How does the StL Index differ from the sample trip count?

Trip Counts provided in your Project downloads represent the raw number of trips in our sample observed at your specified O-D pair. So for example, say you're working with Metrics for an O-D project where Zone A is your Origin Zone and Zone B is your Destination Zone. The O-D Traffic (Trip Counts) Metric will represent the raw number of trips in StreetLight's data sample that traveled between Zone A and Zone B across your entire data period included in your project. So if your project includes the months May through July, the Trip Count value will represent the total sample count across those three months, not a daily average. This Trip Count metric allows you to better understand sample size for a specific O-D pair, rather than the project as a whole. It can also be compared to real world counts (like AADT) to better understand rates of penetration for your specific zones or projects.

The StL Index is a normalized value derived from the trip counts described above. Our Normalization process differs slightly across our different data sources, but ultimately it accounts for variation in our sample across space and time and allows for comparison of Metrics across projects. So for example, if our sample size increased dramatically between the months of May and June, causing a spike in Trip Counts between the two months, the StL Index would account for that variation. Generally, we recommend working with the StL Index values, especially if you're looking to perform comparisons or to understand trip flows expressed as percentages.

Keep in mind that neither the Trip Counts nor the StL Index values represent the absolute number of vehicles traveling from one zone to another. If you're interested in working with estimated volumes, learn more about working with our calibration feature.

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