What is Direction and When do you Use it?
Any Polygon Zone marked as Pass-through or a Line Segment Zone can have a “direction.” Only trips that travel through the Zone in that specified direction (+/- 20 degrees) will be included in the analysis.
The direction is shown as an arrow when viewing Zones in StreetLight InSight. The direction is represented in files (downloaded or uploaded) by degrees with values between 0° and 359°, where 0° is due North, 180° is due South, etc.
Note: To indicate no direction, the value will be null rather than 0.
Direction is useful when trying to analyze traffic going one-way on a two-way road or through an intersection. For example, If you want to analyze Eastbound traffic on a highway, you would add direction to your polygon Zone going East[a] (90°). Direction is not useful when analyzing a one-way road.
What is “Bi-directional” and when do you use it?
Any Zone indicating direction can be marked as “bi-directional.” When a Zone is marked as bi-directional, trips that travel through the Zone in the specified direction and the opposite direction (+180°) will be included in the analysis. The analysis also includes the trips that pass through +/- 20° of each direction.
When a Zone is not marked as "bi-directional", it will be referred to as a "uni-directional" Zone.
Marking a Zone as bi-directional is useful when you’d like to examine traffic moving across one road in an intersection in both directions.
Each Zone returns a single Metric. If you want to see separate Metrics for each direction on a road, create two Zones with opposite directions. If you want to see a single Metric for all traffic on a road, create a single Zone and mark it as bi-directional.
Note: For 2017 AADT Projects, all Zones must have direction and they must be marked as bi-directional.
Implied Direction on Line Segment Zones
Line Segment Zones have an implied direction on the line from the start point to the end point of the segment. This is based on how the line was drawn and may or may not match the direction of traffic for a road. This implied direction is used when StreetLight InSight generates the start, middle, and end gates for the segment as well as when it calculates direction for each of the gates.
The “direction” of the Line Segment Zone can be reversed. This will swap the start and end gates and flip the direction of each gate zone by 180°.
Adding Direction to a Zone
Any Polygon Zone marked as Pass-through will have the option to add a direction to the Zone. Each gate of a Line Segment Zone can have a unique direction, but the bi-directional setting applies to the entire segment.
Step 1: Edit the Zone to add direction.
Step 2: Click “Draw Direction” before you save your Zone.
Step 3: Click once on the map to create a “start” pin, then click again to create an “end” pin. You will see an arrow that designates the direction of travel.
Note: To indicate a different direction for the zone, click the “Clear Direction” button and then start over with Step 8.1.
Note: You can reverse the direction of the zone with the “Reverse Direction” button. This will swap the start and end points for the direction.
Step 5: You will then have the option to choose if the roadway is bi-directional. Choose “No” if you wish to analyze traffic in only the direction indicated. Choose “Yes” to analyze trips that pass through your Zone in either direction.
Using Direction on Complex Road Networks
Sometimes roads are in complex environments. There are a few things to note when adding direction or a polygon Zone to complex roadways or interchanges:
- For off and on ramps, be sure that your gate only includes the exact ramp you want to analyze, and that it does not cover any other roads. Confirm this by turning on the OSM layer. Note that drawing gates on curves is fine.
- In the case shown below, the ramp is one way, so you do not need to designate directionality. Should you choose to use directionality of a curve, make sure to designate the direction of the road as it passes through the gate.
- If you need to analyze a road segment as it crosses another road, you can use the directionality arrows to “filter out” traffic on the crossing road. In the example below, only traffic traveling South-West on Phillips Lane will be captured.
- In cases like Phillips Lane, where roadways on top of each other, we suggest choosing Bi-directional as “Yes.” A Bi-directional Zone designation in this case will allow you to analyze trips that Pass-Through Phillips Lane in either direction, but do not travel along the expressway.
Pro-Tip: One way to confirm that your Zone will cover the intended complex traffic patterns is to turn on the OpenStreetMap layer when drawing your Zones. Again, this layer is what StreetLight InSight uses to lock trips to the existing road network. On occasion, the OpenStreetMap road network might differ slightly from what is depicted on Google Maps or Satellite views. For this reason, it’s always a good idea to reference the layer by checking the box “Show OSM Layer.”