Updated Reference Mean Energy Levels (REMELs) for More Realistic Modeling of Traffic Noise
Community & Cultural Concerns, Environmental Process
Research Idea Scope
The FHWA’s Traffic Noise Model (TNM) software must be used to analyze noise impacts on federally funded highway projects. TNM analysis is based upon a 25-year-old vehicle noise reference emission database (REMELs) that used a very limited measurement approach which lead to inaccurate assumptions to be made for vehicle noise source positioning and distribution. More accurate highway noise measurement technologies, OBSI and acoustic beam-forming, have now been developed and verified by the National Academy of Sciences, NCHRP Research Reports 630, 635, and 842. These newer measurement technologies show the old TNM REMELs assumptions have critical positioning and distribution errors. These legacy errors have a cascading negative bias on the TNM noise barrier analysis. Tire/pavement noise is a primary vehicle noise source, and a huge amount of new knowledge is now known about pavement acoustics and its variability and its direct impact on roadside noise levels. An updated REMELs database that applies the new NCHRP measurement research would greatly improve the reliability of quantifying traffic noise impacts and assist SDOTS in designing more cost-effective noise barriers and noise abatement options. When the REMELs were measured in the 1990’s, there were relatively few measurements at lower speeds. This has led to increased uncertainty about modelling results at low speeds. Also, all-electric vehicles are becoming a larger portion of the vehicle fleet, especially light vehicles and buses, and these electric vehicles are not accounted for in the original REMELs database. Both of these deficiencies would be eliminated with this update as an appropriate amount of data could be collected at relevant speeds and for all vehicle types, including those new to the fleet. The deliverable for this task would be a REMELs database which is ready to be incorporated into future versions of the Traffic Noise Model (TNM) or other FHWA-approved noise modeling software.
Urgency and Payoff
TNM does not accurately predict sound level reductions from low noise barriers. If a wall does not break the line of sight between the receptor and the 12 foot source height, where TNM assumes a large proportion of the noise is emanating from, TNM will not predict that the wall will be effective at reducing traffic noise, but in reality much less energy originates at that height. This leads to conclusions in project noise analyses that walls must be built taller than they might otherwise need to be. Measurements taken by CALTRANS and the Ohio Department of Transportation consistently show that low walls provide greater noise reduction than TNM predicts. There would be a great benefit to state DOTs for the REMELs to be updated with a more realistic distribution of energy, speeds, vehicle types, and implemented within TNM. First, it would allow for more realistic predictions of traffic noise. As a result of this, there would be cost savings for state DOTs from not building oversized walls which are taller than they really need to be. Additionally, accurate modelling with new REMELs would show the benefit of low walls which would give state DOT’s more inexpensive options for abating noise impacts and allow for noise abatement to be provided at locations which currently would not qualify for any noise abatement if a tall wall did not meet the cost effectiveness requirement.
Daniel Burgin and AASHTO Noise Work Group Oregon Department of Transportation 503-509-7415