This guide will show you how to easily create a turning ratio study using StreetLight InSight. This study provides information about the volume of trips that enter an intersection and the share of trips that leave the intersection in each direction.
To accomplish this with StreetLight InSight, you will need to create Zones around your intersection, an O-D Analysis using those Zones, and then either use our interactive visualizations or download the resulting Metrics to get the percentages.
Step 1: Create Zones and Zone Sets
The Zones for a turning ratio study are the entrances and exits to the intersection. Learn more on how to create Zone Sets. For this guide, we will show a simple intersection for two roads from a residential area in San Francisco.
First, create a Zone Set with all the entrances to the intersection. Create these as road polygons, learn more on creating Zone Sets. Make sure to create one Zone for each entrance into the intersection. Note, these Zones do not need to have direction as that will be understood from their use as the origin Zones. But, direction does not hurt if you would like more specific Metrics.
If you decide to include direction, you will need to create a Zone Set for the exits from the intersection as well. You can create a new one or copy the Zone Set and reverse the direction of each zone.
Pro Tips: When creating your Zone Set for a turning ratio analysis, there are a few important tips to keep in mind
- Be sure to draw your Zone close to the intersection (but not in the intersection). This way, vehicles can’t escape the system between the gate and the intersection on some other road.
- Be sure to mark the Zones as pass-through.
- If you want to collect every combination of in and out traffic, do not include directionality on your Zones – but keep the “Is Pass Through” box checked. This will give you all combinations as outputs.
- If you would like to analyze traffic going a specific direction, then add direction to your Zones.
Step 2: Create Projects
After you created your Zone Sets you will then run an “O-D Analysis” project type. Learn more on creating an O-D analysis. When setting up your O-D analysis, you can choose the type of travel you want to analyze and what data source you would like to use--if you are unsure about which Data Source to feel free to learn more. In this case, our Origin and Destination Zone Sets are the same. If you created separate Zone Sets for each direction, add the entrances to the Origin Zone Set(s) window and the exits to the Destination Zone Set(s).
Step 3: Visualize
After your project has completed you can visualize your results in our Interactive Visualizations, learn more about our Interactive Visualizations
When setting up your Interactive Visualization for a turning ratio, make sure to select the Origin Zone that you want to analyze. This allows you to see the volume of trips that go through your Origin Zone and exit through your Destination Zones. You can also choose your day type and day parts to look at in the Visualizations.
For this example, we assessed the “Average Weekday” day type and “All Day” day part to get a general idea of travel patterns through the intersection. I could also change the time of day and the type of day, and get more information.
In this example, you can see that 17% of trips entering the “Arguello S” Zone will then go through the “Arguello W” Zone. This tells you that only 17% of trips coming from the South are turning left.
We can also look on the right-hand side and see that most trips are entering the “Arguello S” and exiting through the “Arguello N.” This tells us that most of the trips are going straight.
Important: If a trip goes into an intersection on Road A, goes out Road B, drives a bit, does a U-Turn, then comes back and passes through the intersection again at Road B, but in the opposite direction, it will show up in Road B twice. This is a rare occurrence, but it is worth noting if the values in your study don’t quite add up.
Pro Tip: You can configure this Project using calibration to get estimated counts. Learn more about how to do this with either your own count data or StreetLight AADT.
Step 4: Further Analysis of the Metrics
To analyze the Metrics outside of StreetLight InSight, download them as .CSV by clicking the download button under the “All Projects” tab.
After downloading, we highly recommend reading the README files to understand what each column in each file means.
From the downloads, you can create pivot tables and use further calculations to analyze the data. For turning ratios, however, you can quickly and easily get most of the information you need from the Interactive Visualizations tool. We recommend giving those a try before moving into the .CSV files.