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

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# '''Three segments''': The U-turn is a reversal of driving direction through three segments: <br>an incoming segment (A), a single median segment (B) and an outgoing segment (C). | # '''Three segments''': The U-turn is a reversal of driving direction through three segments: <br>an incoming segment (A), a single median segment (B) and an outgoing segment (C). | ||

− | # '''Short median''': The median segment (B) is shorter than {{:Routing_penalties/U-turn_minimum}}. | + | # '''Short median''': The median segment (B) is shorter than {{:Routing_penalties/U-turn_minimum}}. ''For safety leave a 1 meter buffer zone, and only use {{:Routing_penalties/U-turn_minimum|less}} or less.'' |

# '''In/out parallelism''': The in and out segments (A and C) are [[#Checking for parallel incoming and outgoing segments|within ±5° of parallel]] to each other. | # '''In/out parallelism''': The in and out segments (A and C) are [[#Checking for parallel incoming and outgoing segments|within ±5° of parallel]] to each other. | ||

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## Revision as of 09:01, 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 52.49 ft (16 m).*For safety leave a 1 meter buffer zone, and only use 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 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) 52.49 ft (16 m) 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 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.

The 14m median segment in the examples above satisfies condition #2 for U-turn 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 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.

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. |

#### 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 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 | 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 may not trigger here if all of its criteria are not met. | |

A median segment longer than 52.49 ft (16 m) 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 52.49 ft (16 m) 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 52.49 ft (16 m) 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. |