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:
- Have two entrances to same exit path but one route is prohibited so can't use a simple Turn restriction
- Have u-turn allowed in one direction of an H-intersection, but prohibited in another
- To override u-turn prevention when the cross segment is less than 45.93 ft (14 m) due to the geometry of the intersections
- Traffic backs up through multiple junction nodes from a direct left turn, an at-grade connector (AGC), an exit ramp, or even through travel caught at a traffic light; where the back up adversely affects the timing for the traffic that is moving
- To better collect traffic data on intricate intersections
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.