Confessions of a Recovering Engineer. Charles L. Marohn, Jr.

Manual on Uniform Traffic Control Devices¹

      Destiny Gonzalez was killed on State Street. There are many reasons we call it a “street” instead of something else. Likely, that is the name given to it in the original plat: the initial layout and design of the city of Springfield. That would make the designation of “street” a tradition more than anything else.

      Some communities use “street” for places that are more residential — or less residential. It is often used in parts of the community that are designed with a network of grids, although not always. When used in places that also use terms like lane, access, boulevard, or drive, it might just be a random choice. For State Street, it could also be a preference for alliteration.

What is not often seen is calling something in the center of town a “road.” I am not saying that it never happens, but when one thinks of a road, it is often in the context of the “open road,” the conjecture of a more expansive kind of space.

      Before I became a civil engineer, this is how I understood things: Streets were in the city and roads were outside them. I grew up on a farm, so we lived on Mapleton Road. My grandmother lived in town on I Street NE. It seemed clear to me that roads were rural and streets were urban.

      Clear, until I began designing transportation systems.

Hierarchical Networks

      Traffic engineers and transportation planners classify streets and roads according to their status in a hierarchy. Classification is determined based on how much traffic the street or road handles, or how much it is expected to handle.

      The smallest of these with the least amount of traffic are called “locals.” They provide access to “collectors,” which collect traffic and funnel it to “arterials.” Sometimes a community will have “major arterials,” which is another step up the ladder of intensity. Theoretically, these different streets form a cascading system with many small streets emptying into fewer large streets.

       Locals

       Collectors

       Arterials

       Major Arterials

      There is an obvious tradeoff in this hierarchy between what engineers call “mobility” and what they refer to as “access.” Consider a cul-de-sac, the ultimate local street. A cul-de-sac provides plenty of access to the properties along it, but it does not provide much in the way of mobility. It is a dead-end street that is not expected to handle many vehicles. In contrast, an interstate is the ultimate major arterial, providing lots of capacity for vehicles to move at high speeds but with limited access to adjacent property.

With these two as the extreme endpoints, a standard classification analysis gently blends the tradeoff between mobility and access as we move from cul-de-sac to interstate. We can impair the mobility of the interstate to provide a bit of access, and we can give up some local access on the cul-de-sac to improve our mobility. This is a simple and comfortable relationship best represented in Figure 2.1.

      The relationship in Figure 2.1 is firmly grounded in the profession's priority values of speed and volume. Note the apparent happy compromise in the middle where we can have our transportation cake and eat it, too. Those are the collectors, where there is a lot of access but still plenty of mobility. At least, that is the way a transportation planner might explain it.

      Another way is to note that collectors combine high travel speed with complexity. Collectors facilitate the flow of traffic at speeds above what is safe for a local street with a lot of access, yet they provide just enough access to ensure that there will be random starts and stops, turning movements, and people walking around outside of a vehicle. High speeds combined with complexity create environments that are extremely dangerous.

Figure 2.1 Relationship of functionally classified systems in serving traffic mobility and land access.

It would be dangerous enough if State Street were designated a Collector. State Street, which runs through the heart of Springfield, is actually a Principal Arterial, a designation often used for highways.

Roads and Streets

      The embedded values of the engineering profession prioritize speed and volume in order to maximize mobility, with the belief that increasing mobility is the ultimate good provided by a transportation system. Discarding these antiquated values also allows one to move beyond the hierarchical classification system to one that creates greater value for a local community.

      The starting point is the pre-automobile understanding of the difference between a road and a street.

       Road: A high-speed connection between two places

       Street: A platform for building community wealth

      Think of a modern road as a replacement of the railroad, which, as its name suggests, is a road on rails. A railroad does not have frontage railroads or drive-through railroads. That would obviously be silly. For a railroad, a passenger gets on at one place and off at another, with the road providing a high-speed connection between those two places.

      A road creates the greatest value by providing the fastest connection between two places that people want to be. This is done by limiting things that slow down traffic along the road. For a railroad, that means reducing the number of stops, the amount of merging traffic, and congestion along the route. For automobile roads, the concept is the same. The higher the sustained speed, the lower the travel time and the greater the value provided by the road.

      While roads connect places, streets are the framework for building a place. Streets provide the greatest value when they create places that people want to be. When people choose to buy land, build something on it, and then maintain and improve what is built over time, they are building measurable wealth within the community. The most accurate measurement for the value of a street is the financial productivity of the land adjacent to it — how much value is created per acre of land that abuts the street?

The tension between achieving mobility and providing access in the current model is rightly replaced with the tension of either building a productive place or connecting productive places. This is a financial constraint more than anything else because the wealth created on the community's framework of streets is what must be tapped to pay for building and maintaining both the streets and the roads.

      A railroad, again, is the easiest way to think of this system. During westward expansion, major railroad companies established cities along their train routes. They would do minimal development work to establish the community, mostly laying out the network of streets and establishing lots along them. The railroad company would then sell the lots to pioneers, builders, and speculators, who would pay a premium because of the presence of the railroad and the exclusiveness of being located near one of the stops. The profit from improving the land is what the railroad company used to cover the cost of the rail line.

      Without a road, a network of streets will have no value. They will not build any real wealth. A city needs connections to other places for the streets to sustain anything beyond a village level of development intensity. The better these connections, the more value they will provide to these places and the more investment the community will attract and retain.

      Similarly, a road that fails to connect places to each other is worthless. A circular train that connects