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Diverging Diamond Interchange designs are beneficial for locations with heavy lefts on and off of freeway ramps.


Diverging Diamond Interchanges and roundabouts can be designed to help all roadway users.

By Meredith K. Cebelak, Ph.D., P.E.

Like many other Americans, getting ready for the recent holiday season meant travel and gift giving. With the advent of e-commerce, the latter has become more convenient, but this convenience comes at a price: more trucks on the roadways. Personally, I enjoy seeing trucks moving goods throughout my city because more trucks mean a thriving economy. However, not everyone shares my attitude. I often hear about how slow they are moving through non-highway systems or how someone was “stuck” behind a truck at an interchange trying to get onto the interstate.

Historically, traditional roadway design has focused on the passenger vehicle with other modes taking a back seat when it comes to design considerations. However, with a shift toward complete streets, transportation engineers are taking a more holistic view of the transportation network and looking for ways to address the needs of all users, including the bicycle, pedestrian, transit, and trucking communities.

The case for freight

Figure 1: Diverging Diamond Interchange Navigation

According to the Bureau of Transportation Statistics’ 2017 Freight Facts & Figures report, 11.5 billion tons of goods were moved via truck in 2015; by 2045 this number is expected to reach 16.5 billion tons. This increase means more trucks using the transportation network to move goods, and while these vehicles utilize the National Highway System, it is important to remember that the last mile is always on a truck.

Annually, the American Transportation Research Institute (ATRI) surveys the trucking community to determine the critical issues the industry is facing. Transportation infrastructure, congestion, and funding made the “Top Ten” list from this report, which notes that traffic congestion and poorly maintained roads create wear and tear on vehicles, increase fuel consumption and emissions, add stress to drivers, and negatively impact productivity. With estimates of $74.5 billion of additional operational costs during 2016 attributed to congestion-related delays, the problem is greater than just being stuck in traffic with trucks.

Moving the trucking community through our transportation network as efficiently as possible requires designers to think about how the type of vehicle operates in a more meaningful way. Two innovative intersection design alternatives have been selected to further explore opportunities to address the needs of the trucking community: Diverging Diamond Interchanges (DDIs) and roundabouts.

Diverging Diamond Interchanges

Figure 2: Diverging Diamond Interchanges within the U.S. and Puerto Rico

In a DDI, vehicles are moved from one side of the roadway to the opposite side for the non-freeway system (Figure 1). These designs are beneficial for locations with heavy lefts on and off of freeway ramps, moderate and unbalanced crossroad volumes, locations with left turn safety concerns, and where there is a need to add capacity without widening the facility. DDIs eliminate left crashes and have been shown to reduce right angle and rear end crashes, as well as to reduce delay and congestion due to the increase in capacity. Having grown in popularity and use in recent years, DDIs are operational, being designed and built, or being studied, in nearly every state in the U.S. (Figure 2).

Figure 3: Roundabouts within the U.S. and Puerto Rico

From the trucking community’s perspective, these interchanges, when designed correctly, easily accommodate large commercial vehicles (even long combination vehicles) and navigating road crossings and ramps is a non-issue. The state of Missouri performed a performance evaluation on its I-44 and Route 13 DDI and found that 83 percent of the truck drivers surveyed felt that maneuvering a large truck through the interchange was easy and not any different from other interchanges.

When designing a DDI with freight in mind, there are a few things to keep in mind. First, engage the community early, especially the freight community. Also think about detour routes; while the interchange may not be heavy with freight traditionally, that may change if it is on a detour route. Design lane widths to accommodate truck movements by widening crossover and turning lanes, consider the single- versus double-lane configuration, and keep path alignments as “straight” through as possible. Finally, consider using rolled-raised curbs, which can help trucks avoid tire damage and blowouts.

Roundabouts

Roundabouts are often used in locations where there is unbalanced flow, high turning volumes, and a need to reduce speeds and/or congestion.

Similar to DDIs, roundabouts have grown in popularity and use throughout the U.S. and Puerto Rico. In fact, every state within the U.S. and in Puerto Rico has at least one roundabout, with some states being home to more than 100 (Figure 3). Keeping in mind that the last mile for freight is always on a truck, it’s likely that a truck will encounter a roundabout at some point.

Often, roundabouts are used in locations where there is unbalanced flow, high turning volumes, and a need to reduce speeds and/or congestion. While these intersections do reduce crashes and delays, and allow traffic to continuously flow, not all users think they’re a great design. From a trucking perspective, roundabouts can be difficult to navigate, as they are often “too small” and have travel path issues. There are times when a truck driver is unaware that a roundabout exists on the route, which creates additional issues.

An ATRI survey on roundabouts and large trucks found that more than 70 percent of the people surveyed found them to be more problematic than other intersections. Respondents noted that trailers often encroach into the center or second lane, elevated or sloped curbs cause problems for 90° turns, and center island aesthetics caused blind spots for drivers. However, all is not lost! There are design considerations that engineers can use that can address the trucking community’s concerns.

Size matters — Traditionally, roundabouts are designed with a central island that is sized for traffic to move around the facility in a circular motion. For the trucking community, the size of the inscribed circle can create significant issues for moving within the prescribed pathway, including vehicles getting stuck and even tipping over, both of which negatively impact the driver and the delivery. By considering road classifications and surrounding land use when adjusting the central island, roundabout designers can provide more space for trucks to negotiate the center island.

In addition to appropriately sizing the central island, using irregular-shaped islands such as ovals and teardrops can also be effective. These size and shape design considerations are particularly important where there may be a large number of trucks that need to make a left or through movement within the roundabout or when longer combination vehicles may be making deliveries. Similar to the DDI, using rolled-raised curbs within the roundabout can be very impactful.

Eyes on the road — We have all seen a roundabout with an art installation, water feature, or fancy landscaping in the middle. While these features certainly add to the roundabout’s aesthetics, there can be significant safety consequences when drivers are focused on these items rather than the roadway. There are even times when these features can limit sight distance and/or provide obstructions that interfere with a truck’s wheel path, limiting its ability to navigate around the roundabout. Limiting the features that are within the roundabout helps all users utilize roundabouts safely.

Another consideration for designing roundabouts that enhances truck mobility is the need to provide enough room for them to maneuver through. In roundabouts that have two lanes, a truck may need to use both lanes in order to negotiate the central island. FHWA’s Human Factors Laboratory is testing signs for use in roundabouts that let motorists know that trucks may need both lanes, that wide turning movements may be needed, and that passenger cars should be aware of being next to the rear wheels of a truck that needs to use both lanes.

The Transportation Research Record: Journal of the Transportation Research Board (TRB) published “Accommodation of Large Trucks in Roundabouts: Motor Carrier Perspective” in 2013. The authors noted the need for all areas to have a designated truck route and, from the motor carrier perspective, the need for these routes to be free of the issues associated with roundabouts. From the carrier’s perspective, drivers are the ones who have to figure out a way around the problem, only adding stress on drivers.

By examining where truck routes exist and looking at potential detour routes, designers can determine if additional accommodations should be made within the design for the trucking community. Additionally, having all parties (state, city, and county governments) look at the long-term plan for a region, including public and private developments, can provide additional insight into the current and future needs of the system for the freight community, as well as if additional design features to support truck mobility are needed.

Keep on trucking 

Moving the trucking community through our transportation network efficiently and effectively requires us to engage, consider, and design for the needs of the trucking community, which we can only do by understanding the trucking community’s needs. When we engage the freight community early and often by reaching out to local and national organizations — such as the American Trucking Association, Association of American Railroads, Council of Supply Chain Management Professionals, National Industrial Transportation League, and Transportation Clubs International — we can better understand who we’re designing for. Engaging with these organizations can include networking events, educational seminars, newsletters, websites, surveys, one-on-one interviews, focus groups, and freight forums. It can also be beneficial to get plugged in with freight advisory groups at the state, regional, and local levels, as well as economic development groups.

As the freight industry continues to grow, freight considerations can no longer take a backseat in interchange design. By understanding the needs of freight drivers, being aware of surrounding land use, considering potential detour routes, and making design accommodations for oversized vehicles, we can create innovative interchanges that keep our freight industry trucking along.


Meredith K. Cebelak, Ph.D., P.E., is a senior transportation system management and operations engineer, associate, and freight enthusiast at Gresham Smith (www.greshamsmith.com). She has a wide range of project experience that includes freight planning studies, signal designs, intelligent transportation systems, and big data. She has been recognized as a Dwight David Eisenhower Transportation Fellow and ENO Fellow, and is active in many Transportation Research Board committees including the Urban Freight Transportation committee.

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