How traffic engineers predict the future, with Adam Millard-Ball

Our track record predicting the future isn’t so great.

Adam Millard-Ball is an assistant professor at the University of California in Santa Cruz. Before that, Adam worked as a transportation planner. So naturally, one of his focus areas of academic research is derived from his experiences.

We’re going to focus on an issue that has huge impacts on the built environment: trip generation.


Gypsy Fortune Tellers Professional Transportation Engineers

Adam recently wrote an article and posted a companion video about trip generation. What’s trip generation, you ask?

Before a property owner builds a project (big box retail store, public school, grocery store, etc.), local government regulations typically require a traffic study. The purpose of the study is to document the impacts that the new development will have on the public infrastructure. Or, how many car trips will be generated by development.

Trip generation is considered science. An if-then type of analysis.

But it’s much closer to fortune telling or gambling. Sometimes the predictions are close to reality and sometimes they’re disparate.

Trip generation analysis is bread and butter work for modern-day traffic engineers. It’s based on the Trip Generation Manual published by Institute of Transportation Engineers (ITE).


Junk Infrastructure

You and I are paying for infrastructure to cater to vehicle trips that never happen. It’s a massive waste of our money.

Status quo transportation planning assumes that every development project will be financially successful and have high occupancy. That isn’t practical.

Not every project is a runaway success.

ITE data doesn’t give local governments any choice whether or not to use the built-in “success bias”. Local governments are locked into ITE’s assumptions. Why? Local or state regulations usually require planners and engineers to embrace ITE trip generation.

So local governments end up building new, expensive infrastructure. Or they require developers or property owners to pay for new, expensive infrastructure.

If local government pays, it’s a waste of taxpayer money. If developers pay, it discourages projects that meet the needs of people. (Adam talks about how this is especially true of transit oriented development projects.)


How traffic engineers make babies.

No need to cover your eyes. I’ll explain this in words. Clearly, but not graphically.

Adam makes an excellent point about what happens between the sheets [of the manual]. Constructing a new elementary school does not mean that new children are suddenly magicked out of thin air. And yet, that’s exactly what trip generation assumptions mean.

Built a school? There must be hundreds of new kids [riding in cars] at this new school necessitating wider roads, new stop lights, etc. (or so the logic goes).

New trips aren’t materializing like that. People — actual human beings — change their habits. Kids go to a new school? Then they stop going to their last school.

Adults shopping at a new grocery store? Then they’re spending less time at the old grocery store.

Building a new courthouse doesn’t generate more arrests or divorces.

This is common sense, and it’s routinely ignored by highly educated planners and engineers.


“Ok, you have my attention. What can I do about it?!”

Make a ruckus. (For ideas on ruckus-making, check this out.)

I ask Adam what he says to professional planners and engineers who want to apply common sense, but feel trapped by the ITE trip generation requirements.

He has two excellent suggestions for you:

  1. Rethink the term “conservative” in the context of infrastructure expansion.
  2. Don’t be wed to the idea that the trip generation estimates are inevitable reality.

You’re going to love this episode. Let me know what you think.


Connect with Adam Millard-Ball

Academic website:

Video interview with Access Magazine: 


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