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 -The exceptions in the AGC wiki do not address SLTL 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 -AGC wiki does not address using AGCs for left turn lanes -SLTL are not a different roadway then 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.
Slotted Left Turn Lane(SLTL)
Slotted Left turn lanes should only be mapped in very limited circumstances-
- When there is a raised curb or median 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.
-or-
- When the turn lane physically separates from the main road well in advance of the intersection
- 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
The routing server does not play nice with SLTLs 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 SLTLs are created.
Left turn lanes should be mapped as one-way, using the same road type as the through lanes, with no name, and the speed limit should be set to SL of the segment feeding into the left turn lane(unless signed otherwise). 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 road type and speed limit of the through road.
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.
Add turn restrictions to prevent U-turn(if applicable), right turn, and to the through-lane left turn(to force use of the left turn lane).