The planners' desk reference from the 1960s reveals the early missteps that led to the automobile-centered built environment of today.
It took me a long time to understand that the reason why so much of the United States looks so monotonously similar: just about everything built since 1970 has been designed according to the same national transportation models which predict and provide for motor vehicles first. Planners and engineers do this because they have an unshakeable faith that land uses, once constructed, magically acquire the power to "generate" travel, and the only way to deal with this is to mitigate with extra lanes and parking spaces. Where did this idea come from? How did this backward idea become the default understanding for an entire profession?
I think the answer can at least partially be found in a book that at one point was (or still may be) stashed away in nearly every planning department across the country. Principles and Practice of Urban Planning, published in 1968, is one of those old documents that provides fascinating insights about the mores of its age, and what constituted "good" planning at that time--it also offers a starting point for understanding the genesis of many of the bizarre policies still on the books in zoning codes today. This text lends a glimpse into the world when the sprawl shown above was still considered a good idea, and a look at the fork in the road before trip generation and transportation demand models became the gospel truth for rank and file planners.
Probably intended for the hapless bureaucrat recently assigned to the planning department, Principles and Practice of Urban Planning provides overviews of a range of topics on just about everything related to planning; here I am only looking at their brief overview of transportation planning and prediction. There are four points from this section which I find especially notable, all of them still widely believed today:
1. Predicting travel--the foundational comparison for is that traffic is similar to water or electricity:
"Travel is to the transportation system what current is to an electrical network or fluid is to a hydraulic network. The design of the system must be based not only on the characteristics and functions of the conductor, but also on the properties of that which passes through the conductor."
Put simply, managing water or electricity effectively is mostly a question of large enough reservoirs, big enough batteries, or enough pipe or conduit. Water and electricity move and change in predictable ways under knowable conditions, and knowing this allows us to plan and predict for their movement in safe and verifiable ways. Unfortunately, people in cars don't really behave like that; they drive differently going to the emergency room than they do going to work, they drive differently going to the beach than going to the store, and they even drive differently while talking on the phone versus sitting in silence. Traffic is made up of drivers making decisions, but "water-based" transportation models can't really account for any of this, as they treat traffic as something that land uses "generate", which then flows outwards like water or power. In reality, traffic is much more complicated than water or electricity--and traffic prediction should consider how and why people make decisions before any other factor. By reducing it so clumsily, planners and engineers blinded themselves to only a few tools for mitigation.
2. Walking isn't considered travel:
"A journey made completely by foot is not considered to be a trip in the typical origin-destination survey."
There isn't much to say about this--if planners didn't even consider walking to be a form of travel, why would anyone ever think about building usable infrastructure for pedestrians?
3. Predicting Travel Based on Land Use is "more convenient":
"A rewarding or a profitable activity is the main purpose of travel and it can be said that such activity generates travel. Whatever the purpose of the trip it can be looked at in two ways: from the viewpoint of the type of activity involved, or from the viewpoint of the person making the trip. For transportation planning, the most accurate description of the activity is land use, and the most accurate description of the person's viewpoint is purpose. Land use is perhaps the more convenient measure of the two, because it enables travel to be related to a tangible, and relatively predictable quality."
Planners and engineers are expected to provide certainty where there is little, provide predictions of future events that no one can really know. Everyone has always hated new congestion, and so naturally there would be a lot of pressure to look for ways to predict and avoid it. If you had to figure out a method to predict something very complicated, which would you choose, a method that requires interviews, psychological study, and behavioral intervention, or something that can be easily quantified and repeated from the comfort of your planning office chair? Land uses provided a simple and quantifiable variable that they could use to make up guesses about what might happen. The result was a disaster, but the convenience is hard to beat.
4. Land Uses Don't Cause Travel, People Do:
"However, it must be thoroughly understood that people make trips, and that land use is only a convenient indirect measure of the type and geographic location of trips made."
This might be the most important aspect of this whole section, because it turns the trip generation paradigm on its head: It isn't land uses that cause trips, but people deciding to travel. If you work in zoning or development this might seem like a dull or trivial point, but I can't emphasize enough how important it is given that the belief that land uses, not people, "generate" trips is the philosophical basis for most of the parking minimums, traffic studies, and ever-expanding roadways that get built every single day. But here, in the 1960s, they were speaking frankly about how the real agency in how travel happens lies with the people making the trips.
If planners and engineers stopped believing that land uses "generate" trips, they would be forced to think about what actually causes travel: people making decisions based on public infrastructural context. This would mean that it isn't the land use like the apartment, store, or factory which causes people to drive, but the roads, the parking spaces, and all the other things that municipalities and DOTs require with every new development. It follows then that how much people drive, or walk, or bike (or ride a horse) is not inescapable or unchangeable, but simply a choice that communities have the full responsibility to make. But taking that responsibility is a very heavy burden, especially for local politicians. Planners could play a role in highlighting this also, but most either don't understand the concept or don't want to upset their long term stability.
There's a lot more interesting stuff in this chapter, including explorations about predicting travel based on income, car ownership, and proximity to downtown (all much better methods, although clearly less convenient), and if you have access to an old planning department library it is really worth picking up a copy. The book seems like it could potentially be a great starting point for a clearer discussion on the assumptions and misconceptions behind many of the backwards transportation models still in use in across the country today. Hopefully this could prompt some sort of useful self-reflection from engineers, planners, the American Planning Association, or ICMA (the original publisher)---but I won't be holding my breath.