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data-event-name="pinnable-header.vector-toc.pin">move to sidebar</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-toc.unpin">hide</button> </div> <ul class="vector-toc-contents" id="mw-panel-toc-list"> <li id="toc-mw-content-text" class="vector-toc-list-item vector-toc-level-1"> <a href="#" class="vector-toc-link"> <div class="vector-toc-text">(Top)</div> </a> </li> <li id="toc-Minimum_drainage_gradient" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Minimum_drainage_gradient"> <div class="vector-toc-text"> <span class="vector-toc-numb">1</span> <span>Minimum drainage gradient</span> </div> </a> <ul id="toc-Minimum_drainage_gradient-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Typical_values_of_drainage_gradient" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Typical_values_of_drainage_gradient"> <div class="vector-toc-text"> <span class="vector-toc-numb">2</span> <span>Typical values of drainage gradient</span> </div> </a> <ul id="toc-Typical_values_of_drainage_gradient-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Effect_of_insufficient_gradient" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Effect_of_insufficient_gradient"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Effect of insufficient gradient</span> </div> </a> <ul id="toc-Effect_of_insufficient_gradient-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Minimizing_insufficient_drainage_gradient" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Minimizing_insufficient_drainage_gradient"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Minimizing insufficient drainage gradient</span> </div> </a> <ul id="toc-Minimizing_insufficient_drainage_gradient-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Maintenance" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Maintenance"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Maintenance</span> </div> </a> <ul id="toc-Maintenance-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Other_resources" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Other_resources"> <div class="vector-toc-text"> <span 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class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Term in road design</div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:200907201000_Vattenplaningsrisk.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/9/9e/200907201000_Vattenplaningsrisk.jpg/220px-200907201000_Vattenplaningsrisk.jpg" decoding="async" width="220" height="147" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/9e/200907201000_Vattenplaningsrisk.jpg/330px-200907201000_Vattenplaningsrisk.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/9e/200907201000_Vattenplaningsrisk.jpg/440px-200907201000_Vattenplaningsrisk.jpg 2x" data-file-width="1134" data-file-height="756" /></a><figcaption>Water pooling at the end of a banked curve, to the left of the median barrier. Also note water collecting in the wheel <a href="/wiki/Rut_(road)" class="mw-redirect" title="Rut (road)">ruts</a>. (Photo taken in Sweden)</figcaption></figure> <p><b>Drainage gradient</b> (DG) is a term in <a href="/wiki/Road" title="Road">road</a> design, defined as the combined slope due to road surface <a href="/wiki/Cross_slope" title="Cross slope">cross slope</a> (CS) and longitudinal <a href="/wiki/Grade_(slope)" title="Grade (slope)">slope</a> (hilliness). Although the term may not be used, the concept is also used in <a href="/wiki/Roof" title="Roof">roof</a> design and <a href="/wiki/Landscape_architecture" title="Landscape architecture">landscape architecture</a>. </p><p>If the drainage gradient is too low, rain and melt water drainage will be insufficient. This results in water pooling on the road surface, thereby increasing the risk for <a href="/wiki/Hydroplaning_(tires)" class="mw-redirect" title="Hydroplaning (tires)">hydroplaning</a> and wet-pavement vehicle crashes. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Minimum_drainage_gradient">Minimum drainage gradient</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Drainage_gradient&action=edit&section=1" title="Edit section: Minimum drainage gradient"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Most road design manuals require drainage gradient to exceed 0.5%, in order to drain water and prevent excessive skid accidents.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>One exception to the minimum 0.5% DG limit can be found in the Norwegian road design manual, where the minimum drainage gradient is 2% instead of 0.5%.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Typical_values_of_drainage_gradient">Typical values of drainage gradient</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Drainage_gradient&action=edit&section=2" title="Edit section: Typical values of drainage gradient"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Typically on straight road sections, the drainage gradient is at least 1–3% due to the normal cross slope of 1–3%. </p><p>In curved sections the drainage gradient is higher, and may often reach 5–12% due to <a href="/wiki/Cant_(road/rail)" class="mw-redirect" title="Cant (road/rail)">superelevated</a> CS that may reach 5–8% in areas with icy roads and up to 12% in areas without icy roads. </p><p>The road's longitudinal slope contributes to a high drainage gradient. However, longitudinal highway slopes steeper than 0.5% are surprisingly rare outside hilly or mountainous areas. </p> <div class="mw-heading mw-heading2"><h2 id="Effect_of_insufficient_gradient">Effect of insufficient gradient</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Drainage_gradient&action=edit&section=3" title="Edit section: Effect of insufficient gradient"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Due to the normal cross slope and the interaction with grade, road sections with insufficient drainage gradient are few and short. Still, they account for an unacceptable number of <a href="/wiki/Skid_(automobile)" title="Skid (automobile)">skid</a> accidents. These hot spots are found at the entrances and exits of banked curves, where the cross slope changes direction in order to create superelevation. As the outside edge of the curve is raised (or superelevated) to create the bank, it passes through a point where the cross slope is absolutely flat. If there is not enough longitudinal grade, water will collect at these spots. This takes place at the beginnings and ends of curves to the left in countries with right hand traffic, and curves to the right in countries with left hand traffic. </p><p>A large study in Sweden has shown that the lanes on the outside of the curve (outercurves) average five times more crashes than lanes on the inner side (innercurves).<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> This finding can partially be explained by the entrances and exits of banked outercurves having insufficient DG.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Minimizing_insufficient_drainage_gradient">Minimizing insufficient drainage gradient</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Drainage_gradient&action=edit&section=4" title="Edit section: Minimizing insufficient drainage gradient"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Roads should be designed so that sections where the cross slope changes direction (and sign) are located where the road is going uphill or downhill. Otherwise there will be a length of pavement with a drainage gradient of less than 0.5%, resulting in unacceptable skid accident risk. </p><p>When designing road curves in a flat landscape, it may be necessary to design long wave undulations on purpose. These "synthetic" longitudinal gradients can then be used to reach a sufficient drainage gradient in sections where the cross slope is close to zero. </p><p>Another option to minimize crash risk due to low DG at the entrance or exit of banked outercurves is to move the superelevation further from the curve and out to a straight road section. This results in a banked straight lane. This design can yield another risk, since the water film (when raining) on an adjacent lane may become thicker. However, this is on a straight section where the lack of road curvature minimizes the lateral forces and thus keeps the skid risk low. </p><p>Another option within the superelevation transition section is to increase the cross slope "tilt rate" within the zone where the cross slope is between −0.5 to +0.5%.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (October 2013)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading2"><h2 id="Maintenance">Maintenance</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Drainage_gradient&action=edit&section=5" title="Edit section: Maintenance"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>As roads are worn down by tire wear, wheelpath rutting often interrupts the DG, and water will collect in the wheelpaths. This is more prevalent on <a href="/wiki/Asphalt_pavement" class="mw-redirect" title="Asphalt pavement">asphalt pavement</a>, but concrete pavements are not immune. Before wet pavement crashes reach unacceptable levels, maintenance actions such as resurfacing or <a href="/wiki/Diamond_grinding_of_pavement" title="Diamond grinding of pavement">diamond grinding</a> should be taken, even if other pavement distress such as cracking is still low. </p> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Drainage_gradient&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">[AASHTO "A Policy on Geometric Design of Highways and Streets" (AASHTO Green Book)]</span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://www.vegvesen.no/binary?id=14121">Håndbok 017 Veg- og Gateutforming</a><sup class="noprint Inline-Template"><span style="white-space: nowrap;">[<i><a href="/wiki/Wikipedia:Link_rot" title="Wikipedia:Link rot"><span title=" Dead link tagged December 2016">permanent dead link</span></a></i><span style="visibility:hidden; color:transparent; padding-left:2px">‍</span>]</span></sup></span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://publikationswebbutik.vv.se/upload/1436/2002_109_analys_av_singelolyckor_med_dodlig_utgang_pa_det_statliga_vagnatet_exklusive_motorvagar_1997_2000.pdf">Analysis of fatal single crashes on Swedish State Highways except expressways 1997-2000, Swedish Road Administration publ 2002:109</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20110825082453/http://publikationswebbutik.vv.se/upload/1436/2002_109_analys_av_singelolyckor_med_dodlig_utgang_pa_det_statliga_vagnatet_exklusive_motorvagar_1997_2000.pdf">Archived</a> August 25, 2011, at the <a href="/wiki/Wayback_Machine" title="Wayback Machine">Wayback Machine</a></span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://www.roadex.org">Roadex III report on "Health issues related to poorly maintained road networks"</a></span> </li> </ol></div> <div class="mw-heading mw-heading2"><h2 id="Other_resources">Other resources</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Drainage_gradient&action=edit&section=7" title="Edit section: Other resources"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="http://books.nap.edu/openbook.php?record_id=6357&page=126">NCHRP Web Doc 16 Improved Surface Drainage of Pavements: Final Report</a></li></ul>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1&useformat=desktop" alt="" width="1" height="1" style="border: none; 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