How Waze determines turn / keep / exit maneuvers: Difference between revisions Discussion View history

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A somewhat simplified description of this is covered in the [[Junction_Style_Guide#Controlling_Turn_Instructions|Junction Style Guide]]. Additional routing information is covered in the article [[Routing server]].


== General comments ==
The description below fits a right turn in a [[right-hand traffic]] country (e.g. not England). Left turns are symmetrical to right turns.
== Definitions ==
* s-in: the segment going into the junction; this is the segment the driver is on before reaching the junction
* s-out: the segment going out of the junction; this is the segment we want the driver to traverse to
* s1, s2 … sN: all the segments connected to the same junction, which includes s-out
* Best continuation is one of the segments, and is the one that Waze determines is what drivers would consider the "no turning path" through the intersection - see expanded description below
== What is the 'best continuation'? ==
In order to determine if s-out is the 'real' continuation of s-in, we do the following:
* if s-out has the same road type & street name it is selected as the real continuation.
* if not, we look at the other segments: s3, s4 … sN. One of those will be chosen as a better continuation than s-out if both following conditions are met:
:* this segment has a better match street name & road type wise than s-out (e.g. it has the same street name and s-out doesn't; it has the same road type as s-in, and s-out doesn't). Street name is more important than road type.
:* the angle between this segment and s-in is smaller than the angle between s-out and s-in
== The algorithm / list of conditions ==
The algorithm iterates over a list of conditions. As soon as a condition is met, the relevant instruction is determined, and the algorithm terminates.
The list of conditions:
1. if the junction has only 2 segments, the instruction is: 'CONTINUE'.
1.1. In some cases, T junctions could be considered as only 2 segments. See explanation on 'T junctions' below.
2. if the angle between s-out and the best continuation is larger than 45 degrees (and less than 180), the instruction is: 'TURN RIGHT' - this is because we assume that on primary roads (minor highways, major highways and freeways), turn angles which are no more than 45 degrees (no sharp turns on higher-throughput roads); therefore, you never have something called an "exit" that has such an angle, and the instruction should be TURN, not EXIT
3. if s-out is determined to be the best continuation of s-in (explanation on 'best continuation' above), the instruction is: 'CONTINUE', which implies that the driver is not turning (i.e., going straight through the junction)
4. if s-in is a primary road and s-out is not a primary road, the instruction is: 'EXIT RIGHT'
5. if s-in is a ramp/exit and s-out is neither primary nor ramp/exit, the instruction is: 'EXIT RIGHT'
6. if none of the above conditions is met, the instruction is: 'KEEP RIGHT'
== T junctions and one way roads ==
In some cases, a node could have more than 2 segments, but the routing server will consider only 2 of them as valid and therefore, the maneuver will be 'Continue straight'.
Nodes can become locked on specific scenarios - see [[Roundabouts/USA]] for more information.
For example, in this case (image below) - assuming the node is locked - heading south, the right turn will be considered 'Continue straight' as the routing server has no other option and there's only one possible segment to be s-out.
If the node was not locked, most likely that waze would tell you to turn right. Driving against the direction is a high [[Routing_penalties|penalty]], and so is a left turn where a turn is not allowed. However, it would still have been an option, which is why the routing server would have called it 'turn right'.
[[image:Right_turn_1.png|700px|left]]
This could be confusing, especially since there's no external indication on whether or not a node is locked at the moment. We will add it to the Papyrus soon.

Revision as of 17:46, 22 June 2014

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