Routing penalties/Controlling U-turn penalties Discussion View history

Median U-turn penalty

For the latest discussion on this section please visit the wiki forum thread.

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

Segment too short to allow two left turns

The Waze routing algorithm assesses whether to apply a penalty for a double-left or -right turn. This can be used to prevent U-turns at box- (#) and H-style intersections on divided highways and parallel roads.

Preventing median U-turns

U-turns via a median segment are heavily penalized when all three of the conditions below are met. This prevents U-turns at box- (#) and H-style intersections on divided highways and parallel roads.

1. The forbidden U-turn is composed of one incoming segment, one median segment, and one outgoing segment.
2. The length of the median segment is shorter than 15 m (49 feet).
3. The portion of those two segments closest to the median segment must be within ±5° of exactly parallel.

Allowing median U-turns

To allow a U-turn through a median, ensure that any one of the above criteria are broken:

• To allow the median U-turn in both directions, it is simplest to make the median segment longer than 15 meters. (Breaking condition #2)
• To allow the U-turn in only one direction, it is simplest to adjust the turn angles in only for the segments of the allowed U-turn so they are wider than the ±5° allowance of exactly parallel. This is easily achieved without affecting the overall look of the map using a Micro-dogleg.

Checking for parallel segments

A convenient way to check if the two side segments are parallel is to use another segment as a reference tool. If the median segment has no geometry nodes, it can be the reference segment. Otherwise you can draw a new segment as a reference tool with no geometry which connects the A and B nodes of the median segment. The reference segment can be deleted after the check is complete, if appropriate.

Check two angles:

1. X — the angle between the incoming one-way segment and the reference segment
2. Y — the angle between the outgoing one-way segment and the reference segment.

If the sum of these two angles is within 5° of 180° (between 175-185°), then the roads are considered parallel.

In the example below, angle X is 102°, and angle Y is 75°. The sum is 177°, which is within 3° of 180°. Therefore these roads are considered parallel.