When to use
Junction boxes are suitable only for specific locations where the above-listed functions of junction boxes are required. An example would be to prohibit the U-turn in one direction but allow it in the other such that a 45.93 ft (14 m) center segment is forcing a necking down of the normal flow of a road and micro-doglegs are used to allow the one direction's U-turn. If problems with data collection or turn restrictions are experienced then, a junction box may be used to solve the problem. If problems are easily solved with other features such as turn restrictions, then a junction box should not be used. For routes through H intersections that have one u-turn allowed and one u-turn prevented then use a simple junction box instead of or U-turn penalties and using micro doglegs to break the u-turn prevention in one direction. Existing intersections with micro doglegs or abnormal geometry can be changed to a junction box when being edited for other reasons, but you shouldn't go out hunting to eliminate all of them if they are already working properly.
Appropriate times to use a junction box:
- To allow routes across multiple junction nodes:
- Where there can be two entrances to same exit path, but a route is prohibited. A simple turn restriction cannot adequately set the correct restrictions.
- In an H-intersection, when u-turns are allowed in one direction, but prohibited in the opposite direction.
- When the cross segment is less than 45.93 ft (14 m) due to the geometry of the intersection, and u-turn prevention needs to be overwritten to allow u-turns.
- To set a turn instruction override in these situations:
- A single turn instruction at the first junction node, and no additional instructions for a route.
- To advance an
- To better collect traffic data when there is a speed differential between various traffic lanes through multiple junction nodes, such as:
- At-grade connectors (AGC) and parking lot roads cutting an intersection into multiple segments and nodes
- Auxiliary lanes: entry ramps that extend to the next exit ramp: drivers that are merging on/off the freeway will have a different set of speeds than those that were already on the freeway and continuing on it
- Regular, long backups: backups that extend past turn pockets and the exit ramp
Junction boxes create another limitation that should be considered:
- Segments connected to junction boxes have some attributes that cannot be edited without first deleting the junction box.
Edits not allowed are:
- Direction of travel (two way/one way)
- Turn restrictions that are not in the junction box for segments that extend outside the junction box's boundaries
- Moving the location of a junction node within a junction box
- Edits that change a segment's identification number
- Adding or deleting a junction node inside the junction box
- Adding or deleting a junction node on a segment that extends outside the junction box (ex. You have to delete the junction box to add a parking lot road to a segment that is included in a junction box)
For these reasons, junction boxes should not be used for every intersection.