Why not use AGC rules here?
- AGC rules apply to right-turns that have a dedicated roadway that is separate from the roadways the AGC connects.
- AGC wiki does not address left-turn lanes
- The exceptions in the AGC wiki do not address LTL except for a brief old picture that uses ramps in the Signed, Numbered exceptions
- All other exceptions use Ramp type roads for any of the left turn options
- LTL are not a different roadway than the through road.
- The ONLY reason they are mapped for is improved turn instruction timing due to the raised curbs between the lanes, otherwise in appearance they are part of the through road.
- Use the same road type of the through road because the LTL is part of the through road not part of the road it connects to.
Left-turn lane(LTL)
Should the left turn lane be mapped?
First and foremost, the need to have the left turn lane mapped needs to be established. As the driver approaches an intersection, Waze has usually given at least one warning that a turn is imminent, and we can assume that the driver is already looking for turn lanes. For most intersections, that is enough to guide the driver into the proper lane and to turn at the proper time; in other words, it is enough that the two roads intersect without separately mapped turning lanes.
The routing server does not play nice with LTLs and will often create unnecessary right turn/ U-turn or U-turn/ right turn fiascos to route through these intersections. Often a junction box is required to help with routing when LTLs are created.
There are only a few situations in which left turn lanes are called for in an intersection:
- When the turn lane physically separates from the main road well in advance of the intersection
- When there is a raised curb between the left turn lane and the through lanes for a long distance before the intersection that the driver might overshoot while waiting for a delayed "turn" audible.
- When the area is "busy" enough that the driver may not get any advance warning that a turn is coming up--in which case a left turn lane lets you notify the driver a little earlier
- When there are user complaints (Update Requests) that the audible was too late, or that the map should have shown a left turn lane
When in doubt, leave it out. Each left turn lane complicates the map and adds to your workload when the time comes to check connectivity, direction, name, etc.
Road type
Essentially these left turn lanes are the same road as the through road, we are just adding a segment for better turn instruction timing, so they should be mapped using the same road type as the through road.
- Ex: Street to Minor Highway should be Street type.
- Ex: Minor Highway to Primary Street should be Minor Highway type.
- Ex: Primary Street to Primary Street should be Primary Street type.
Road name
In most cases, checking None for the Street name will be sufficient. Navigation instructions will simply use the name of the next segment to which the left turn lane segment connects.
If there are specific signs at the intersection which are confusing or contrary to the destination road segment name, then a name can be applied to the left turn lane.
Geometry
In general using geometry is sufficient for the desired turn instructions- keep left instruction entering the turn lane and turn left instruction at the turn. When geometry won't work use TIOs for these instructions.
Turn restrictions
Add turn restrictions to the left turn lane segment to prevent U-turn(if applicable) and right turns.
Add turn restrictions to the adjacent through lane segment left turn(to force use of the left turn lane).
Speed Limit
Left turn lanes should be mapped using the speed limit of the through road unless signed otherwise.