Quiet Moves

A war is being waged on noise generated by pavements and highways.

By Tom Kuennen, Contributing Editor

Today, highway noise is considered an undesirable emission, just as if it were a noxious gas out of the tailpipe. But road designs can do much to attenuate highway noise.

These include active methods to quell noise created at the pavement-tire interface, such as variable transverse groove patterns on concrete and the new Next Generation Concrete Surface (NGCS), and thin and thick open-graded or porous asphalt friction courses.

But they also include passive methods, such as sound walls, vegetation screens, earth berms, recessed pavements, or combinations of the foregoing.

And even as the techniques of active and passive highway noise suppression are refined, methods used to measure noise are getting more sophisticated amid a political climate of less tolerance for highway noise in our cities and neighborhoods.

Keeping it Down

New solutions are coming in to play to control noise from America’s Interstate highways, primary highways and arterial streets. Road agencies are spending more on noise mitigation. On new or capacity improvement projects, sound walls that were once considered an extravagance are now standard procedure.

Engineers are finding that the best solution to highway noise is a combination of sound wall, appropriate vegetation and a quieter pavement surface. Any combination of the three elements will help, because noise barriers can cost an average of $3.9 million per mile, according to current estimates by the Washington State DOT, with lower costs for rural barriers, and higher for urban.

Producing a low-noise diamond ground surface – the NGSC – requires creating uniform and consistent negative-land profiles

Highway noise barriers can be of many configurations, including recycled plastic, wood, evergreens, gabion walls and precast concrete panels. Trees — such as stands of thick evergreens — have the potential to replace noise barriers, and are aesthetically pleasing, but are effective only in deep stands, requiring additional strips of right-of-way, as much as 100 feet wide.

Efforts to limit highway noise have focused on barriers. But because most of the noise originates at the tire-pavement interface, use of “quiet pavements” to quell noise there makes sense.

For portland cement concrete pavements, texture is added to improve friction and driver control, but done incorrectly it can add to pavement noise. Tine or groove depth, width, spacing and orientation are all major factors affecting tire-pavement noise. Transverse tinings with uniformly spaced tines a half inch or greater have been found to produce an objectionable tone, with pressure spikes at specific frequencies, that users interpret as a tire “whine.” Randomly varying the transverse tine spacing, or skewing it, can reduce the tonal-quality problems.