ENVIRONMENTAL NOISE ANALYSIS - ANTELOPE ROAD WIDENING PROJECT. Sacramento County, California. Prepared For

Transcription

1 f, - ENVIRONMENTAL NOISE ANALYSIS, - - ANTELOPE ROAD WIDENING PROJECT Sacramento County, California I Prepared For Sacramento County Public Works Agency Transportation Division 906 G Street, Suite 510 Sacramento, CA February 2, 1996 Prepared By Brown-Buntin Associates, Inc. Fair Oaks, California BBA 5150 Sunrise SuUe D-3 Fair Oaks. CA (916) Fax: (916)

2 INTRODUCTION The acoustical consulting firm of Brown-Buntin Associates, Inc. (BBA) was retained by the Sacramento County Department of Transportation to study Antelope Road traffic noise levels before and after paving with rubberized asphalt. The purpose of this study was to~uescribe the reduction in Antelope Road traffic noise levels which resulted from the use of rubberized asphalt concrete. The project study corridor extended from Auburn Boulevard to Old Auburn Road, as shown by Figure 1. In recent years, Brown-Buntin Associates, Inc. (BBA) has prepared similar studies of the noise reduction provided roadway resurfacing projects at Alta Arden Expressway and Bond Road. Those studies are on file with the Sacramento County Transportation Department and Department of Environmental Review and Assessment. METHODOLOGY The basic methodology employed to determine the effectiveness of rubberized asphalt in reducing traffic noise levels was to take the difference between traffic noise levels measured before and after the repaving. It is important to note that there are several factors which influence traffic noise generation which need to be carefully considered in this type of analysis. Those factors, which include roadway geometry, noise level measurement equipment location and configuration, atmospheric conditions, and traffic volume, speed, and heavy truck usage, are discussed below. Roadway Geometry: The repaving of Antelope Road with rubberized asphalt was performed following a roadway widening project on this roadway. 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 repaving. However, due to the widening, the near travel lane generally moved closer the noise measurement sites. It is not specifically known to what degree the change in roadway geometry. affected the noise measurement results. It is likely, however, that the post-paving noise levels were marginally higher than had the widening not occurred. 1

3 f Elapsed Time Between Measurements: Due to the time required to complete construction on the project (which was significantly increased due to a series of storms and flooding), a period of 16 months elapsed between the "before" and "after'? noise measurements. The asphalt overlay was installed apprjximately 10 months into this period. A potential consequence of this delay (other than changes in traffic volumes), could be that some compaction of the asphalt overlay occurred between the time the asphalt was installed and the noise level measurements were conducted. Compaction of the asphalt overlay reduces the porosity of the road surface, which is believed to account for the noise reduction properties of the rubberized asphalt pavement. This compaction is more representative of the long-term condition of the road surface than the more porous condition occurring immediately after the resurfacing. Therefore, the noise level measurements are expected to be more representative of the long-term benefits of paving with rubberized asphalt than had the measurements been conducted right after the resurfacing. Equipment Location and Configuration: The noise level measurement survey consisted of continuous (72-hour) measurements at two locations along the project corridor, and short-term (I5-minute) measurements at three locations. Figure 1 shows the locations of the continuous and short-term traffic noise level measurement sites. The continuous noise level measurements were conducted to evaluate the differences in noise levels over three consecutive 24-hour periods before and after the repaving. Those measurements were conducted at a distance of approximately 75 feet from the Antelope Road centerline, at a microphone height of 5 feet above ground. A benefit of the continuous noise level measurements is that a statistically large sample of noise level data is obtained by which the effects of the rubberized asphalt can be evaluated. However, it is not practical to monitor and account for all of the factors which affect the measured noise levels over the entire 72-hour continuous sampling period. Therefore, the findings based on the continuous sampling should be considered approximate. The short-term noise level measurements were conducted at various distances from the roadway centerline, at a microphone height of 5 feet above ground. These measurements provide a statistically smaller sample of data by which to evaluate the effects of rubberized asphalt than do the results of the continuous monitoring, but traffic counts conducted during the short-term 2

4 samples allow normalization of the measurement data. 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 asphalt. 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 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 measurement periods, based on BBA field observations. Temperatures ranged from 70 to 85 degrees Fahrenheit, winds varied from calm to 5 mph, and relative humidity was moderate, with partly cloudy skies. Due to the close proximity of the continuous noise level measurement microphones to the roadway centerline (75 feet), variations in weather conditions between the before and after continuous 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 mid-week at an interval of sixteen months. Given the relatively long period between the before and after measurement periods, the traffic volumes could have varied significantly. Therefore, continuous traffic counts were conducted by Sacramento County during both the before and after measurement periods. The resllits of those counts indicate that traffic volumes did not vary significantly between the two noise level measurement periods. The traffic counts conducted by BBA during the short-term samples also indicated that heavy truck traffic accounted for a very low percentage of the total traffic on Antelope Road during the shortterm measurement periods. This finding is important in that heavy trucks generate considerably 3

5 ,...,~ more engine and exhaust noise than automobiles. 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 which should be isolated for this type of study. Average vehicle spe ds were observed to be higher after the repaving at the 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 BBA observations, by speedometer checks while driving with the flow of traffic, and by a speed survey conducted by Sacramento County Transportation Division. RESULTS The results of the before and after traffic noise level measurement surveys indicate that the rubberized asphalt perceptibly reduced Antelope Road Traffic noise. A discussion of the specific findings for the continuous and short-term samples follows. Continuous Noise Level Measurement Results: The results of the continuous noise level measurements are shown numerically in Table I, and graphically on Figures 2 and 3. The Table I data imply that traffic noise level reductions of 2.7 and 6.1 db Ldn resulted from repaving with rubberized asphalt. However, at site A it is believed that the change in roadway speeds and geometry resulted in an increase in traffic noise levels at that location, thus offsetting some of the noise reduction obtained by the resurfacing. Based on observed speed changes at measurement Site A, as well as changes in roadway geometry at that location, the estimated reduction in traffic noise associated with the rubberized pavement is estimated to be approximately 4 db at that location, rather than the measured reduction of 2.7 db. As stated previously, these results should be considered approximate because the conditions affecting the measured noise levels were not monitored continuously throughout the noise measurement period. Figures II. and III illustrate the differences between the measured noise levels before and after the repaving. Those figures illustrate that the average noise reduction was fairly uniform across the entire 24-hour period. 4

6 TABLE I Continuous Traffic Noise Measurement Results Antelope Road - Before and After Repaving./ '.. Measured ~,!db,75f~t froi1l,.,..., A~t~I[)~~R.~~~2enterline Location A Antelope B Antelope Before Repaving - June 21-23, After Repaving - October 17-19, Short-Term Noise Level Measurement Results: The comparison of before and after short-tenn noise level measurements is more complicated than simply taking the difference between the measured noise levels. The reason is that the traffic counts conducted during the short-tenn measurements allow nonnalization of the noise level measurement results. The nonnalization process was perfonned by using the Federal Highway Administration Highway Traffic Noise Prediction Model (FHWA-RD-77-l 08). The FHWA model is the analytical method currently favored for traffic noise prediction by most state and local agencies, including the California Department of Transportation (Cal trans). The model is based upon the CAL VEND noise emission factors for automobiles, medium trucks and heavy trucks, with consideration given to vehicle volume, speed, roadway configuration, distance to the receiver, and the acoustical characteristics of the site. The FHWA model was developed to predict hourly Loq values for freeflowing traffic conditions, and is considered to be accurate within 1.5 db.. Short-tenn noise measurement results were compared to the FHWA model results by entering the observed traffic volumes, speed and distance as inputs to the FHWA model. The results of this comparison are shown by Table II. By comparing the relationship of the measured to modeled traffic noise levels before and after the repaving (and adjusting for speed changes where appropriate), a reasonable estimate of the noise level reduction resulting from the use of rubberized asphalt can be obtained. 5

7 The far right column in Table II contains the normalized differences in noise levels as a result of the repaving with rubberized asphalt. That column indicates that the traffic noise level reduction ranged from 2.6 db to 4.1 db at the short-term noise measurement locations. However, the most significant realignment of Antelope Road occurred at Site 3. As a result of this realignment, traffic noise levels were expected to increase at that location due to increased proximity to the roadway ljijd increased vehicle speeds. Therefore, the actual reduction in traffic noise which is attributable to the use of rubberized asphalt at Site 3 is estimated to be closer to 4 db, rather than the computed value of2.6 db. 6

8 TABLE II Comparison of FHW A Model to Measured Noise Levels Antelope Road - Before and After Repaving Site A Condition </i... pbseryed Vehicl"S/fIQl!l"...\ Autos.... ' \<solldd Levt!!s:dl3 "..... i...c Medium Heavy Distance Measured Vs. libefore V $5 Measured L., Modeled L., Trucks Trucks (feet) Modeled... A I Before j< After Ii '. 2 Before i,, After I... i 3" Before '<_2.6//. After < J ",,,, A- The noise level measurement sites are shown on Figure I. B- The most significant realignment of Antelope Road occurred at Site 3. As a result of this realignment, traffic noise levels were expected to increase at that location. Therefore, the actual reduction in traffic noise which is attributable to the use of rubberized asphalt at Site 3 is estimated to be closer to 4 db. ",I. 7

9 CONCLUSIONS The pavement of Antelope Road between Auburn Boulevard and Old Auburn Road with rubberized asphalt appears to have resulted in a traffic noise level reduction of approximately 4 db at the shortterm noise measurement locations. The measured reduction in Ldn values over a 3 day period ranged from 3 to 6 db follo\ying repaving of the roadway. :; The noise level reduction was found to occur primarily in the 500 to 4,000 hertz frequency bands, which is the typical range of frequencies affected by the interaction of tires and pavement. In closing, it appears that the use of rubberized asphalt resulted in clearly noticeable decrease in Antelope Road traffic noise levels, even though the improvement project increased roadway capacity and vehicle speeds. Respectfully Submitted, "",,,,,,,,_,I ~. 8

10 Figure 2 Measured Antelope Road Traffic Noise Levels Before and After Paving with Rubberized Asphalt 7591 Antelope Road,I ~J ~.,_4.1, :: Sound Level, db ~~ ~ 65 E -8_.. L 60 '''',I - "I 1---, ':-=+-,. s:t: 55 T=- 1 I, /:f= 50 Midnight 4am 8am 12 noon 4pm 8pm 11 pm Hour ofoay Mean Measured Leq --. Before (June 21-23, 1994) + After (October 17-19, 1995) 11,1. BB

11 Figure 3 Measured Antelope Road Traffic Noise Levels Before and After Paving with Rubberized Asphalt 7737 Antelope Road,.\,j ~.''''''. 75 Sound Level, db Midnight 4am Bam 12 noon 4pm Bpm 11 pm Hour of Day Mean Measured Leq --e- Before (June 21-23, 1994) + After (October 17-19, 1995) 1\ 01: BBA

12 APPENDIX A ACOUSTICAL TERMINOLOGY AMBIENT LEVEL: CNEL: DECIBEL, db: NOISE The composite of noise from. all sources near and far. In this context, the ambient noise level constitutes the normal or existing level of environmental noise a given location. Community Noise Equivalent Level. The average equivalent sound level during a 24-hour day, obtained after addition of approximately five decibels to sound levels in tbe evening from 7:00 p.m. to 10:00 p.m. and ten decibels to sound levels in the night before 7:00 a.m. ami after 10:00 p.m. A unit for describing the amplitude of sound, equal to 20 times the logarithm to the base 10 of the ratio of the reference pressure, which is 20 micropascals (20 micronewtons per square meter). Day-Night Average Sound Level. The average equivalent sound level during a 24-hour day, obtained after addition of ten decibels to sound levels in the night after 10:00 p.m. and before 7:00 a.m. Equivalent Sound Level. The sound level containing the same total energy as a time varying signal over a given sample period. ~ is typically computed over 1, 8 and 24-hour sample periods. Note: CNEL and L dn represent daily levels of noise exposure averaged on an annual basis, while Lc:q represents the average noise exposure for a shorter time period, typically one hour. L...,.: L.: The maximum sound level recorded during a noise event. The sound level exceeded "n" percent of the time during a sample interval. ~o equals the level exceeded 10 percent of the time (L;o. Lso, etc.) BBA

13 I A-2 ACOUSTICAL TERMINOLOGY NOISE EXPOSURE CONTOURS: "< SEL OR SENEL: SOUND LEVEL: Lines drawn about a noise source indicating c(;jnstant levels of noise exposure. CNEL and L dn contours are frequently utilized to describe community exposure to noise. Sound Exposure Level or Single Event Noise Exposure Level. The level of noise accumulated during a single noise event, such as an aircraft overflight, with reference to a duration of one second. More specifically, it is the time-integrated A-weighted squared sound pressure level for a stated time interval or event, based on a reference pressure of 20 micropascals and a reference duration of one second. The sound pressure level in decibels as measured on a sound level meter using the A-weighting filter network. The A weighting filter de-emphasizes the very low and very high frequency components of the sound in a manner similar to the response of the human ear and gives good correlation with subjective reactions to noise. BBA