Episode 39 – Trucks

Topics: Trucks



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Hello and welcome to another edition of Talking Traffic. My name is Bill Ruhsam and I host this podcast and its sister website, talking traffic dot org. Today is Monday, June 27, 2011. This is episode 39 of Talking traffic.

Today’s topic is about trucks. All kinds of trucks. But before I dive into the nebulous term that is “trucks” let me throw some engineer-speak at you.

When we are designing or analyzing roads, we talk about the “Design Vehicle”. The design vehicle is the largest type of vehicle that is most likely to use that road. For example, if I’m working on a residential subdivision, I dont’ need to design the road to allow a tractor-trailer to turn around in a cul-de-sac. No, a typical large vehilce for a subdivision would be a UPS or FedEx truck. Now, the occasional tractor trailer might come into that roadway, maybe a moving truck, but it only happens occasionally, and the inconvenience caused by having to back up a tractor-trailer to turn it around is small compared to the inconvenience of designing the subdivision to allow the truck to drive around as if it were an interstate.

It’s important to determine at teh very beginning what your design vehicle is because it will affect many different things in your roadway project.

Now, let’s talk about trucks. When I, as a traffic engineer say “truck”, I mean some specific types of vehicles. I’m not talking about pickups or SUVs or dooleys or anything that you migt see in a Ford commercial. I’m talking about larger vehicles that are intended to carry freight. These trucks break down into two cateogries. Single Unit trucks, and Multi Unit trucks.

Single Unit trucks are trucks that don’t articulate, that have a single frame to which the wheels are attached. UPS and FedEx trucks are good examples of these. dump trucks and garbage trucks and smaller moving trucks are all examples of the single unit truck.

Multi-unit trucks include the typical tractor-trailer combination that you see everywhere. These come in various sizes measured from the center of the front axle to the center of the rear most axle. So, when I’m throwing engineering speak at people. I would refer to the WB-62 design vehicle, which means a multi-unit tractor-trailer combination that has a 62 foot wheel base. This truck is actually about 69 feet from nose to tail. There are also the WB-40 and the WB-50 and the WB-65. For trivia purposes, the WB-65, which is 74 feet from nose to tail, is the vehicle used when designing interstaets and interstate ramp terminals.

The importance of the design vehicle becomes clear when you start putting together the design of intersections and sharp curves. Larger trucks need more room in order to make turns. The rear wheels of a truck–well, of any vehicle, really–will run to the inside of the front wheels. This wider path made during a turn is called overtracking, and its why you see large trucks swing out really wide when they’re making right angle turns at intersections. The distance and width needed to ensure that the rear wheels of the design vehicle stay off the edge of pavement, or out of the adjacent lanes, can add a lot of cost to a design project.

Next time you’re walking in an area that has curbs and you come to an intersection, look at the corners. Do you see tire tracks up against the curb faces? Do you see tire tracks on top of the curb, or on the sidewalk? Are the corners, maybe including the pedestrian ramps, broken and cracked? If any of these are true, it means most likely that truck drivers are running their rear wheels up and over the curbs when they’re trying to make turns. This could be becuase the driver isn’t very good, but more likely it’s because there’s not enough room for the trucks to clear when taking their overtracking into account.

Large vehicles are also important to consider when we’re designing intersections and considering the time it takes to accelerate. For example, the distance you need to be able to see to your left, or to your right, when you’re trying to turn onto a roadway. On a 45 mile per hour road, with no other consideations, that distance should be 500 feet if you’re driving a car. If you’re driving a tractor trailer, then I have to add another 270 feet! A football field worth of view, basically. That means no hills, curves or bushes to block the view of that truck driver trying to make a left turn.

Similarly, if I’m working on signal timing, and I know that large trucks are going to make a significant percentage of my traffic stream, then I have to include the additional time it’s going to take to move each truck through the intersection. I can’t just make an assumption based on cars or I’ll end up with a huge congested mess and people calling me and asking what the hell I was thinking.

Then of course, trucks impact how we design pavement! Trucks make a much larger impact on the asphalt and concrete than a typical car for one simple reason: they’re much heavier. So pavements have to be designed to handle the types and quantitiels of trucks that are expected.

Lastly, at least for this podcast, is noise and air pollution. Trucks make up a big part of the noise and air pollution that we transportation engineers have to take into account nowadays. It is possible that when designing a new roadway, the additional noise may be so great that we have to install soundwalls in order to avoid impacting the surroundings. Trucks add a lot to that noise level, so again, it’s important to know what type of vehilce is being designed for.

And that’s it about trucks. There is more that I could discuss, getting into the juicy engineering details, but that covers all the basics.

Thanks for listening to talking traffic. If you like what you heard, or didn’t, be sure to let me know by leaving a comment on the show notes or sending an email to bill at talking traffic.org.

The music you’ve been listening to is by five star fall and can be found at magnatune .com. This episode is released under a creative commons attribution share alike 3.0 license. Feel free to distribute and/or modify this podcast, but please link back to me and to talkingtraffic.org.

Until next time, have a great week.

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4 Responses to Episode 39 – Trucks

  1. jack brown says:

    Wb 65 is that a tractor with sleeper birth and 53 foot trailer? Which one is is tractor and two pup trailers or 24 foot trailers? What is the most common signal time red lights?and determines a green wave?

  2. Bill Ruhsam says:

    The WB 65 is typically a sleeper cab with a 53′ trailer. The tractor + two trailer combination is a WB-67D, I believe (I don’t have the reference material in front of me: AASHTO Green Book, under Design Controls).

    As for the last two questions, they’re a bit vague. Can you be more specific?

  3. jack brown says:

    Why wb67d what is the d? Why do traffic enginer do green wave? What is the average singal time green to red? How long is the green light a verage three light signal

  4. Bill Ruhsam says:

    Jack: The “D” in WB67D is a mystery to me. I assume it was some designator applied by the first authors of the AASHTO Green Book when they were naming the different types of vehicles typically found on American roadways. It’s possible there is meaning there, but I do not know it.

    A green wave is an intentional bit of signal coordination so that as you progress through a series of traffic signals, you are not repeatedly stopped. This allows for greater throughput, lower emissions, and less frustration for drivers.

    As for your questions with respect to average times on signals, I don’t know. A typical signal cycle (Green to the next Green) can be almost anything depending on the area. a “typical signal cycle” could be 60 seconds, or 240 seconds. Around here in Atlanta, 180 and 240 are not uncommon cycle lengths. In the middle of nowhere, you might not *have* a cycle length because the signal is fully actuated and defaults to the mainline as quickly as there is no more calls on the side street.

    However, for a three signal coordinated network, I would guess a signal cycle around 120-150 seconds. You have to progress traffic in *both* directions and if the cycle is too long, you can generate that driver frustration I mentioned.

    Hope this helps!

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