Walking the Walk

Urban design from the front lines

Kevin Klinkenberg

Using urban design to make our lives more enjoyable and create wealth

This site is for all those interested in the making of cities and towns, and especially the lives of the humans that inhabit them. Kevin Klinkenberg is an architect and urban designer who's practiced from coast-to-coast. 

The Path to Prosperity during rush hour: A final note on road design

It's true, I'm not an engineer - I'm an architect. However, with all of our work in community planning over the years we've had to work hand-in-hand with engineers of all kinds. As a result, we’ve learned a great deal about how our transportation systems are planned. For my engineering friends - yes, I hope to get my "honorary engineer" badge one day. At any rate, it's not the intent of this series or blog to talk solely about transportation and road design issues. However, it's in that arena where an awful lot of decisions get made that impact the long-term health and character of our communities, and also where tremendous sums of money are spent daily. So, we need to pay attention, and learn not just the techniques but the philosophies.

So today, here’s a note on the philosophy behind much road design. One of the consistent beefs I've had with the way our transportation systems, especially roads are sized is based on what I like to call the "worst-case scenario" school of design. That is, much of the emphasis in traffic engineering is placed on how a road operates at the busiest time of day, with the highest amount of traffic.

For example, when a road is being studied for its current and future traffic levels, several variables are plugged in - current and future development patterns, other roads in the network, intersections, etc. And, what is often the controlling factor is what engineers call the "PM peak hour". In English, that's the amount of cars coming along a roadway during the afternoon rush hour (afternoon is typically assumed to be busier than morning rush hour). Additionally, since intersections or interchanges are always the "choke points" for traffic, the ultimate variable is how an intersection functions at the PM peak hour. Engineers assign a letter grade to this measure, called Level of Service. This grade ranges from “A” to “F”. An "A" grade essentially means that traffic flows freely without delay and an "F" grade indicates that there will be delays of 60 seconds (or more) at an intersection.

At this point, it all sounds very logical and, well… engineered.

Here's the problem in a nutshell: we are fundamentally solving for the few hours a week when traffic is the worst. In a city where rush hour is 2 hours long, that would mean we are designing intersections to account for the traffic that happens during 10 of the 168 hours of any given week (20 if you allow for morning rush). For those doing math, 20 out of 168 hours is 12%. This is a little like designing an entire house for the few hours a week when it's the most chaotic. Or, you might say, designing a home for the occasional 100 person party that you host. Sure, for a few folks with means, they can afford that. But for the bulk of the population it's unnecessary and expensive. We arrange our lives for the day-to-day normalcy, and deal with the exceptions.

And, that's fundamentally the difference we often have with those who size our roadways. Contemporary practice sizes for the *worst* condition, and then we are left with oversized roadways for the 148-158 hours of the week. That's not only expensive to build, but it's also dangerous by allowing for much higher speeds in those other hours. Plus, it makes any other form of travel non-competitive. Importantly, it also factors out the reality that these are human beings operating vehicles, not water running through a pipe. We can and do often change our behaviors as the conditions around us change - we might take a different route, might adjust our lifestyle or get around in other ways.

As we deal increasingly with limited resources for transportation, we should re-orient our thinking to design for the normal condition, and manage the exceptions. What would that mean in practical terms? Any number of things are possible: narrower (and cheaper) roadways, longer driving delays during rush hour, more transit usage/carpooling to help with rush hour conditions, or a change in land-use patterns. But the benefits would be many-fold: less infrastructure burden to build and maintain, streets that are pleasant and safe for all users (pedestrians, bicyclists, etc), and ultimately more choice in how to get around.

  • Here's an example of an "oversized road" in downtown Austin, Texas: