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

2 The Difference Between a Road and a Street

       The following words and phrases, when used in this Manual, shall have the following meanings: 225. Street — See Highway

— 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.

      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.