Six years of accident data show convincing evidence that Permeable Friction Courses (PFC) can significantly reduce the occurrence of wet weather accidents. This conclusion is drawn from accident data collected (between 2001 and 2007) on RM 1431 near Jonestown, Texas in the Austin District of TxDOT. The amount of wet weather accidents on this highway decreased by over 93% in the 3 years after the PFC mixture was placed in February of 2004 compared to the 3 year period prior the placement of the overlay. The PFC mix was placed under an emergency contract in order to address the high occurrence of wet weather accidents and fatalities that were occurring along a two mile section of this highway. This section of highway is often referred to as ¨Dead Manˇs Curve〃 by both the media and local residents due to its reputation for wet weather accidents.

Despite the fact that wet weather accidents, injuries, and fatalities have dramatically decreased, this project still has some safety issues primarily related to the geometrics of the project. While wet weather accidents decreased over 93% in the 3 years after the PFC overlay, dry weather accidents decreased less than 9% in the same time frame. In August of 2007, TxDOT awarded a contract ($11 million plus) to reconstruct this section of highway. The reconstruction will include right of way acquisition, realigning the roadway and a number of other safety improvements.

Background
For a number of years the Austin district has taken proactive measures in an effort to reduce the number of accidents along RM 1431. Some of these measures include:

劔 Reducing the posted speed limit
劔 Increasing police enforcement of the speed limit
劔 Installing warning/caution signs throughout the project
劔 Installing rumble strips at the beginning of the project
劔 Installing raised pavement marking along the centerline of the project

Despite all of these efforts by the Austin District, wet weather accidents continued to occur on a frequent basis and the project continued to receive negative publicity from the media. In 2003, a total of 5 fatalities were reported on this two mile section of highway and the true number may have been even higher since police records do not capture what happens away from the scene of the accident. For example: an ¨incapacitating injury〃 would be a serious injury where a person is transported to the hospital. If that person dies at the hospital or on the way to the hospital, the police report does not capture the event as a fatality. It is believed that some of the 13 ¨Incapacitating Injuries〃, reported between 2001 and 2003, later became fatalities as evidenced by the fact that there were 10 different crosses photographed along the roadside of the project in February of 2004.

Causes of Wet Weather Accidents
Research has shown that the vast majority of wet weather accidents occur on horizontal curves, vertical curves and at intersections. RM 1431 has all three of these geometric features as well as being an undivided highway. It is worth noting that all of the fatalities reported on RM 1431 between 2001 and 2003 occurred as the result of one vehicle hydroplaning in a horizontal curve, crossing the centerline and being struck by a vehicle traveling in the opposite direction. Analysis of the police reports showed that all of the accidents involving fatalities had the following in common:

劔 Pavement was wet and slick
劔 Hydroplaning coming out of a curve
劔 Vehicle slid into oncoming traffic
劔 Driver failed to control speed to avoid accident
劔 Driver failed to adjust to roadway conditions
劔 Other vehicle did not have time to take evasive action

As engineers and designers, we sometimes look at highway fatalities as being something we have little control over. Common attitudes are that ¨people just need to slow down and adapt to the conditions.〃 It is very true that almost all accidents are avoidable if drivers slow down and adapt to the conditions, but the reality is that drivers are human and are therefore prone to making mistakes like not slowing down on wet pavements. Fortunately, we design highways with many redundant safety features such as:

劔 Bridge rail
劔 Guard rail
劔 Rumble strips
劔 Shoulders
劔 Median barriers
劔 Crash cushions
劔 Safety end treatments for culverts
劔 Break away signs

One could reasonably argue that none of the safety features listed above would be needed if drivers would ¨just slow down and adapt to the conditions.〃 However, it is difficult to imagine that we would design highways without such safety features. Evidence suggests that PFC can be used as a redundant safety feature when it comes to reducing wet weather accidents.

One Solution
As a short term solution to the safety problem, the Austin District let an emergency contract to place a PFC overlay on the project. John Wagner, P.E. was the area engineer in charge of this project. The PFC overlay was placed in February of 2004. It was anticipated that the PFC would reduce the number of wet weather accidents based on the results of another PFC project placed in 2002. That particular project was an Asphalt Rubber PFC mix placed as an overlay on a concrete pavement on IH 35 in the San Antonio District. The accident data from the San Antonio IH 35 project showed that the number of wet weather accidents was reduced by over 50% in the year after the asphalt rubber PFC overlay was placed compared to the year prior to the overlay.

PFC overlay dramatically improved the safety on this section of RM 1431. There was a dramatic reduction in wet weather accidents, injuries, fatalities and total accidents after the PFC was placed. This is not to say that the PFC mix alone is responsible for all of the safety improvements but it obviously plays a significant role. It is worth noting that wet weather accidents went from comprising 58.7% of the total accidents (64 out of 109) between 2001 and 2003 to comprising only 8.9% of the total accidents (4 out of 45) between 2004 and 2006.

Unlike traditional dense graded mixtures, PFC mixes are designed to let the water pass through to the underlying layer. This allows the water to be removed from the pavement surface much quicker than dense graded mixes which in turn dramatically reduces the risk of hydroplaning. Prior to the emergence of PFC, TxDOT would have likely placed on overlay on the project comprised of a traditional dense graded mixture with high frictional characteristics (high skid resistance). Good skid resistance alone is not always sufficient to address wet weather accidents because to have ¨skid resistance〃 the tire must remain in contact with the pavement surface. However, in the presence of hydroplaning (tire floating on water), the frictional characteristics of the pavement are of little consequence.

We can conclude that the accident data collected from RM 1431 supports the claim that PFC mixes are effective at reducing the occurrence of wet weather accidents. Since 2003, TxDOT has let more than 150 PFC projects and more are on the way. Although some tout noise reduction and smoothness as reasons for using PFC, the primary reason for the rising popularity of PFC is the unique safety benefits it provides. The primary benefit of using PFC is to reduce the risk of hydroplaning which in turn reduces the risk of wet weather accidents. PFC may not be appropriate for use everywhere, but the use of PFC should be strongly considered in areas prone to wet weather accidents such as undivided highways with significant amounts of horizontal or vertical curves or intersections. Safety is a very high priority within TxDOT and the use of PFC mixes has now proven to be an effective tool to reduce wet weather accidents.