Traffic noise analysis for transportation projects: Computer Noise Modeling – Part II

July 13, 2015

Justin K. Kellogg, M.S., QEP, Environmental Engineer's Part II blog post about computer noise modeling as it relates to transportation projects.

There is certainly more to computer modeling than data entry and model run times.  Actually, I have to say, that computer modeling has definitely advanced with the advent of faster processors.  Twenty years ago, I was entering long strings of data into formatted input files using a font that looked like it was typed on a typewriter.  Like the characters behind the matrix, computer models of yesteryear had no graphics or pictures, just long strings of seemingly random letters and numbers to the casual observer (remember Microsoft DOS).   Computer noise modeling these days uses a graphical user interface (GUI) in coordination with computer animated drafting and design (CADD) files and spreadsheets.  Instead of entering coordinates into a text based input file, the user of modern computer noise modeling programs can either import CADD files or click on mapped roadway centerlines and structures. 

Currently, the main computer noise model is “TNM” (for Traffic Noise Model).  This is the noise model that is accepted by the Federal Highway Administration (FHWA).  Beyond the obvious graphical interface improvements offered by TNM, there are numerous improvements to the acoustic algorithms “under the hood” as well.   TNM yields more accurate results than older models since TNM takes into account more of the variability and site specific conditions that are found in the “real world”.

As stated on the FHWA website, the FHWA TNM contains the following components:

• Modeling of five standard vehicle types, including automobiles, medium trucks, heavy trucks, buses, and motorcycles, as well as user-defined vehicles.
• Modeling of both constant-flow and interrupted-flow traffic using a 1994/1995 field-measured data base.
• Modeling of the effects of different pavement types, as well as the effects of graded roadways.
• Sound level computations based on a one-third octave-band data base and algorithms.
• Graphically-interactive noise barrier design and optimization.
• Attenuation over/through rows of buildings and dense vegetation.
• Multiple diffraction analysis.
• Parallel barrier analysis.
• Contour analysis, including sound level contours, barrier insertion loss contours, and sound-level difference contours.

While I am sure you are now dazzled by all of the buzz words and computer lingo, you may be asking, how do you know that the computer model can accurately predict noise in the complicated “real world”?  Well, as I explained in Part I of this blog, noise models are tested under project specific conditions with site specific traffic data obtained during the field noise measurements.   Essentially, if the results of a noise model reasonably match the results obtained from the field noise measurements, the noise model is good-to-go.

Once a site specific noise model is complete and ready for use, a transportation project’s proposed design alternatives can be input into the model to allow for an approximation of what future conditions may be at a specific location and date of concern.  In New York, the procedures followed for this modeling conform to the policies presented in NYSDOT’s The Environmental Manual (TEM), Section 4.4.18, "Noise Analysis Policy and Procedures," prepared by the NYSDOT Engineering Division - Office of Environment, which, in turn, conforms to the Federal noise regulations, Procedures for Abatement of Highway Traffic Noise and Construction Noise, 23 CFR 772 and the Highway Traffic Noise Analysis and Abatement Policy and Guidance.  Generally, unless special circumstances apply, only projects that match the parameters for a 23 CFR 772 Noise Type I project undergo detailed computer noise modeling.

For Noise Type I projects, noise model inputs incorporate proposed roadway alignments and future traffic conditions expected for the project’s design year.  The projects design year is the estimated-time-of-completion (ETC)+20 for roadway projects and ETC+30 for bridge projects.  Design year volumes and speeds are used in an attempt to capture worst case results for the traffic-generated noise levels produced by the Build Alternatives and No-Build Alternative. 

For each alternative, the noise levels predicted using the design year traffic and speeds are compared to the FHWA Noise Abatement Criteria (NAC).  NAC are found in 23 CFR 772 and in the table below.  Noise Type I project receivers at which predicted noise levels approach (within 1 dBA) or exceed the NAC level are identified as requiring consideration of noise abatement measures.  In accordance with the NYSDOT TEM, the future noise levels are also compared to existing noise levels to determine the net increase in noise levels.  Increases of 6 dBA or more over existing levels also require consideration of noise abatement measures.

NOISE ABATEMENT CRITERIA (NAC)

HOURLY A-WEIGHTED SOUND LEVEL - DECIBELS (dBA)

Leq (h):   Equivalent sound pressure level.

¹    Includes undeveloped lands permitted for this Activity Category.

For the areas meeting the NAC or 6 dBA comparative criteria described above, a full range of noise abatement measures are considered for acoustic effectiveness, cost, desirability, and feasibility.  Abatement measures generally include:

1. Traffic management measures, such as traffic control devices and signing for prohibition of certain vehicle types, time-use restrictions for certain vehicle types, modified speed limits, and exclusive lane designations.
2. Alteration of horizontal and vertical alignments.
3. Acquisition of property rights (either in fee or lesser interest) for construction of noise barriers.
4. Construction of noise barriers (including landscaping for aesthetic purposes) within the highway right-of-way.
5. Acquisition of real property or interests therein (predominantly unimproved property) to serve as a buffer zone to pre-empt development that would be adversely impacted by traffic noise. This measure may be included in Type I noise projects only.
6. Noise insulation of publicly owned school buildings that are off the highway right-of-way in connection with a Department construction project undertaken with Federal-aid. For this measure to be recommended, the NYSDOT Commissioner must determine that it is in the best interest of the State considering, among other factors, the cost and feasibility of other alternatives.  Post-installation operation and maintenance costs of noise insulation are not eligible for Federal-aid.

Abatement measures are recommended for impacted sites on projects where measures are found to be effective, feasible, and reasonable.  For alternatives that are found to be feasible and reasonable, TNM noise models inputs are then modified to predict whether noise abatement measures would be effective.