AFFECTED ENVIRONMENT

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SOUNDSCAPES AND NOISE

This section describes the current noise environment in the project corridor.

NPS Management Policies (2001) states that the National Park Service will strive to preserve the natural quiet and natural sounds associated with the physical and biological resources of parks. Activities causing excessive or unnecessary unnatural sounds in and adjacent to parks will be monitored, and action will be taken to prevent or minimize unnatural sounds that adversely affect park resources or values or visitors’ enjoyment of them.

A qualitative noise analysis was conducted for the rehabilitation of Highway 62 West. This study was based on the type of land use found in the park, the distance to nearby receptors, traffic noise, and noise generated by construction equipment.

For this noise assessment, two types of noise descriptors were used. In discussing fluctuations in noise levels, ambient noise levels were described in terms of dBA. In discussing hourly average noise levels, the descriptor was Leq(h), or hourly equivalent noise level. The hourly equivalent noise level is a sound pressure level that, if constant over a specified time period, would contain the same sound energy as the actual sound that varies in level with time (Cowan 1994).

Traffic noise from roadway vehicles along Highway 62 West is generated by the engine, tire-roadway interaction, brakes, vehicle vibration, and air disturbance. Roadway traffic noise is influenced by vehicle speed, volume, auto-truck mix, and roadway grades. The effects of traffic noise on surrounding areas depends on the noise levels generated, background noise levels, intervening terrain, and nature of land uses.

Ambient noise levels along Highway 62 West within the project corridor are generally low (reflected in the light traffic volumes), are heavily wooded, and the undeveloped and unpopulated nature of the park comprises much of the surrounding landscape.

In order to estimate noise levels, the Federal Highway Administration’s Traffic Noise Model Look-Up Tables were utilized. These tables calculate noise levels, when given the parameters of traffic volume, type of vehicle, number of vehicles, and speed for a flat straight roadway section. Therefore, the predictions at the reference locations are representative of most locations along Highway 62 West and do not represent any one location. During the summer, the average daily traffic is 1,130 vehicles, while during the winter months, the average daily traffic is 300 vehicles. A conservative 30% of the average daily traffic was considered as the peak or design hourly volume of traffic.

Using this method on a typical summer day, with a peak hour traffic volume of 339 vehicles per hour at 45 mph, the estimated noise level at 50 feet (known as Reference 1) was 61.4 dBA Leq(h), while at 100 feet (known as Reference 2) the noise level was 54.9 dBA Leq(h). To consider vehicles driving around curves known as a switchback, a 20 mph speed was used and the estimated noise level was estimated to be 51.7 dBA Leq(h ) at Reference 1, and 46.0 dBA Leq(h) at Reference 2. Although the estimated noise levels at Reference 1 at the 45 mph speed was predicted to exceed the noise abatement criteria, this receptor does not represent a noise sensitive use and is outside the proposed wilderness area.

For a typical winter day, with a peak hour traffic volume of 90 vehicles per hour at 45 mph, the estimated noise level at Reference 1 was 55.6 dBA Leq(h), while at Reference 2 the noise level was 49.1 dBA Leq(h). At 20 mph, the estimated noise level was 45.9 dBA Leq(h) at Reference 1, and 40.2 dBA Leq(h) at Reference 2. These noise levels are well below the noise abatement criteria.