Noise is generally measured using A-weighted decibels (dBA), which are adjusted to include only frequencies that humans can hear, as summarized in the following box.
Measuring Noise
Noise is measured in decibels (dB), a logarithmic scale. A 10 dB increase represents a doubling in noise level. Decibels A-weighted, (indicated “dB(A)”) units emphasize the frequency sensitivities of human hearing, and correlate well with subjective impressions of loudness. Common noise levels range from 30 to 90 dB(A). Decibels are an instantaneous measurement, so various indexes are used to measure noise over a period of time:
Decibels Examples
130 - Threshold of pain
120 - Loud car horn close by
110 - Busy airport
100 - Inside underground train
90 - Inside diesel bus
80 - Busy residential road
70 - Conversational speech
60 - Background music
50 - Quiet office
40 - Quiet bedroom
20 - Silent room
10 - Threshold of hearing
· Leq represents the equivalent continuous sound level in dB(A) for a specific time period. Leq (8 hours) is used in many traffic noise standards established by OECD and WHO.
· L10 represents the dB(A) level that is exceeded 10% of a time period (often one hour). Analogous measurements, L01 L05, L50, refer to noise levels exceeded 1%, 5% and 50% of the time period. L10(18 hours) is the mean of the hourly values taken over an 18-hour period, typically from 6 a.m. to midnight. L10 is often used to define traffic noise.
· MNL (Maximum Noise Level) is the loudest noise during a certain period. Some researches consider this index to correlate with noise annoyance better than Leq and L10, but does not address the number of noise events, and is not widely used.
Because decibels are a logarithmic scale, an increase of 10 decibels (dB) is perceived as a doubling of noise volume. The number of decibels from multiple sources cannot simply be added to get the total number of decibels. If two trucks that each produce 90 dB of noise are passing the same location at the same time, they will produce a total of 93 dB, not 180 dB. As an approximation, use the following table to add two decibel amounts; the table is accurate within 1 dB of the exact value.
Addition of Two Different Decibel Values
Source: Hendricks, Rudy. Technical Noise Supplement: A Technical Supplement to the Traffic Noise Analysis Protocol. California Department of Transportation (Caltrans), October 1998.
Transportation agencies have different standards for measuring noise. For example, the Federal Aviation Administration (FAA) uses a standard called the day-night sound level (DNL), which measures sound exposures (such as a plane flying overhead) and combines them into a single decibel measure, with nighttime exposures weighted more heavily. Many federal agencies use the DNL standard to measure noise.
In contrast, the Federal Highway Administration (FHWA) and many state highway agencies, including Caltrans, use one of the following standards:
L10, the sound level that is exceeded 10 percent of the time during a given period
Leq, the total amount of sound during a given period of time averaged over that period
Caltrans offers many publications about traffic noise that provide detailed instructions for measuring noise.
Many projects, such as a new street, will increase noise for nearby residents and businesses. Other projects may shift noise from one area to another; for example, a traffic diversion project would reduce noise on streets from which traffic was diverted and increase noise on other streets.
State and federal agencies have computerized models that can estimate how much a project will change noise levels in an area. Multi-modal transportation projects often require a different model for each mode.
The Federal Highway Administration (FHWA) model, called the Traffic Noise Model® (TNM), assesses the noise impact of different types of vehicles, different pavement types, graded roadways, and other variables. Some state transportation agencies use TNM for their own projects.
In California, the Caltrans Division of Environmental Analysis has developed its own modeling program, called Sound 2000, based on the FHWA model. Unlike TNM, Sound 2000 is available for free. Caltrans also provides computer-based training videos that show how to use Sound 2000 effectively.
A number of studies have attempted to monetize traffic noise costs, that is, to calculate the dollar value that people place on a change in their exposure to traffic noise, for economic evaluation. These mostly use hedonic pricing method, which measures how a change in traffic noise affects nearby residential property values.
Most older U.S. studies indicate that traffic noise has relatively low costs, typically averaging a fraction of a cent per vehicle-mile traveled (Delucchi and Hsu 1998; FHWA 1997). However, these studies tend to be biased in various ways that underestimate traffic noise costs, particularly under urban conditions. Most U.S. studies measured the marginal noise cost of each additional highway vehicle, and so are inappropriate for evaluating the noise costs of traffic on urban area surface streets. Their noise level thresholds they used were arbitrary, the data used are often incomplete, they assumed that home buyers have accurate knowledge of noise exposure at each location, and they do not account for non-residential noise impacts (such as on businesses and pedestrians). Verhoef (1994) concludes that conventional highway noise cost studies only account for approximately 1/8th of the total traffic costs.
More recent studies tend to indicate much higher noise costs for urban driving, typically one to three cents per vehicle-mile, and even higher for noisy vehicles (motorcycles, large trucks and buses) and for night conditions. The table below compares noise cost estimates of various studies, with all values converted to 2007 U.S. dollars.
Noise Studies Summary Table – Selected Urban Values (Litman 2009)
This table summarizes urban vehicle traffic noise cost estimates from various studies, with costs converted to 2007 U.S. dollars per vehicle-mile for comparison sake. Older U.S. studies indicate relatively
Information Resources
N. Becker and D. Lavee (2003), "The Benefits and Costs of Noise Reduction," Journal of Environmental Planning and Management 46(1), pp. 97-111.
B. T. Berglund, T. Lindvall, and D. H. Schwela (2002), Guidelines for Community Noise. World Health Organization.
BTCE & EPA (1994), “The Costing and Costs of Transport Externalities: A Review,” Victorian Transport Externalities Study, Vol. 1, Environment Protection Authority - Victoria, Australia (www.epa.vic.gov.au).
California Department of Transportation (Caltrans). "Some Frequently Asked Questions About Highway Traffic Noise Analysis & Abatement." Available at: http://www.dot.ca.gov/hq/env/noise/pub/FAQ.pdf.
Mark Delucchi and Shi-Ling Hsu (1998), “External Damage Cost of Noise Emitted from Motor Vehicles,” Journal of Transportation and Statistics (www.bts.gov/publications/jts),Vol. 1, No. 3, pp. 1-24.
FHWA (1997) 1997 Federal Highway Cost Allocation Study, USDOT (www.dot.gov), Table V-22. Available at www.fhwa.dot.gov/policy/hcas/summary/index.htm.
R. Hendriks (1998),Technical Noise Supplement: A Technical Supplement to the Traffic Noise Analysis Protocol. California Department of Transportation (Caltrans). Available at: http://www.dot.ca.gov/hq/env/noise/pub/Technical%20Noise%20Supplement.pdf.
Federal Highway Administration (1995), Highway Traffic Noise Analysis and Abatement Policy and Guidance. Federal Highway Administration Office of Environment and Planning, Noise and Air Quality Branch. Available at: www.fhwa.dot.gov/environment/polguid.pdf.
Todd Litman (2010), "Noise," Transportation Cost and Benefit Analysis, Victoria Transport Policy Institute (www.vtpi.org). Available at www.vtpi.org/tca/tca0511.pdf.
M. Maibach, et al. (2008), Handbook on Estimation of External Cost in the Transport Sector, CE Delft (www.ce.nl). Available athttp://ec.europa.eu/transport/costs/handbook/doc/2008_01_15_handbook_external_cost_en.pdf.
Matthew McCallum-Clark, Rochelle Hardy and Malcolm Hunt (2006), Transportation and Noise: Land Use Planning Options for a Quieter New Zealand, Land Transport New Zealand Research Report 299 (www.ltsa.govt.nz); at www.ltsa.govt.nz/research/reports/299.pdf.
Ståle Navrud (2003), "State-of-the-Art on Economic Valuation of Noise." Paper presented at the WCE/WHO Pan-European Program on Transport, Health, and Environment, Stockholm. Available at: http://www.fhi.se/pdf/navrud.pdf
H.A. Nijland, E.E.M.M. Van Kempen, G.P. Van Wee, and J. Jabben (2003), "Costs and Benefits of Noise Abatement Measures." Transport Policy 10, pp.131-140.
Paul Schomer (2001), "Assessment of Noise Annoyance." Schomer and Associates, April 2001. Available at: http://www.nonoise.org/library/schomer/assessmentofnoiseannoyance.pdf.
Erik Verhoef (1994), “External Effects and Social Costs of Road Transport,” Transportation Research A, Vo.28.
WHO (2010), Burden of Disease from Environmental Noise: Quantification of Healthy Life Years Lost in Europe, The World Health Organization (www.euro.who.int); at www.euro.who.int/__data/assets/pdf_file/0008/136466/e94888.pdf.
Anming Zhang, Anthony E. Boardman, David Gillen and W.G. Waters II (2005), Towards Estimating the Social and Environmental Costs of Transportation in Canada, Centre for Transportation Studies, University of British Columbia (www.sauder.ubc.ca/cts), for Transport Canada. Available at: www.sauder.ubc.ca/cts/docs/Full-TC-report-Updated-November05.pdf.