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83 5. HYDROPLANING MITIGATION SOLUTIONS This section presents a critical review of existing solutions for sites with high HP identified by a low PM, including estimated cost, potential drawbacks (if any), and estimated benefits. Since several mitigation strategies are often available to improve the safety performance of road sections identified to have a potentially high potential for hydroplaning, identifying specific countermeasures would require a project-level assessment. However, this section intends to present available options that could assist in the decision process and investment allocation. A fundamental part of the assessment is optimizing the geometric design and pavement surface characteristics to reduce the accumulation and thickness of water in the road lanes while considering comfort, safety, and drainage. It is important to note that while geometric modification provides a relatively permanent impact on pavement runoff characteristics, road surface modifications often provide only a temporary impact that begins to diminish immediately after construction and that may need periodic retreatment for effective performance. Other non-engineering measures can also help and are mentioned in this chapter for completeness. 5.1. HIGHWAY ENGINEERING When geometrical problems of an existing road have been identified as causing water ponding, certain measures need to be taken. Examples of these causes include insufficient cross slope in a transition from a tangent to a horizontal curve (or vice versa), poor drainage in very wide pavement sections (three or more lanes), and rutted asphalt pavement surfaces. NCHRP 1-29 conducted a survey of highway agencies and identified a portfolio of measures for improving surface drainage that were classified into three broad groups: 1. Optimization of geometric design parameters such as cross slope; 2. Road Surface Improvements, such as the use of milling and grooving; and 3. Improving the drainage through the use of internally draining wearing courses and/or reduction of the distance that the water must flow (flow path) by installing drainage appurtenances. The following sections investigate these strategies. 5.1.1. Optimization of the Road Geometric Design Geometric design improvement are clearly important, especially at the highway design phase, and the AASHTO Policy on Geometric Design of Highways and Streets manual mentions hydroplaning as an important consideration (AASHTO 2018). However, geometric design improvements are usually expensive once the road has been constructed and they are only considered as a last recourse as a mitigation strategy. Therefore, it is important that designs are checked for areas of potential high hydroplaning risk at the design phase. These include areas of high water accumulation but also areas where deficient features may negatively impact vehicle performance (e.g., deficient superelevation or incorrect cross slope). The elements that should be considered include the following: ⢠Sags and vertical alignment. Vertical curves are the link between segments with constant grades. Adequately designed crests or sags provide safe transitioning curves for drivers by
84 considering the minimum required sight distance needed for passing other drivers and stopping when there is an object ahead. To minimize sight distance problems, the K-value controls the rate of vertical curvatures, which is the length of the curve (L) per percent algebraic difference in intersecting grades (A), expressed by the equation K = L/A. The suggested K-values for sag and crest are less than or equal to 167 ft., with a minimum 0.3% grade (AASHTO 2018). ⢠Cross slope and superelevation. Cross slope is important to facilitate the drainage of the water to the sides of the road. Superelevation is determined by climate, terrain, type of area, and slow-moving vehicles. Special attention should be taken in the transition areas (runoff) that are located ahead of the superelevation. The reason for this is that there is a greater chance of having water accumulation due to the transition from a normal crown to an inverted crown that results in flat areas (see section 3.2.3). In general, drainage can be improved by increasing grade and cross slopes. This provides faster and more efficient removal of water from the pavement. However, in some designs greater longitudinal slope can increase the drainage path and increase water accumulation impacting the HP. Adjusting the road configuration can also shorten the traveling distance of the water (Anderson, et al., 1998). 5.1.2. Road Surface Improvements Designing new road surface is easier than modifying the geometry of existing roads. Improving the road surface is the alternative for some existing road. Examples of intervention treatments for correcting geometric design or pavement deterioration problems (e.g., rutting), include the following: ⢠Milling (cold planing, asphalt milling, or profiling) of asphalt pavements is widely used to remove existing pavement to a desired depth (e.g., to fix specific problems such as rutting, where water can accumulate and cause hydroplaning), to restore the pavement surface to a specified grade and/or cross slope, and to help improve the rideability of an existing surface. It can be applied at the small scale (hotspots) or large-scale (road corridors). This procedure is typically applied for leveling the road geometry or repairing damaged pavement, but it also restores drainage flow, is environmentally friendly, and is relatively low cost compared to other procedures (Epps, 1990). ⢠Diamond grinding of concrete pavement produces a similar effect to milling in asphalt pavements. Diamond grinding is a technique to improve pavement irregularities, such as faulting, curling, roughness, and warping of the slabs, by using closely spaced diamond saw blades that gently abrade away the top surface of the concrete. It is used mainly to provide a smoother ride but can also result in noise reduction, better friction, and lower HP. ⢠Cutting of grooves across the entire width of the lane to help channel the water from the traveling lanes of the pavement can also help reduce HP. In the U.S., grooving is applied mostly to Portland cement concrete surfaces to create channels that drain the water out of the surface. In other countries, grooves are also used on asphalt surfaces; however, the groovesâ effectiveness decreases rapidly with traffic. To enhance its effectivity, grooves should be applied parallel to water flow. For the best result, grooves should be parallel to
85 the slope of pavement. The grooves are typically 0.25 in. deep, 0.125 inches wide, and 0.75 inches separate (Hoerner et al., 2003). Laboratory studies indicate that grooves are more effective at channelizing rain falling in the upstream flow path than the downstream path since water is carried in the grooves (Anderson, et al., 1998). Furthermore, some limited empirical evidence has suggested that any type of texture would increase the contact between the tire and the pavement on wet pavements and thus, enhance the handling capabilities and available grip (e.g., Flintsch et al 2010). ⢠Overlay and cross section overbuild for asphalt pavement, preferably after a milling operation, to even the surface and allow good bonding of the new layer, and correct any cross slope problems that might be present. The paving is then completed with an even depth on the entire depth of the overlay. Overlays are used mainly as a rehabilitation or preservation treatment, but they can also help provide adequate drainage. The overlay design can improve slope and crown to allow runoff water to drain to the shoulders, into a ditch, and out from the surface. Cross slope overbuild is usually applied after milling to correct the slope and even the surface. 5.1.3. Drainage Improvements The portfolio of mitigation strategies also includes measures focusing on reducing water accumulation through the use of permeable pavements and the installation of drainage appurtenances. ⢠Permeable wearing courses are becoming more popular, especially in states that do not have severe winters. Although both permeable and concrete pavement surface have been used in parking lots and streets, only porous or permeable asphalt layers are commonly used in roadways. The primary benefit of these permeable surfaces is the potential to reduce WFT because the water can flow though the layer (Noyce, et al., 2005), which reduces the HP. In the U.S., porous hot mix asphalt contains 10% to 13% air voids and a maximum depth of 19 mm. The European mixes usually consists of 20% air voids and a thicker depth of 25 mm. In general, it provides greater texture and more internal drainage (Chaithoo & Allopi, 2012; Balmer & Gallaway, 1983). Locations with porous pavement were found to reduce wet crashes by 20% in fatal and injury crashes in Europe (Hein & Croteau, 2004). ⢠Inlets. An alternative to solving the problem of water ponding and sheet flooding on multilane facilities is installing inlets between lanes. While fewer lanes allow water to travel shorter distances and exit the pavement surface faster, multilane facilities account for higher water paths and higher WFT. Increasing the slopes will drain the water faster; however, when suggested slopes cannot provide adequate drainage for safe roads, installing inlets can be an alternative to improve road drainage when constructing new roads. However, construction of any form of inlet within the traveled way is almost invariably problematic because all inlets require periodic maintenance and lane closures.
86 5.2. ENFORCEMENT AND TRAFFIC CONTROL Although outside the scope of the study, if potential hydroplaning problems requiring some type of construction solution are identified, temporary signs alerting drivers to reduce speed or that the pavement is slippery when wet can be put in place as soon as possible. This will also help to provide a safer work zone for the construction crews when the repair operations begin. The use of variable speed limits based on changing weather conditions may also be appropriate. 5.3. ADVICE AND EDUCATION Another critical element in the development of hydroplaning mitigation is driversâ understanding of the phenomenon and their reaction to the conditions in which hydroplaning may occur. The development and use of the Hydroplaning Potential Assessment Tool developed can give a better fundamental understanding of hydroplaning, allowing this phenomenon to be better described to the driving public. Simulation results can be used to demonstrate to drivers the importance of maintaining the tread depth on their tires while also educating them about the effects of steering and braking commands in wet conditions. With a better understanding of hydroplaning and a sound fundamental model, the differences and similarities between driving on flooded pavements and ice/snow can be conveyed to the driving public. Specifically, drivers can be instructed about how to adapt their driving strategy on wet pavement based on how they should adapt their driving to ice/snow. On a larger scale, this information should be shared with tire and vehicle manufacturers to aid them in product design. Since tire tread has a major impact on HP vehicle manufacturers may be able to refine Electronic Stability Control protocols based on the results of this study.