Episode 14 – Functional Classification

Topics: Functional Classification

Talking Traffic Episode 14 – Functional Classification

Hello! Welcome to Talking Traffic. My name is Bill Ruhsam and I’m here to bring you insider information about transportation and traffic engineering, and those who practice it. Today is Monday, February 25, 2008. This is episode number 14 of talking traffic and I’m going to talk about the phrase “Functional Classification.”

I’m sure that you have noticed that different roads have different uses. This is obvious, even if you haven’t given it much thought. A residential street is there to provide access to individual homes, townhouses, apartments, or alleys. It is not for high-speed drivers or delivery trucks trying to beat a deadline. The street can be windy and narrow. An Interstate Highway on the other hand is intended to move large numbers of vehicles at high speeds over long distances. Access points are extremely limited. You may only enter and exit an Interstate highway at grade-separated interchanges that are constructed for the purpose.

Whoops. Let me take you on a quick aside: I’m guilty of using Jargon right there. The term Grade-Separated refers, as I’m sure you are aware from the context, to two roads that pass over or under one another. The vertical positioning of the road is called the Grade or Profile, and if they are separated in the vertical dimension then the intersection is “grade separated.” If two roads intersect at the same elevation, i.e. if they’re a normal stop- or signal-controlled intersection, they are said to be “at grade”.

Back to discussions of Functional Classification. I have illustrated the two extremes of the functional classification system. First, I described the Local Road, which is designed and intended for full access to the surrounding properties. Second, I described the Interstate Principal Arterial, which is designed and intended for absolutely no access whatsoever to the adjacent properties. Another way to describe these two extremes is to say that the Local Road is intended for limited “Mobility” while the Interstate is for maximum mobility.

Mobility and Access are two diametrically opposed features of a road. If you have high mobility, it is nearly impossible to have a large number of access points, be they driveways or cross streets or interchange ramps. Vice Versa, with a large number access points, mobility drops. This is because if you have vehicles turning onto and off of a road, the friction they cause will slow the vehicles in adjacent lanes, reducing the volume and speed of the traffic.

We don’t design roads to have both high mobility and frequent access. Instead, we apply the functional classification *system* to intelligently (hopefully) address the needs of the area the roadway is going through. The system starts at the top with the Interstate Principal Arterial, then as it goes down the list, with decreasing Mobility and increasing Accessibility, we have the Principal Arterial, the Minor Arterial, the Major Collector, the Minor Collector, and finally, the Local Road. In a perfect world, to get from A to B, you would start your journey on a Local Road or Minor Collector, drive to a Major Collector, then to an Arterial. The Arterial would take you to the vicinity of B where you would turn onto a Major collector, then a minor collector or local road, and finally arrive at B. The world isn’t perfect, however, and we can’t build small streets off of rural arterials in order to serve every house, which is why you will still see driveways with mail boxes off of roads that are signed 55 MPH. This is not a good idea, but it’s hard to avoid in the real world.

As I mentioned just now, functional classification is a tool used to design roadway networks. Its purpose isn’t to tell you what a roadway *is*, it’s to tell a roadway designer what a road *should be*. In other words, if I’m building or upgrading a road in a particular location, I had better know what the designated functional classification of that road is, and then design a set of plans that constructs pavement to serve that purpose. If it’s supposed to be a local road or minor collector, I shouldn’t be designing it to be a 55 MPH, six lane thoroughfare. The intended purpose of the road will conflict greatly with the actual use it will receive once construction is complete, leading to phone calls that I’d rather avoid.

But! Did I mention that in the real world, things are bit less ideal? I have found on numerous occasions that a road is being used in a fashion entirely different from what the official classification indicates. Case in Point: My day job has me working on a roadway improvement and widening. This road crosses an Interstate without an interchange, but serves several factories and other warehouse facilities. It also serves as an access road for numerous homes that need to use another arterial to head south to the next interchange. The functional classification of this roadway is officially a Local Road. While it is serving the function of a local road by being accessible at all locations, the traffic volumes and truck percentage have grown to the point where it is operating more like a major collector. Once our design is built, this roadway will operate like a minor or principal arterial because it will serve as a major shipping bypass of a congested route. It is entirely likely that this road will be reclassified once it is rebuilt, but *right now* it’s still a local road and I’m making design decisions that indicate it’s an arterial. That is a dichotomy that should not exist. The problem here is that functional classification is something decided by planners, and planners tend to be dissociated from the design process, in my experience. But, (again with the buts) it isn’t necessarily a bad thing that this roadway might be reclassified. Where the problem lies is that we aren’t providing ANY access points to the new roadway because, as I mentioned way back at the beginning, mobility is opposed to access. This road is intended to provide freight mobility, not access, and now we’ve potentially set ourselves up for conflicts with property owners who want their driveways back and I’ll be forced to say “No.” We’ll provide them driveway access, but it won’t be on the new road. Trying to explain to a property owner why their driveway is going away, and using terms like “Functional Classification” and “Increased Mobility” tend to get you in trouble. It requires a great deal of patience to win an owner around to the merits of the situation.

I’ve been concentrating on access rights along roadways by mentioning Driveways. However, there are different kinds of access. What about left-turn access? Business owners become incensed when they discover that a road project is going to build a median in the road in front of their business. What they fear, is that if you can’t turn into or out of their business, they’ll be forced to close. Before I address that, let’s talk about why a median is being built in the first place.

Typically, unless a road is planned to have a center median from the beginning, widenings and eventual median construction are due to increases in traffic volume and congestion. Up to a certain point, a road will serve greater and greater vehicular volumes, then a sharp drop in quality will occur, resulting in high amounts of congestion. This is not a smooth curve. Congestion is dependent on a number of factors and it is the proverbial camel; just a few additional straws will cause serious issues.

One way to address these issues is to remove unrestricted left-turning vehicles from the traffic stream by building a center median. This smoothes out the traffic flow and increases the capacity of the roadway. It also removes the ability for vehicles to turn into and out of businesses along the route which is perceived by the business owners as a death knell. Several studies have looked at this and found that for the vast majority of businesses, sales and customer throughput either remain the same or go up, after completion of construction. Inquiries made to the customers themselves indicate that they are happier with the median than without it, regardless of any potential U-turns that they are forced to make. The lesson here is that while construction will hamper business, after the median is in place, the increased traffic volume, lower congestion, and generally nicer road environment should help a business grow. The one dark point several studies have identified is for businesses that are primarily auto-oriented, such as oil-change and service stations. These generally have shown a drop in business numbers. A caveat that every study mentions, though, is that hard data is extremely difficult to come by. Business owners are not exactly going to run out and volunteer their financial information for before/after studies. Other methods are used and are therefore a bit suspect. However, the trend indicates that medians do not hurt businesses, and most of the time, they help.

This brings me back to Functional Classification. While the presence or lack of a center median has nothing to do, directly, with functional classification, it does have an impact on the accessibility of a roadway. Thusly, a local road or minor collector is probably not going to have a median, otherwise it isn’t serving its designed purpose. Medians are a way of controlling the access onto and off of a roadway, and they properly belong to the major collectors and arterials inside the functional classification system.

What have we learned today? Functional Classification is a system that places roads into various buckets depending on how they are supposed to be used. A road with a higher functional class is supposed to provide higher mobility and less access. One with a lower functional class will provide more access, and less mobility. The system is supposed to work together so that people move from high-mobility roads to high-access roads in order to arrive at their destinations. We also learned that the functional classification system doesn’t always match what is on the ground. We learned that Access and Mobility are opposed to each other: Higher access means lesser mobility and vice versa. Functional Classification is a planning tool that determines what sort of facility should be constructed and maintained in a particular location, not necessarily how that roadway ends up being used. That is a much more organic process.

Thanks for listening to Talking Traffic. If you have any questions or comments, please send an email to Bill @ talkingtraffic.org. Or you can leave a comment on the show notes at www.talkingtraffic.org. I promise to address them, even if it’s with an “I don’t Know.” I have no problem admitting my own ignorance; I think that makes me a better engineer.

Today’s episode is released under a creative commons 3.0 attribution non-commercial no-derivatives license. If you don’t know what that means, please check out the Creative Commons at creativecommons.org. For day to day purposes, you may copy and distribute this podcast as long as you give credit to me, you don’t change it, or sell it. In fact, I encourage you to tell as many people as possible about these episodes. You can even copy them wholesale and give them away. I don’t mind. The whole point behind Talking Traffic is to educate the public. “Public” means all the people who aren’t my colleagues; they don’t qualify as “public”. That way we’ll all know what the terms and phrases mean when we’re talking about transportation projects.

In case anyone who listens to these episodes is in the Atlanta Area on March 1st, we’re having a late spring party at our house. If you want to come, send me an email and I’ll hook you up with directions. This is our quadrennial February 30th Party, and we’ve invited pretty much everyone we know, so a few more won’t hurt.

The music you heard on this podcast is by Five Star Fall at Magnatune.com.

Until next time, have a great week!

Theme Music: Five Star Fall, Mercurial Girl, Magnatune.com

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