Tuesday, March 30, 2010

Better Pavements for Safety and Low Noise

On a recent trip in a driving rainstorm on Interstate 80 and I-680 (California), there were a variety of pavement types. Some seemed to collect a thin layer of water that was tossed into the air by cars and trucks, creating a blinding fog, which made for near-zero visibility. However, on some blacktop sections with equally heavy rain, there was no layer of water and no wall of spray coming off other vehicles. The visibility there was excellent. Why the difference?

The latter is open-graded asphalt, which Caltrans (the California Department of Transportation) now uses on most paving projects, and it is wonderful. It allows water to drain off the surface and into a top permeable layer — almost eliminating the spray kicked up on older pavement.

This asphalt uses pieces of gravel 3/8 of an inch thick. Underneath are 2 inches of rubberized asphalt, flexible material that withstands the wear and tear of heavy big rigs twice as long as conventional asphalt.

The uniform pieces of gravel on top do not bind as tightly as varied sizes used in other paving, so water seeps through the first few inches of pavement and drains off.

When open-graded asphalt was used on a curvy stretch of Highway 17 a few years ago, crashes dropped 41% in the first three months. On I-880, where a 26-mile repaving job ended in 2002, crashes fell 15%, from 3,172 in 2000 to 2,696 in 2004.

When you saw water being kicked up by traffic, you probably ran into an older section of I-80 that will get the better pavement this spring.

(Source: http://www.mercurynews.com/news/ci_14779911?nclick_check=1)

Unfortunately neither open-graded asphalt or rubberized asphalt are common practice on Oahu.

There is an additional advantage of open-graded asphalt that Caltrans did not mention: Noise reduction.

Environmentally, open-graded asphalt reduces road noise for drivers, passengers and those who work, live or play near a busy highway. Studies have found that open-graded asphalt pavements, when compared to traditional dense-graded asphalt pavements, reduce road noise by 3 to 7 decibels (dBA) which make them a viable alternative to noise walls for areas with moderate noise problems.

Source: Asphalt Intitute, http://www.asphaltinstitute.org/public/engineering/PDFs/Construction/Open_Graded_Asphalt_Surfaces_Offer_.pdf)