It was useful for preventing the sequence of two turns that would make up the Uturn, while still allowing the individual turns. However, if either of the two turns making up the Uturn was illegal or impossible, it would disallow that turn instead. The Uturn would then not be routed, and the following penalty would not matter.
Preventing median Uturns
An extremely large Uturn penalty was set for a doubleleft or doubleright turn when all three of the conditions below were met:
 Three segments: The Uturn 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.

It was possible for this method to fail to prevent a Uturn 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 Uturns 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 Uturn from only one direction, you would first setup the intersection to allow Uturns in both directions. Then, add a Junction Box, and restrict the prohibited Uturn route. A rank 4 user or above is required to install a Junction Box.

Previously, unidirectional Uturn prohibition on a divided road was set up by breaking condition #3 of the Uturn prevention mechanism through the use of a microdogleg. 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 Uturn 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 Uturn:
 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.

The short 14m median segment in the above examples satisfies condition #2 for Uturn prevention.

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 insegment (A) junctions with the median segment (B) and where the median segment (B) junctions with the outsegment (C)). Check the angles using this new straight segment, and delete the new segment afterwards.

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.

Additional examples
Example

Tip

Details


Cutting a median segment into two may have unintended consequences

Uturns are legal at this intersection. Even though Uturn prevention will not be triggered here, Waze will rarely route a Uturn at this intersection. When the railroad was junctioned with the median segment, it cut a single median into two. The twosegment 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).


Cutting a median segment into two may have unintended consequences

Uturns are not legal at this intersection, but Waze may route a Uturn here anyway. Cutting the median breaks condition #1, so Uturn prevention will not trigger. Furthermore, BDP might not trigger here if all of its criteria are not met.


A median segment 52.49 ft (16 m) or longer will always allow a Uturn

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 Uturn segment is 52.49 ft (16 m) or longer by adjusting geometry. A median segment of any shape can trigger a Uturn penalty between parallel roads if it is not long enough.


To ensure proper routing between two closely spaced parallel roads, connect the two with a segment 52.49 ft (16 m) or longer

Uturn prevention triggers for both doubleleft and doubleright turns over any road type. If the three conditions for median Uturn 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 doubleleft and doubleright turns can be penalized, regardless of country, and can occur for one way or two way segments.
