User:Benandjerry/thai wiki/landing page/uturns

U-Turn Constructions

Preferred method to handle dedicated u-turn lanes on a divided road.

The following guidance applies only to dedicated U-Turn lanes that allow traffic to turn around to access addresses on the opposite side of a divided road.

• Concept: use a segment of the same road type as the connecting type, without city and street name, connected with a small departure angle of 170 degrees or higher, to trigger the "make a u-turn" instruction, coming from both directions. Notes: 1. U-Turn Segment length has to be 16 m. or longer. 2. Segment start to prevent recalculation of route. Example

To ensure proper construction and angles, it is essential that you have the Junction Angle Info tool installed before attempting to create a U-turn construction.

There are 2 variations possible: a small and a large version. The difference lies in the instructions received by the driver.

Small U-turn construction

This construction is used where the total length of the construction is less than 150 meters (measured from the startpoints of the U-turn lanes). This type of U-turn will give the driver a "make a U-turn" instruction. This type is typically found on Primary Roads, Rural Roads and minor Highways.

This variation BEGINS with the >170° angle:

Large U-turn construction

This construction typically measures more than 250 meters, and the U-turn lanes are more than 100 meters long. By reversing the U-turn segment compared to the above version, the driver will first receive a "Keep Right" instruction, followed by a "make a U-Turn" instruction. This type can be found on all types of roads, but most likely on Freeways and Major Highways.

This variation BEGINS with the small departure angle (<10°) followed by the >170° angle:

U-Turn / side street combinations

If there are also side streets connected to the U-Turn area, a special construction is needed to provide correct routing instructions for all situations. The starting situation will look like this:

In order for the construction to work, it must meet the following criteria:

- the divided road segments (N-S direction) must be within 5° of parallel to each other

- the direct distance between the 2 nodes (A and A) must be 14 meters or less.

First create the (2-way) segment used for left/right turns. Note the length of the segment.

Now it is time to create the U-turn segment (2-way as well).

When saving, you will get an error message saying "There might be something wrong with this edit. Intersecting segments with the same elevation". Ignore this message and save.

Once saved, there will be an error showing for the left/right turn segment: "Same endpoints drivable segments (slow)". In order to remove this, 'break' the u-turn segment. Do NOT break the left/right turn segment.

With both segments in place, it is time to set/verify the turn restrictions. Basic concept: the U-turn segment should only be used coming from the North or South, and when leaving the U-turn segment it should only be in South or North direction. Any other turn should be routed through the left/right turn segment.

(Technical background: while in theory the routing engine could use either the uturn segment or the left/right segment to make the U-Turn, it is the parameters of the construction that prohibit this: <14meter, and N/S segments parallel. For further explanation see Avoiding U-turns)