It was 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 was illegal or impossible, it would disallow that turn instead. The U-turn would then not be routed, and the following penalty would not matter.
Preventing median U-turns 
An extremely large U-turn penalty was set for a double-left or double-right turn when all three of the conditions below were met:
- Three segments: The U-turn is a reversal of driving direction through three segments:
an incoming segment (A), a single median segment (B) and an outgoing segment (C).
- Short median: The median segment (B) is equal to 45.93 ft (14 m) or less.
- In/out parallelism: The in and out segments (A and C) are within ±5° of parallel to each other.
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It was possible for this method to fail to prevent a U-turn when it was the first segment of a route, or it immediately followed a reroute. Previously, if you observed this method to fail in other situations when it is implemented properly, you would report it in this forum thread. |
Previously, to allow median U-turns in both directions, you would break condition #2 by making the median segment (B) 52.49 ft (16 m) or longer.
Previously, to allow a U-turn from only one direction, you would first setup the intersection to allow U-turns in both directions. Then, add a Junction Box, and restrict the prohibited U-turn route. A rank 4 user or above is required to install a Junction Box.
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Previously, unidirectional U-turn prohibition on a divided road was set up by breaking condition #3 of the U-turn prevention mechanism through the use of a micro-dogleg. As of 2017[update], Waze staff has expressed a preference that map editors use Junction Boxes in these situations. |
Checking for parallel incoming and outgoing segments
Condition #3 for U-turn prevention requires that the in and out segments (A and C) are within ±5º of parallel to each other. This is checked by using a straight median segment as the reference and the Junction Angle Info (JAI) browser extension to measure angles. JAI may be set for absolute or turn angles; using either is OK.
Straight median segments
If the median segment is straight, sum the two angles making up the U-turn:
- The angle between the incoming and median segments (between A and B)
- The angle between the median and outgoing segments (between B and C)
The in and out segments (A and C) are considered parallel if the sum is between 175º and 185º.
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| The short 14m median segment in the above examples satisfies condition #2 for U-turn prevention.
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Curved median segments
If the median segment (B) has been shaped with geometry nodes, it is not straight and cannot be used for determining parallelism. Instead, draw a straight segment connected at the same in and out junctions (where in-segment (A) junctions with the median segment (B) and where the median segment (B) junctions with the out-segment (C)). Check the angles using this new straight segment, and delete the new segment afterwards.
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Be sure the junctions are not locked above your rank. Otherwise, drawing a new temporary segment connected to locked junctions may produce a save error even if it is deleted prior to saving.
Click Expand for details on workaround options when any segments connected to either junction node cannot be edited.
- Disable the WME setting "Draw new roads with all turns allowed" before drawing the new segment.
- Draw the new segment disconnected from any other segment, then drag the end nodes to connect to the junctions.
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Additional examples
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Tip
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Details
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Cutting a median segment into two may have unintended consequences
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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 Big Detour Prevention, BDP).
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Cutting a median segment into two may have unintended consequences
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U-turns are not legal at this intersection, but Waze may route a U-turn here anyway. Cutting the median breaks condition #1, so U-turn prevention will not trigger. Furthermore, BDP might not trigger here if all of its criteria are not met.
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A median segment 52.49 ft (16 m) or longer will always allow a U-turn
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While condition #3 requires a straight segment for determining parallel in and out segments, condition #2 is the measure of the length of the median segment itself. Be careful when the space between the divided road is small. Ensure the U-turn segment is 52.49 ft (16 m) or longer by adjusting geometry. A median segment of any shape can trigger a U-turn penalty between parallel roads if it is not long enough.
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To ensure proper routing between two closely spaced parallel roads, connect the two with a segment 52.49 ft (16 m) or longer
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U-turn prevention triggers for both double-left and double-right turns over any road type. If the three conditions for median U-turn prevention are met, a large penalty will be applied, irrespective of whether the turn is onto the same road in the opposite direction or onto a completely different road. Both double-left and double-right turns can be penalized, regardless of country, and can occur for one way or two way segments.
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