# Difference between revisions of "Routing penalties/Controlling U-turn penalties"

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===== {{@|Straight median segments}} ===== | ===== {{@|Straight median segments}} ===== | ||

If the median segment (B) is straight, sum the two angles making up the U-turn: | If the median segment (B) is straight, sum the two angles making up the U-turn: | ||

− | :# The angle between the in-segment (A) and median segment (B) | + | :# The angle between the in-segment (A) and median segment (B). |

:# The angle between the median segment (B) and out-segment (C). | :# The angle between the median segment (B) and out-segment (C). | ||

The in and out segments (A and C) are considered parallel if the sum of these angles is {{font color|red|between 175º and 185º}}, | The in and out segments (A and C) are considered parallel if the sum of these angles is {{font color|red|between 175º and 185º}}, | ||

<br> | <br> | ||

[[File:U-turn_Parallel_Check_Straight.png||680px]] | [[File:U-turn_Parallel_Check_Straight.png||680px]] | ||

− | |||

===== {{@|Curved median segments}} ===== | ===== {{@|Curved median segments}} ===== |

## Revision as of 07:54, 30 September 2015

### Median U-turn penalty

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

The Waze routing algorithm sets a penalty for some double-left or double-right turns. This can prevent U-turns at H or # style intersections on divided and parallel roads.

#### Preventing median U-turns

An extremely large U-turn penalty is set for a double-left or double-right turn when * all three* of the conditions below are 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 shorter than 15 meters (49.21 feet).**In/out parallelism**: The in and out segments (A and C) are within ±5° of parallel to each other.

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 |

#### Allowing median U-turns

To allow a double-left or double-right turn through the median segment, break any one of the above conditions. For example:

- To allow median U-turns in both directions, break condition #2 by making the median segment (B) 16 meters (52.49 feet) or
*longer* - 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 not parallel. A micro-dogleg can be used to change the angle without changing the appearance of segment.

#### 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 (B) is straight, sum the two angles making up the U-turn:

- The angle between the in-segment (A) and median segment (B).
- The angle between the median segment (B) and out-segment (C).

The in and out segments (A and C) are considered parallel if the sum of these angles is between 175º and 185º,

##### 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 median (B) and median (B) junctions with out-segment (C)). Check the angles using this straight segment and delete the segment afterwards.

*Note: Be sure the junctions are not locked above your rank, otherwise connecting a temporary segment to locked junctions will produce a save error even if it is deleted prior to saving.*

#### Additional examples

Example | Tip | Details |
---|---|---|

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 will 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 | 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. Unlike the railroad example above, BDP will not trigger here either, since the divided one ways do not have matching primary nor alternate names. | |

A median segment longer than 15 m (49.21 ft) will always allow a U-turn | 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 longer than 15 m (49.21 ft) by adjusting geometry. A median segment of any shape can trigger a U-turn 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 16 m (52.5 ft) or longer | 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. |