A Primer on Trip Generation

As a key factor in determining rates for Portland’s proposed street utility fee, the Institute of Transportation Engineers’ (ITE, henceforth) Trip Generation Manual has gotten a lot of love lately among local transportation wonks. It is worthwhile, then, to take a quick trip through the weeds of the manual to better understand where the opportunities and complications lie when it comes to using this data as the backbone of our fee structure.

First, a bit of context. The art/science/guesswork of predicting the trip generation of various land uses is the first of four steps undertaken in transportation forecasting. Along with the ensuing three steps—determining where, generally, the trips will originate and end; choosing which mode they will utilize; and identifying the time, route choice, and other properties of the individual trips—the goal is to understand the future needs of the transportation system based on current land use and development patterns. Because most jurisdictions have a concurrency requirement—a stipulation that roads and intersections must have adequate capacity to accommodate new demand concurrent with a proposed development—trip generation is of particular interest to folks in my line of work so that we might identify what developers must do to meet this requirement, blissfully ignoring any notion of induced demand.

To this end, ITE has been aggregating and disseminating trip generation data since the first edition of the manual was published in 1976 (the current edition is the ninth). The context manifests in the data in myriad ways. For any of the 172 land uses listed in the manual, the robustness of the dataset is likely a function of both how often that particular land use arises, and how much NIMBY-ism it’s likely to inspire. The well-known suburban bent of the data owes to the fact that most development over the last 40 years has occurred in the suburbs or exurbs, so this is where the vast majority of studies have been conducted.

The trips described by the manual are one-way trips, so what one might colloquially describe as “a trip to the grocery store” is actually two trips: the trip from home to the store, and the return trip home. This is important for analysis purposes—the “trip to the grocery store” will indeed traverse an intersection along the way twice—but results in quantities that are twice as high as what one might intuit. This means that in a closed system each trip is double counted, as both the home and the grocery store would be credited with generating both an inbound and an outbound trip in this example.

The manual provides two general mechanisms for determining the trip generation of a given land use. The first is a mathematical function derived through what’s called regression analysis, which attempts to fit the cleanest possible curve to a set of disparate data points. The second is the trip rate, often expressed as a number of trips per thousand square feet. But it’s important to recognize that square footage is often not the best (or even a viable) independent variable for predicting trip generation. For example, student enrollment is a much better predictor of trip generation for schools, employee count is a better predictor for offices, and the number of ‘fueling positions’ is a better predictor for gas stations.

Interestingly, there’s another predictor for trips generated by a gas station that works better than floor area: the amount of traffic using the street that it’s located upon. That’s because gas stations generate a large number of pass-by trips, which are trips that are ultimately headed to another destination (this destination is credited with generating the primary trip) but stop at a business located directly along the way. A similar type of trip—a diverted trip—is also a trip ultimately headed elsewhere, but in this case the pit stop entails a small amount of out-of-direction travel. There are also internal capture trips, which describe trips that take place entirely on roads and facilities located within a mixed use development. Note the suburban bias there; in the city, this would likely take the form of a person parking once and visiting several locations on foot, using the public streets.

And here we have arrived at the biggest failing of the Trip Generation Manual with regard to our purposes: The manual implicitly considers only vehicular trips. Assuming that only a nominal number of trips are non-automotive might work for the ‘burbs, but this often causes the stated trip rates to be wildly inaccurate in the city. Recent work by Professor Kelly Clifton’s research group at Portland State University confirms what we might have suspected: The more “urbany” a built environment, the more inaccurate the assumption that all trips are automotive is likely to be.

Thus, there is a need to distinguish between the vehicular trips quantified by the ITE manual and what are generally called person trips, which include all trips regardless of mode. Fortunately, as Clifton verifies, the latter seems to be relatively consistent regardless of the built environment. So perhaps by considering only vehicular trips, but doing so primarily in auto-centric locations, the manual has inadvertently provided a good proxy for estimating the differences in person trips generated from one land use to the next. There are exceptions, of course—the manual will understate the person trips generated by a school compared to other land uses, for example, due to the prevalence of buses in travel to and from schools. But it seems that basing a potential street fee on person trip rates inferred from Trip Generation Manual data is defensible and keeps with the spirit of the residential fee in being mode-independent. Basing the fee on vehicular trips, by contrast, would be far more complicated to implement and would leave unsolved many of the issues with the gas tax that Chris Smith wrote about last week.

The manual offers a lot of utility with regard to predicting trip generation, but really it’s just one piece of a puzzle that fits together differently from one land use to the next, and one business to the next. To accurately model the trip generation of a particular business requires a heck of a lot more than the published trip rate, which does not consider countless predictors and is often derived from a small sample size. While using these rates as the basis of the street fee would hardly be the first or most egregious misuse of the data, it seems inevitable that it will result in some businesses substantially overpaying and others substantially underpaying. As luck would have it, that seems to fit with the spirit of this fee quite well.

11 responses to “A Primer on Trip Generation”

  1. This all seems very complicated, for the something that really doesn’t need to be. A vehicle-mile tax with a curb weight multiplier seems like the only fair way to properly charge people for infrastructure use and damage.

    • I think your proposal gets us a lot of the way there.

      To your proposal, I would like to add charging contractors for moving heavy equipment around Portland. Compare the damage of a flat bed trailer hauling a bulldozer vs. the damage of a Honda Civic. We need to capture some $ from area contractors- especially those that just bring their carbon-belching heavy equipment in and out of PDX while enjoying the tax benefits of a Washington County or Clackamas location.

      The trip generator seems like a good tool- for a different job. It wasn’t designed for tax policy, IMHO. Don’t saw wood with a screw driver. Use a saw.

  2. +1 Chris I. And in the meantime while the details are worked out and as electric vehicles continue to increase but are still only a rounding error in terms of actual contribution to road use, we have this wonderful existing user fee called the gas tax.

    Raise it and index it to inflation, ban studded tires or impose much larger fees, and increase registration fees for heavier cars. Trip generating modeling is ridiculously complicated and more importantly a back breaker for small businesses who already operate on thin profit margins.

    • I hear you folks, and I’m not in love with the fact that the fee considers all ‘trips’ equally without regard to the mode, damage caused, etc. In doing so, it does nothing whatsoever to discourage driving.

      But what I hope we’re beginning to fund here is a dramatic shift away from driving. You simply cannot do that on revenues that depend on driving.

      I think we need an all-of-the-above approach where we (significantly) raise the gas tax and parking fees to better cover the true cost of driving and further incentivize large scale modal shifts. But we also need that component of revenue that can be unapologetically spent on the 8-80 bike infrastructure that I hope we build while I’m still young enough to enjoy it, as well as other transit and active transportation infrastructure. As I wrote here a few weeks back stealing from Churchill’s wit, I think a street utility fee is the worst way of doing that except for all of the other ways that have been proposed.

      • “But what I hope we’re beginning to fund here is a dramatic shift away from driving. You simply cannot do that on revenues that depend on driving.”

        This is where I fundamentally disagree. While it may be true for a few select neighborhoods in NW Portland and the inner east side, the reality is that cars are here to stay. It is likely the gas cars will be replaced with more and more electric cars over time (hence the need for a VMT fee), but cars themselves aren’t going anyway. Most people in the Portland metro do not live in the the 3 areas above and aren’t going to; regardless of how many apartment buildings you want to pack in like sardines. The urban growth boundary will never be reduced; there is only talk of expanding it. Portland is simply too spread out for this utopian ideal of turning the metro area into Manhattan. If we want more revenue that doesn’t depend on driving, we can start with more enforcement on the streetcar and max and then most importantly, implement a SALES TAX. This affects all people including out of town visitors. The bottom line: the street tax is an absolutely terrible idea that will go to the ballot and should be defeated if passed by Ms. Fritz.

      • We are never going to shift away from driving as the primary mode if we double-down on driving subsidies. Cars and trucks (and busses) damage the roads, so their operators should pay for road maintenance. Once we start seeing major drops in funding for PBOT due to decreased driving levels, then we can start looking at new fees. Total miles traveled in Portland is still on the rise, even though per capita VMT has dropped.

  3. Is it possible that the trip generation tool is a good tool for deciding where to put new transit but a terrible tool for assessing a tax?

    It’s one thing to generalize about what neighborhoods need in terms of transit- but this business of charging all single family homeowners more than the owners of all multi-family buildings based on trip generation- unfair & arbitrary.

    It’s one thing to generalize about my neighborhood (OM’s neighbors need X transit) and another thing to tax my neighbors differentially based on gross generalization about all SFH vs. all multi-family buildings. A cottage in Lents should not pay more than a condo that’s worth 5 times as much in the Pearl.

    More and more I think we just need to assess the street fee as an income tax- at least on businesses.

    Also- if the street fee is supposed to manipulate transit choices while fixing the streets- it will be taking on too much. Just raise 90 million and find 10 million in cuts.

  4. We need to raise the price of driving, yes, but we also need to make it feel like everyone has some skin in the game for the next wave of bike improvements.

    If we just raised a gas tax or weight-miles tax, that money would likely only go towards repaving. To do the street-reconfiguration style projects, we need to get a funding source that isn’t coming solely from vehicles

    • Two months ago, I would have agreed. But now, if you consider why we’re considering a fee in the first place, a gas tax is the most fair way to maintain our roads and make them safer.
      If there were no cars on the road, how fast would the roads wear out? If there were no cars on the road, what would the safety hazards be? It is because of the cars on the road, and the number of cars on the road, that the roads need to be maintained as they do and that other users of the public rights of way are placed at risk. Therefore, it is the responsibility of those that use the roadway to mitigate their contribution to the degredation of the roadway materials and general population safety. The external cost of auto use must be internalized to the person that chooses to use that form of transportation.
      We have the transportation system we have because those that came before us failed to do this. Will we fail as well and just leave the mess to our children?

      • Are you saying that without cars, roads would maintain pristine condition forever? This is simply not true. Weather also takes a toll on roads

        • He is ignoring the damage that Tri-Met buses, heavy equipment and trucks take on the roads, perhaps to further an anti-car agenda (don’t know).

          Sorry, but that little Honda ain’t the problem. It’s the beer trucks and the
          #14 that damage the roads.

          We all have to pay for the road because there are practically no people who do not use the goods and delivered by trucks.

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