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30 Test Procedure Implementation cedure once differences are identified and understood. This and Enhancement rodeo testing should determine the reproducibility among users and the reasons for differences greater than 1 dB in over- The test procedure provided in Attachment 1 is recom- all level. mended for adoption as a national standard. To re-enforce the In addition, issues such as the effect of roadway curvature, application of the findings of this research, technical presenta- roadway grade, banking, roadside reflecting surfaces, and the tions should be made to professional organizations and the presence of other vehicles near the probe may be of concern pavement industry and the results communicated to interested but are not currently documented. For undriven tires, curva- groups. ture, grade, and banking may not be significant issues under Although there was no technical preference for the SRTT, moderate conditions, however, they are currently undefined the SRTT should be specified as the primary test tire for the in terms of actual data. Strict adherence to the data quality OBSI procedure because of its expected long-term availability; indicators is recommended to avoid data that are influenced additional light passenger vehicle tire types such as winter tires by the presence of sound-reflecting surfaces or nearby vehicles. should not be considered at this time. Periodically, the choice of the SRTT in regard to issues that could not be investigated in this research such as tire-to-tire variability and consistency Other Recommendations of noise generation over time should be reviewed. Initial inves- tigation of these issues has been recently reported in other For SPB and CPB standard measurements, a 25-ft micro- research that indicated minimal tire-to-tire variation and little phone distance from the center of the lane of vehicle travel difference between new and used tires (23). These preliminary should be considered. Although even at this reduced distance findings should be verified over a longer time period. site-specific differences were found, the 50-ft distance intro- Effects of temperature were indicated to some degree in the duced additional site-specific variation. Also, for the 25-ft dis- parameter testing. However, the results were too limited to tance, clean passby events could be more easily acquired due develop any trends or potential corrections to account for to the greater signal-to-noise ratio. No adverse effects for either air and/or pavement temperature. The effects of tem- heavy truck passby events were noted in the measurements. perature should be investigated using the OBSI test proce- Current procedures for SPB should be further evaluated to dure developed in this research, either through collective identify means to minimize site-specific effects. Procedures experience of multiple users or directed research. Similarly, for SPB and CPB measurements should be developed for tire rubber hardness due to tire aging could not be addressed consideration and adoption as a national standard. in the current research and should be further documented by The results of passby measurements obtained in this work users of the recommended procedure. In the testing per- should be evaluated relative to the REMELs database to pro- formed in this research, no adverse effects of ambient wind vide insight into how to calibrate OBSI data to those data. Such conditions were noted on the OBSI data. However, recent calibration could facilitate using OBSI data in traffic noise pre- wind tunnel testing (19) indicated that some effect may occur diction such as is done in the FHWA Traffic Noise Model. under specific speed and cross wind conditions. Therefore, the effect of ambient wind speed and direction should be moni- Suggested Research tored in future work to determine if testing should be restricted due to wind conditions. To enhance the results of this research the following topics In order to establish the expected reproducibility of OBSI are suggested for further research: measurements from one user to another, comparative "rodeo" testing should be done between users following the recom- Investigations of the effects of temperature and wind con- mended procedure. This research focused on repeatability for ditions on OBSI measurement. This research will deter- a single user/measurement combination. In application, tire- mine temperature limits and/or correction factors as well to-tire, data acquisition system-to-system, and fixture-to- as limits for wind conditions. fixture variation may create a wider variance than this research Investigations of the effect of porous (acoustically absorbing) indicates. As an example, in this research, a small, but con- pavement on sound propagation from the OBSI measure- sistent difference (-0.3 to -0.6 dB) in OBSI level was noted ment point to wayside receiver locations and incorporation between data taken with the horizontal single probe and verti- of such effects in traffic noise prediction models. cal dual probe fixtures. In comparing one user's implementa- Investigations of the effects of site-specific variables on SPB tion of the OBSI procedure to another, such subtle differences and CPB measurements to recommend limits for the more may combine with other differences due to tires and instru- important variables or otherwise site corrections. mentation to produce user biases on the order of 1 dB. Some Development of procedures for CPB and SPB for consid- other variables may require further specification in the pro- eration and adoption as a national standard.