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Traffic engineers use trip generation analysis to determine the traffic impact new developments will have on the existing road network. A higher traffic count may mean the local government will require the developer to include traffic improvements such as new turn lanes, intersection control, or other forms of traffic mitigation in their plans to account for the increased traffic their project will bring to the road. This seems straightforward until you consider mixed-use developments where people park once and visit multiple businesses on foot.

If I park in front of my office then grab coffee next door, buy a sandwich down the street at lunch, and pick up something from the grocery two blocks away before heading home, how many trips did I take? How this question is answered defines the development’s traffic impact. Traffic engineers use internal capture rates to account for non-vehicular trips or combined trips within the mixed-use site. This can result in a lower traffic impacts to the external road network when compared to the impact if each land use within the development was considered separate and accessible only by car (as in more suburban sites).

But what about a single-use development adjacent to or built within an existing dense urban area? These “downtown” areas or dense urban districts act like mixed-use sites. However, engineers or agencies may apply single-use trip generating methods to the impact analysis. Let’s look at why applying internal capture rates in dense areas would benefit cities, developers, and users.

Traffic in a Mixed-Use Development A mixed-use development combines land-use types such as residential, office, commercial, retail, cultural, and entertainment uses into one space. They’re often referred to as “live-work-play” developments and can even include some type of public park or walking trails. The individual spaces are physically and functionally integrated to provide non-vehicular connectivity. To quantify the traffic reduction benefits of the mixed-use type development, internal capture rates can be applied. These rates typically apply to a percentage of trips (pedestrian or vehicular) that are made within the development without using roads that are external to the site. They ultimately show that mixed-use developments produce lower traffic counts than their single-use counterparts combined.

Using our example above, if each of these sites (work, coffee shop, sandwich shop, and grocery) were considered single use, I’d be generating eight trips (four outbound and four inbound). But in a mixed-use development, I’m only considered to be generating two trips since I only engage the external road twice (to and from my office).

Single-Use Developments in Downtown Areas Now consider an established downtown area. There may be restaurants, retail stores, offices, a park, and residential spaces. The sites may be single use, separate developments, but they certainly behave like a mixed-use space. People may park once and visit multiple sites on foot, bike, or public transit. If you put a large single-use residential development within an established downtown area with a variety of land use types, it’s fair to say residents will likely walk to several adjacent sites rather than drive – much like our mixed-use example.

Applying a trip generation analysis based on a suburban-based single-use development isn’t as accurate in this scenario because the site acts like a mixed-use development. Consider that a person isn’t going to leave the residence via car and drive two blocks for coffee then return, then drive across the street for lunch and return, then drive to get groceries three blocks away and return. They’ll likely drive to their residence then visit many of these additional sites on foot or bike. Using internal capture rates for the new residential site will produce a more accurate traffic count, even though it’s designated as single land-use.

Benefits of Using Internal Capture Rates for Downtown Developments So, why does it matter to get an accurate traffic count on single-use downtown developments? Who does it really impact if a site’s traffic count is unnecessarily high?

  • Developers: As previously mentioned, high traffic counts may trigger a local government requiring traffic improvements such as new turn lanes, intersection control improvement, etc. The developer is responsible for these improvements, and it increases the cost of the project. If a single-use site will realistically generate fewer trips due to its location, it saves the developer time and money to reflect this in the traffic count.

  • Local Governments: Using a higher count than warranted can result in local governments overbuilding their roadways and traffic facilities. This results in more long-term maintenance costs and less efficient networks. For City or local government planners, using internal capture for downtown areas offers a more realistic data set to use in creating smart growth strategies.

  • Users: When unnecessarily high trip generation data triggers traffic improvements in a dense area, it can take the focus off multi-modal transportation solutions in favor of solutions focused on vehicular capacity. A person living downtown and walking or biking to adjacent sites may be better served with creative pedestrian and bike planning rather than road improvements designed for vehicles.

As a side note, it’s important to also be aware of pass-by reductions to the traffic count. Pass-by trips are considered trips that are already on the road network. For example, a new coffee shop may generate 1,000 trips. One might assume 500 of those trips to be pass-by from vehicles already traveling that road. The remaining 500 are “net new” trips generated by the coffee shop. Fast food restaurants, gas stations, coffee shops, etc. may have high pass-by rates of 50%, whereas other land uses like retail shopping are lower, and residential, offices, etc. don’t have pass-by rates at all. The total count may still be applied to the driveway, but the pass-by reduction more accurately reflects what might be needed (or not) in improvements since only a portion of the counted trips are new to the road network.

While there aren’t hard and fast rules for applying mixed-use internal capture rates to single-use downtown sites, it’s worth exploring. The Journal of Transport and Land article Adjusting ITE’s Trip Generation Handbook for Urban Context reports vehicle trip rates tend to decrease as the context of the surrounding area becomes more urban. The Florida Department of Transportation has also produced a report for trip generation recommendations that addresses the impact of developments in established urbanized areas. Ultimately, having a more realistic idea of a development’s impact on the road network in a dense urban setting offers too many benefits to ignore.

About Stevie Berryman, PE

Stevie Berryman, PE is a Project Manager for Foresite Group’s Traffic Engineering Division in Peachtree Corners, GA. Stevie graduated from the Georgia Institute of Technology with a Master’s degree in Construction Management. In his spare time, Stevie is a coach for a youth travel baseball program.


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