Median U-turn penalty 
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For the latest discussion on this section please visit the wiki forum thread. |
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It is possible for this method to fail to prevent a U-turn when it is the first segment of a route or reroute. If you observe this method to fail when it is implemented properly, please report it in this forum thread |
The Waze routing algorithm assesses a penalty for a double-left or -right turn, which can be used to prevent U-turns at box- (#) and H-style intersections on divided highways and parallel roads.
Preventing median U-turns
A large U-turn penalty is applied for a double left/right turn when all three of the conditions below are met:
- The U-turn is composed of an incoming segment, a single median segment, and an outgoing segment
- The length of the median segment is shorter than 15 m (49.21 feet)
- The incoming and outgoing segments are within ±5° of parallel to each other
Allowing median U-turns
To allow a double left/right turn through a median segment, break any one of the above criteria.
For example:
- To allow median U-turns in both directions, break condition #2 by making the median segment longer than 15 m (49.21 ft)
- To selectively allow a U-turn from only one direction, first setup U-turn prevention as you would for both directions. Then, for the side where U-turns are allowed, make the incoming and outgoing segments sufficiently non-parallel. (Tip: consider using a Micro-dogleg to discreetly adjust the angle without affecting appearance)
Checking for parallel incoming and outgoing segments
Condition #3 for U-turn prevention requires the incoming and outgoing segments to be within ±5º of parallel to each other. These two segments are considered parallel if their angles with respect to a straight median segment sum to 180±5º (i.e, between 175º and 185º).
Angles can be easily checked with the Junction Node Info script add-on, which can be set to report absolute angles or turn angles ("departure"). For the purpose of determining parallelism, summing either measurements will work.
However, if the median segment has geometry nodes and is not straight, it cannot be used as a reference. In this case, temporarily add a straight segment to the same junctions as the existing curved median. Check the angles using this straight median and delete when done.
