By Kevin McGlumphy, P.E.
Wet weather crash reduction is a topic that at times can be a very slippery subject. Highway safety in our country is a critical issue and accident prevention is an important part of the goals of the Federal Highway Administration (FHWA), state departments of transportation and local governments.
Based on data from the FHWA in a report on “How Do Weather Events Impact Roads?” nearly one-quarter of the 5.8 million vehicle crashes per year in our country are weather related. The report goes on to cite 73 percent (920,000) of the weather-related crashes, occur on wet pavements and 46 percent (or 425,000) occur during actual rainfall.
Merriam-Webster defines hydroplaning as “sliding on a wet road because a thin layer of water on the road causes tires to lose contact with it.” Looking at the statistics and this definition one can easily see that the lack of friction during wet weather events has a significant impact on driver and public safety.
What can materials engineers do to contribute to the safety of the American motorist and impact the American economy? One way is to select paving materials in areas that provide characteristics that decrease potential for hydroplaning while providing long-term performance. If we can significantly reduce wet weather accidents using engineering design for safety we can also increase mobility and increase productivity.
How can that be done? Proper design, selection and construction of open graded friction course (OGFC) pavements will and have reduced hydroplaning. OGFC is an asphalt mixture comprised of a high percentage of coarse aggregates that are almost uniform in size, higher than normal amounts of performance graded asphalts that are normally polymer modified, and a small amount of finer material to act as a mastic to increase longevity. This mixture provides materials characteristics that can drastically reduce the potential for hydroplaning.
In utilizing the appropriate mixture designs and placement characteristics the designer can improve the ability of the driving surface to drain water, prevent ponding, retain friction and maintain driver visibility. All four of these are key in preventing hydroplaning incidents and create added safety benefits as follows:
1. Increased drainage potential
OGFC mixtures have a much higher percent air void content than more conventionally used dense-graded mixtures. More importantly, these air voids are interconnected so that water can drain vertically through the mixture and laterally to the shoulder or drainage collection areas of the roadway. This mixture design feature increases the mixture’s ability to remove water from the driving surface and drain water away from the roadway. The increased ability for OGFC pavements to allow water to flow to shoulders and drainage areas decreases the potential for water to pond on the surface and decreases hydroplaning events.
2. Improved storage capacity
OGFC mixes are generally designed and constructed in one- to two-inch lifts. This thickness coupled with the amount of voids available in the mixture give the roadway the ability to store excess water within the pavement layer while it drains laterally. Storage capacity of these mixtures can be much higher than other types of surface mixes. In heavy rain events water can enter the pavement much faster than it can surface drain to the shoulder. This storage capacity allows for a buffer of time for water to drain by absorbing it into the surface and keeping water from pooling at the interface of the tire and the roadway surface. The storage capacity also aids in decreasing hydroplaning quickly by getting the water away from the pavement surface.
3. Increased friction
Due to the micro texture of the aggregates used and the macro texture of the surface, these mixtures are designed to create and retain surface friction throughout the performance life of the layer. In many cases as the pavement wears the friction gets better. This is due to the quality of the aggregates selected in the design phase and the surface texture of the compacted mixture. This is very similar to the differences you may see in coarse-grit and fine-grit sandpapers. OGFC would be the equivalent of a long-performing coarse grit sandpaper, thus enhancing the frictional components in wet and dry weather at the interface of the tire and road surface. Increased friction in turn decreases the tire’s ability to slip away from the surface and prevents hydroplaning.
4. Improved visibility
OGFC mixtures drain water faster than other mixes and provide water storage during rain events. These two characteristics vastly decrease water collecting on the surface which significantly decreases splash and spray from nearby vehicles during rain events. This improved visibility allows drivers to remain confident, to see and avoid potential hazards and improves safe traffic flow. These improvements to driver awareness allow for drivers to make better decisions and allow for increased reaction times which in turn remove driver error and decreases hydroplaning.
Combining these four key benefits of open graded friction courses result in one massive improvement in highway and driver safety. South Carolina recently reported a 23 percent decrease in wet weather accidents on roads that utilized OGFC as a surface layer. This reduction is even more impressive when you compare it to a 21 percent increase in accidents documented on all other mixtures during the same timeframe and weather.
Engineering controls are a huge part of safety and by utilizing available methods in cost effective manners our industry is making a difference in the safety of the driving public.
McGlumphy is director of materials and research at Associated Asphalt, Inc. in Cumming, Georgia.