Left-turn lane(LTL)
Should the left turn lane be mapped?
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. Wisconsin DOT calls these slotted left-turn lanes [1] (b). The main reason to map these in Wisconsin is the DOT has built many slotted left-turn lanes.
-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 LTLs and will often create unnecessary right-turn/ U-turn or U-turn/ right-turn to route a left through these intersections. Often a junction box is required to help with routing when LTLs are created.
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 and Turn restrictions
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.
Divided road
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).
Non-divided road
Add turn restrictions to the left turn lane segment to prevent right turns.
Add turn restrictions to the adjacent through lane segment left turn(to force use of the left turn lane) and prohibit U-turns.
Speed Limit
Left-turn lanes should be mapped using the speed limit of the segment feeding into the left-turn lane(unless signed otherwise).