Results of I-95 Sound Study

The study showed that the Federal Highway Administration noise abatement criteria are exceeded every hour of the day.

Using funds raised through member contributions, Got Noise? retained KM Chng Environmental Inc. to measure the highway traffic noise levels in Westport along Interstate 95, and to compare those measurements with the federal and State criteria that determine where noise abatement projects should be considered.

The Study

Noise levels were measured for 24 hours, starting September 23, 1999, at the following four locations:

  • Along Greens Farms Road, just north of Exit 18, 265 feet from the center of I-95
  • Along Maple Lane, north of the Greens Farms train stations, 155 feet from the center of I-95
  • Along Charlotte Place, on the northern bank of the Saugatuck River, 130 feet from the center of I-95
  • Along Hiawatha Road, just south of Exit 17, 110 feet from the center of I-95

Noise levels were measured continuously over 24 hours at the Greens Farms Road location. At the other sites, noise was measured during various times of the day and night.

At these sites, KM Chng measured:

  • The average hourly equivalent noise level, known as the Leq(h) -- this is the metric used by the Federal Highway Administration and ConnDOT to determine whether noise abatement should be considered for the location.
  • The noise level exceeded 10 percent of the time, known as L10 -- this measure is typically used to identify heavy truck traffic.
  • The noise level exceeded 90 percent of the time, known as L90 -- this measure is used to describe the ambient background level.
  • The day-night noise level, known as Ldn -- this measure is a statistically-adjusted rating that accounts for noise from 10 PM to 7 AM, when people are more sensitive to noise.

KM Chng also measured the type of traffic on I-95 along Westport.

The Criteria

Federal and state guidelines recommend that an area
be considered for noise abatement when the average highway noise level over a one-hour period exceeds 66 decibels.

The Federal Highway Administration (FHWA) has established noise limits for various land use along roadways, and the State of Connecticut has adopted the federal guidelines. For residential areas, the guidelines recommend that an area be considered for noise abatement when the average highway noise level over a one-hour period -- the Leq(h) -- exceeds 66 decibels (66 dBA).

The Department of Housing and Urban Development (HUD) has a separate recommendation, using Ldn, the measure of noise over a 24-hour period adjusted to reflect our increased sensitivity to noise at night. HUD recommends that noise abatement be considered when the Ldn exceeds 65 decibels (65 dBA). HUD's standard is not used by the FHWA or the State of Connecticut.

KM Chng's analysis also reflected acoustical standards that define the acceptable noise levels for sleep interference. According to the Noise Criterion curve for sleep interference (NC-30), noise will interfere with sleep when the noise ranges from 30 to 40 decibels (dBA). Inside a house with double-glazed windows, the exterior noise is generally reduced by 25 to 30 decibels. As a result, exterior noise will interfere with sleep in such a house when it exceeds 55 to 70 decibels.

The Results

Inside a house with double-glazed windows, exterior
noise will interfere with sleep when it exceeds 55 to 70 decibels.

The study showed that the Federal Highway Administration noise abatement criteria are exceeded every hour of the day:

  • The average daytime Leq level ranged from 71 to 73 decibels (dBA)
  • The average nighttime Leq level ranged from 70 to almost 74 decibels (dBA)
  • The highest noise level recorded during the study was 86 decibels (dBA)

The study showed that while the ambient background noise (L90) decreases during the night, when car traffic decreases, the L10 noise level remains fairly constant, reflecting the heavy truck-related traffic along I-95. KM Chng observed traffic ranging from 300 to 500 heavy trucks per hour during the daytime, representing approximately 14% of the total traffic volume on the highway, in both directions.