests were conducted on different roadway geometries. Because of this change in geometries, the noise reducing properties of the rubberized asphalt on that roadway may have been slightly understated. The changes in noise reduction of the rubberized asphalt on Alta Arden and Antelope noted between the tests conducted shortly after the paving and those conducted several months and years later (1 dB drop in noise reduction), is believed to be due to compaction of the roadway surfaces.

Table of Contents | Executive Summary | Introduction | History of Noise Reducing Pavement | The Process of Producing Rubberized Asphalt | Current Uses of Rubberized Asphalt | Tables | Studies of Rubberized Asphalt Outside of Sacramento County | Sacramento County Rubberized Asphalt Noise Studies | Conclusions of the Studies Conducted in Sacramento County | Appendix A - Acoustical Terminology | Appendix B - Noise Standards Commonly Applied to Projects in Sacramento County