From Wazeopedia
Fixed JB rank error (was 5, now using Rank template) in "Allowing median U-turns" section
{{anchor|Avoiding U turns|Avoiding U turns in box and partial box intersections}}
{{hatnote|This article discusses double -turn U-turns, for a . For standard one or two segment U-turn turns, adjust the [[Interface_guide#Junction_Arrows|junction arrows]].}}{{MboxForumLink|type=forum|style = background: #cadceb;|text=For the latest discussion on this section please visit the wiki forum thread [ |<nowiki>[Page Update] Intersections / U-turns under 15m</nowiki>].|section=yes}}{| style="float: right; width: 360px; border: 0px;text-align: center; line-height: 1; font-size: small;"|-| [[File:U-turn_Roads.png||360px|right]]|- | style="padding-bottom: 20px; padding-left:10px;" |''Note:'' The road types displayed in the images might not match the standards for where you edit. Please check your local wiki and with local experts to learn the correct road type to use for '''median segments''' in your area.|}
The Waze routing algorithm has a method of [[Routing penalties#Road_configurations|penalizing]] some double-left and double-right turns. This can prevent U-turns at H or # style intersections on divided and parallel roads.
It is useful for preventing the sequence of two turns that would make up the U-turn, while still allowing the individual turns. However, if either of the two turns making up the U-turn is illegal or impossible, disallow that turn instead. The U-turn will then not be routed, and the following penalty does not matter.
# '''Three segments''': The U-turn is a reversal of driving direction through three segments: <br>an incoming segment&nbsp;(A), a single median segment&nbsp;(B) and an outgoing segment&nbsp;(C).
# '''Short median''': The median segment&nbsp;(B) is shorter than {{:Routing_penalties/U-turn_minimum}}.<br>''As a safety margin, leave a 1 m buffer zone by using equal to {{:Routing_penalties/U-turn_minimum|less}} or less.''
# '''In/out parallelism''': The in and out segments (A&nbsp;and&nbsp;C) are [[#Checking for parallel incoming and outgoing segments|within ±5° of parallel]] to each other.
{{Mbox|type=important|text=It is possible for this method to fail to prevent a U-turn when it is the first segment of a route , or it immediately follows a reroute. If you observe this method to fail in other situations when it is implemented properly, please report it in [ this forum thread]}}
==== {{@|Allowing median U-turns}} ====
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To allow median U-turns in both directions, break condition #2 by making the median segment&nbsp;(B) {{:Routing_penalties/U-turn_minimum|greater}} ''or longer''
To allow a double-left or doubleU-right turn through from only one direction, first setup the median segmentintersection to allow U-turns in both directions. Then, break any one of the add a [[#Preventing median Junction Box]], and restrict the prohibited U-turnsturn route. A rank {{Rank|jb}} user or above conditions]]is required to install a Junction Box. For example: * To allow median {{mbox|text=Previously, unidirectional U-turns in both directions, break turn prohibition on a divided road was set up by breaking condition #2 by making 3 of the U-turn prevention mechanism through the median segment&nbsp;(B) use of a [[Glossary#mDL|micro-dogleg]]. {{:Routing_penalties/U-turn_minimumAs of|greater2017}}, Waze staff has expressed a preference that map editors use Junction Boxes in these situations.}} ''or longer''<!-- OLD INFORMATION * To allow a U-turn from only one direction, first set up U-turn prevention for both directions. Break condition #3 on the side where U-turns are allowed. Increase or decrease the angles so that the in and out segments (A and C) are [[#Checking for parallel incoming and outgoing segments|not parallel]]. A [[Glossary#mDL|micro-dogleg]] can be used to change the angle without changing the appearance of the segment. [[File:U-turn selective Uturn prevention.png|center]]-->
The in and out segments (A&nbsp;and&nbsp;C) are considered parallel if the sum is {{font color|red|between 175º and 185º}}.
{| style="border: 0px;text-align: center; line-height: 1; font-size: small;"
| [[File:U-turn_Parallel_Check_Straight.png|680px]]
| style="padding-bottom: 20px;" | The short 14m median segment in the above examples satisfies condition #2 for U-turn prevention.
<includeonly>Please see the [[Routing penalties/Controlling U-turnsturn penalties#Additional examples|Controlling U-turnsturn penalties]] article for more advanced details, and additional examples.</includeonly><noinclude>
==== {{@|Additional examples}} ====
| [[File:U-turn_example_2mediansegs-bdp.png|243px]]
| Cutting a median segment into two may have unintended consequences
| U-turns are legal at this intersection. Even though U-turn prevention will ''not'' be triggered here, Waze will rarely route a U-turn at this intersection. When the railroad was junctioned with the median segment, it cut a single median into two. The two-segment discontinuity between divided roads of Major Blvd can trigger a lower penalty mechanism designed to discourage detours off the main road (see [[Detour_Prevention_Mechanisms/USA#Big_Detours|Big Detour Prevention]], BDP).
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| [[File:U-turn_example_2mediansegs-allowed.png|243px]]