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How to Conduct a Gate OD Analysis
How to Conduct a Gate OD Analysis

Conduct a gate OD analysis by supplying Replica with a file containing the links/gates of interest to your project.

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Written by Charlie Bailey
Updated this week

Example research question: how many vehicle trips use any combination of on ramps, toll gantries, and off ramps on a limited-access tollway?

Replica describes the OD table generally used to answer research questions like the one above as a "gate analysis." More broadly, a gate analysis counts the number of trips using any combination of designated entry points, middle filters (if applicable), and exit points. Replica describes these entry/middle/exit geographies as the network links underneath them, and not the zones encompassing them. Thus, if you're looking to run a gate analysis with Replica, you'll need to designate the specific network links for which you want OD counts and then use this tool to run the analysis.

You can use GIS or Replica Studio to select the links for analysis. Instructions for both methods are below.

Using GIS

Prerequisites

  1. Ability to open, read, and edit a shapefile in GIS software (e.g., ArcMap or QGIS)

Steps

  1. Download the network file for the geography and season of interest and open it as a shapefile in GIS.

  2. In GIS, select the network links corresponding to the entry links (in this example, on ramps), middle filters (if present; in this example, toll gantries), and exit links (off ramps) of interest. Note: a "middle filter" is any link not considered an entry or exit link.

  3. Invert your selection and delete the rest of the network.

  4. Each network link has a stableEdgeId (denoted as stableEdge in the attribute table of the network file). Replica will use this value to query our backend trip tables and count the origin-destination combinations.

  5. Add two string columns to the attribute table: name and type.

  6. In the name column, give each link a name that a consumer of the output of the analysis would understand. The streetName field you may see in the attribute table will likely not have sufficient detail, which is why a name column is important. In this example, you might name an on ramp "NB 63rd St. On to I-10 WB" and a toll gantry "B WB."

  7. In the type column, indicate which links are entry points, exit points, and middle filters. Use only the words "entry," "middle," and "exit." Note: if your gates serve as both entry and exit points (origins and destinations) depending on the OD pairs you're looking to count, you should create a row for each type with the same stableEdgeId.

  8. You may delete all other columns besides stableEdge, name, and type.

  9. The attribute table for your gate file should look like the one in this example. Export it as CSV and run it through the gate analysis tool.

  10. The tool will return tables of results with counts of vehicle trips by entry link, exit link, and middle filter combination. The results will look similar to the below:

Using Replica Studio

Prerequisites

  1. Knowledge of the process of network link selection and export from Replica Studio

Steps

  1. Create a Study and select the desired network links. You'll indicate each link as an entry, middle, or exit point in a future step.

  2. Download the CSV of the volumes on the filtered links.

  3. Each network link has a stableEdgeId (denoted as networkLinkId in the CSV download). Replica will use this value to query our backend trip tables and count the origin-destination combinations.

  4. Open it in Excel. Your version of Excel may automatically convert the values in the "networkLinkId" column to scientific notation; we cannot accept the file if this occurs, so you can circumvent this by following these instructions.

  5. Rename the networkLinkId column to "stableEdge" and add two columns to the CSV: name and type.

  6. In the name column, give each link a name that a consumer of the output of the analysis would understand. The streetName field you may see in the CSV will likely not have sufficient detail, which is why a name column is important. In this example, you might name an on ramp "NB 63rd St. On to I-10 WB" and a toll gantry "B WB."

  7. In the type column, indicate which links are entry points, exit points, and middle filters. Use only the words "entry," "middle," and "exit." Note: if your gates serve as both entry and exit points (origins and destinations) depending on the OD pairs you're looking to count, you should create a row for each type with the same stableEdgeId.

  8. You may delete all other columns besides stableEdge, name, and type. See a CSV example here.

  9. Save the results as a CSV and run the file through the gate analysis tool.

  10. The tool will return tables of results with counts of vehicle trips by entry link, exit link, and middle filter combination.

Gate selection best practices

  • Keep in mind that gate OD analyses are generally best suited for project areas with high traffic volumes, such as freeway interchanges or intersections of roadways of higher functional classes.

  • Select one origin (entry) link and one destination (exit) link for each OD pair you'd like to count. Avoid selecting links immediately downstream or upstream from one another, as this will add unnecessary rows to the OD output.

  • If your gates serve as both entry and exit points (origins and destinations) depending on the trips passing through them, you should create a row for each type with the same stableEdgeId.

  • Include middle filters (gates of type "Middle") only when you need to ensure you only count trips passing through specific links on their way from your origin (entry) links to your destination (exit) links. For example, if your project area includes freeways and surface streets in an urban area and you only want to count ODs for trips using the freeways, you may want to select certain freeway links as middle filters. Remember to include a middle filter to represent each desired passthrough point between your entry and exit links.

Below is a map of an example set of gates labeled by their type. Note the inclusion of middle filters in the central freeway interchange to exclude trips that use surface streets to get from any entry to exit point.

Find the CSV of the gates in the example above here.

Running the Gate OD Analysis

Once you've created your gate file using either of the methods described above, upload that file to the tool linked here and run the tool. It will return various downloadable OD tables for the gates you supplied, including hourly and mode breakdowns and daily trip totals.


What if I want to count trips between a set of gates and a set of polygonal zones?

Some use cases involve counting trips starting or ending in a set of zones and passing through a set of gates. Use this tool to build that OD table. It requires a gate file as an input just like the one described above, but it also requires you to upload your zones to Replica as a custom geography first before using it. You'll also need your organization's internal Replica Org ID to find your custom geo, so reach out to your point of contact or through our in-app chat to obtain that value before using this tool.

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