User:PesachZ/algorithm

A somewhat simplified description of this is covered in the 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

To better be able to put the algorithm into words, we have to name each of the road segments that connect to a junction. These are subjective names, they are relative to which segment the junction is being approached from, and which segment the routing server wants to navigate to after passing through the junction.

Start with the juntion in the middle. 

1. s-in: The segment the vehicle is on as it approaches the juntion will be called s-in.
2. s-out: The segment the vehicle is being routed onto after it passes through the junction will be called s-out. This can be any of the other segments connected to the junction, which will also be one of the segments named below as s1, s2, ... etc..
3. s1, s2, s3, s4, etc.: All the segments connected to the same junction, (including s-out) will be each be named consecutively as s1, s2, etc., for as many segments as there are… sN. One of these numbered segments will also be s-out.
4. Best Continuation: One of the segments leaving the junction will be considered the 'Best Continuation'. This is the segment that Waze determines is what drivers would consider the "no turning path" or "going straight" through the intersection. The criteria Waze uses to determine which segment is the 'Best Continuation' is explained below.

http://i.imgur.com/59WbakW.jpg

Best Continuation

In order to determine if s-out is the 'real' continuation of s-in, we do the following:

1. If s-out has the same road type & street name it is selected as the real continuation.
2. If not, we look at the other segments: s1, s2 … sN (excluding s-out). 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
3. If both condtions are not met, then s-out is considered to be the 'best continuation'.

Special Considerations

• In some situations a Y-Intersection (where there are more than one segment leaving the junction node at less than a 45° angle), navigation to either segment will generate a 'KEEP RIGHT/LEFT" instruction, even if one of those would otherwise be the 'best continuation' and has a departure angle of 0° from s-in. This is also mentioned in the wayfinder section.
• A segment (s1, s2, ...sN) leaving the juction which has the trun into it from s-in restricted in WME will not be considered in this algorithm as a 'best continuation', even if it has a departure angle of 0° from s-in.http://i.imgur.com/TUEyvEZ.jpg

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 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 (mH, MH, FW), the instruction is: 'EXIT RIGHT'.

5. If s-in is a ramp/exit and s-out is neither a primary road (mH, MH, FW), nor a 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'.
Turns can be restricted or unrestricted, in specific scenarios you may not be able to see this in WME - see RevCons for more information.

For example, in this case (image below) - assuming the node is restricted properly - 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 restricted, most likely that waze would tell you to turn right. Driving against the direction is a high 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'.

https://wiki.waze.com/wiki/images/0/00/Right_turn_1.png

This could be confusing, especially since there's no external indication on whether or not a node is restricted at the moment. There are external scripts which show this information (and some allow it to be easily corrected) - see RevCons for more information.