By: Tori Clifford on November 3rd, 2016

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Don’t Neglect External Trip Estimation. Do Collect Good Data.


Transportation planning has always been an important task of local government. Improving community accessibility, safety, and health are just some of the objectives. But external trip estimation is a facet of planning that’s often overlooked, especially by small- and medium-sized communities.

However, pass-through trips (also called external-external trips or external trips) are a trip type that may make up a large portion of travel in your municipality. The map in this article gives you an indication on how heavy pass-through travel is in counties across the United States.

Data from pass-through trips – those in which drivers pass through your city or region without stopping – is important to understanding your community’s entire transportation landscape.


How External Trips Paint Your Entire Transportation Picture

External trip estimation isn’t just an undertaking for improving your community. It’s also important for battling the negative consequences that pass-through travel can have. From congestion and wear and tear on your local roads to greenhouse gas and criteria emissions, external-external trips need to be accounted for.

Being able to analyze how this traffic impacts your community affects critical decisions such as whether to add new public transportation lines and determining how to decrease pollution. It can also provide key insights into how to maximize business opportunities for your local economy.

Our view is that communities should continue to assess internal, inbound, and outbound travel -- but not forget to collect the data about the external transportation, which is just as important to your city.


Collecting Meaningful Pass-Through Trip Data With A Limited Budget

Traditionally external trip data is collected through license plate surveys. But this method of estimation has many flaws.

Traditional collection:

  • Often produces only very small sample sizes
  • Bears the risk of human error
  • Takes a very long time
  • Is too costly

Even if you created the most efficient process using license plate surveys, the data itself is generally more difficult to analyze, and it involves handling private citizens’ DMV records. It’s not ideal.

Today, the leading planning organizations are using mobile data to analyze trips instead. Researching GPS data and the like is a far more cost-effective method to forecast external trips. It provides a much larger sample size and takes much less time.

In fact, these methods were used in the first-ever analysis of the percentage of pass-through trips nationwide.


Modern External Trip Estimation In Practice

The Napa County Travel Behavior Study was an insightful undertaking using mobile data collection of nearly 900,000 daily trips.

Directed by The Napa County Transportation and Planning Agency (NCTPA), the study was conducted as a basis for multiple planning efforts by the NCTPA and other Napa County stakeholders. The ultimate goals were to learn about the travel behavior of visitors, employees, residents, and students who make work and nonwork trips in Napa County.

In another study in the city of Lafayette, CA, the community wanted to determine the ultimate causes of congestion, and forecast future traffic conditions. It previously calculated congestion using surveys and traffic counters, but the data did not explain the cause of the traffic.

Using GPS data, the municipality learned that pass-through trips accounted for roughly 30% of trips in the downtown area. And of those trips, nearly 40% used a specific road in the community.


These fruitful studies have become more and more commonplace. And because of the valuable insights, their studies have provided, the organizations that have directed them are learning how to make a bigger impact in their communities.


How many travelers just pass through your community? Click here to read our county-by-county analysis.



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