How to Create a Before and After Corridor Study

This guide will show you how to easily create a before and after corridor study using StreetLight InSight. This type of analysis allows you to understand the effects of a infrastructure or policy change along a corridor. The effects may include changes to speed, trip duration, and free flow factor. For example, a change in the road configuration, addition of HOV lane, or new tolling rates can all impact speed, duration, and free flow along a segment. This guide will help you analyze what those impacts are.

For this guide, we chose to study Kooser Road in San Jose, California. In mid-2017, the City of San Jose implemented a road diet along Kooser Road, turning the existing four-lane road into one travel lane and a bicycle lane in each direction. With StreetLight InSight, we can easily analyze the effect of the project on vehicle speeds along the corridor before and after the road diet change.

Step 1: Create Zones and Zone Sets

For a corridor before and after study, the Zones used are line segments of the relevant roads. Learn more about how to draw or upload a Line Segment. Your Zone Set needs to meet these requirements:

  • Must include one Line Segment of a road you want to analyze. 
  • Direction: Optional
  • Bi-directional: Optional

For our example, we will use a Line Segment on Kooser Road.

Pro-Tip: Make sure you turn on the Vehicle OSM layer when reviewing or drawing a Zone. The OSM layer is what our system uses to lock trips to the road. Sometimes the OSM layer does not match google maps, so viewing the OSM layer will ensure that your Zones capture all trips on the road.

Step 2: Create Projects

To conduct a before and after study of a corridor, you will need to run a pair of Segment Analysis projects. For a Segment Analysis, you will want to use Personal for the type of travel and Navigation-GPS for the data source. We recommend Navigation-GPS for your Data Source because it pings more frequently, which will provide more data for your Speed and Trip Duration comparisons.  

The important piece of setting up a before and after study is that you will create two separate projects. The two projects will have two different time periods--the before and after periods for the corridor you are analyzing. The projects will ultimately have the same project set up, so we recommend copying your first project (before), and then changing the time period in the copy version to represent after. Learn more about copying a project.

For our project, we ran our first project for March 2017 and then ran our second project for October 2017--after the road diet occurred. By running the two projects we can compare the speed during both time periods.

Step 3: Visualize

After your project has completed you can visualize your results in our Interactive Visualizations, learn more about our Interactive Visualizations

Our Example

For our study, we looked specifically at the change in speed before and after the road diet. We can see in the visualizations, that the volume of trips that traveled 20 mph and under increased after the road diet by about 6%.

Before the Kooser Rd. Road Diet

 After Kooser Rd. Road Diet

Step 4: Further Analysis of Metrics

4a. Download your Metrics. Learn more about downloading Metrics.

4b. Open the folders you downloaded and the CSVs and shapefiles you selected will be there. There will also be a README and Project files text. Open the file named “[Your Project]_zone_seg_all” for the Before study, and “[Your Project]_zone_seg_all” for the After study.

Images of the two zipped folders downloaded for this project.

4b. The easiest way to analyze the changes in speed, trip duration, and congestion along the corridor is to create a pivot table for each project. Select the entire worksheet and insert a new pivot table. If you have never worked with pivot tables before, here is an introduction to pivot tables by Microsoft.

Note: If you are trying to compare volumes of trips between two time periods in your before and after study, please reach out to the Support Team at support@streetlightdata.com.

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