Research examines planning options for highway reconstruction

Construction projects on major urban highways can dramatically impact travel options for thousands of travelers as well as the businesses that rely on the affected roadway for customer access. So when the Minnesota Department of Transportation opted to completely close a section of Trunk Highway 36 in Minneapolis for up to five months for reconstruction and upgrading instead of keeping the road partially open, the decision was the subject of intense public debate and scrutiny.

Minnesota Traffic Observatory (MTO) director John Hourdos and graduate student Feili Hong worked with Mn/DOT on a study of the closure, comparing the effects of the five-month full closure to an alternative multi-year partial closure scenario in terms of construction costs and costs to road users. Hourdos presented findings from the study at an ITS Institute seminar April 30, 2009; a streaming video webcast of the seminar is available.

The reconstruction of TH 36 was the largest full closure of a highway in Minnesota history; however, it was eclipsed by the collapse of the I-35 bridge over the Mississippi River, which resulted in the year-long closure of an even larger and more central highway segment. Like the bridge replacement, the TH 36 project was completed on time and within budget, but public fascination with the bridge replacement has kept the media spotlight away.

Hourdos traced the project’s complex history—from plans for the addition of a single  pedestrian bridge in 1997 to its final form, which included the reconstruction of several interchanges—and gave an overview of the innovative contracting provisions implemented by Mn/DOT to assure timely completion. The agency required that delays and other road user costs be explicitly considered in the contracting phase of the project. In the end, Hourdos said, the full-closure option was the only economically feasible option given budgetary restrictions facing local and state agencies at the time.

In the Minnesota Traffic Observatory, Hourdos and Hong used data from the project to create a test case that would allow them to compare the performance of different planning tools and approaches. The researchers compared three tools—the Federal Highway Administration’s QuickZone deterministic queuing model, the Cube Voyager transportation demand model used by the Twin Cities’ regional planning organization, and a specially developed microscopic traffic simulation based on the AIMSUN simulation package—in an effort to determine which would be the most efficient tool for understanding the effects of the highway closure.

The three tools, Hourdos said, are different by design and also require different levels of effort to set up, calibrate, and run. QuickZone, with its simplified and deterministic modeling approach, can produce results in a few days; Cube Voyager, the product of decades of refinement, is more difficult to use and optimized for the analysis of larger areas; the AIMSUN model required several months to design, calibrate, and validate.

In the end, all three models produced comparable results once they had been properly calibrated, and confirmed that a partial closure would have been more expensive. But the differences in complexity and data requirements, Hourdos said, were significant. He noted that the project had broken new ground in the analysis of full closure, while confirming the value of engineering judgment and uncovering deficiencies in current procedures and tools.

Watch the seminar online.

Intelligent Transportation Systems

Beyond the loop: Researchers develop the next generation of vehicle detectors

In order to manage traffic, you have to know where it is. For nearly 50 years, one of the most widely used tools for measuring traffic flows has been the loop detector—a large coil of electrically charged wire embedded beneath the pavement surface. When a large metal object passes overhead, the coil’s electrical inductance changes, causing a change in the flow of electricity running through it and triggering the detector.

Today, new technologies are available that offer sensing capabilities beyond those of traditional loop detectors—as well as substantially lower costs. With support from the ITS Institute, several independent research groups at the University of Minnesota are developing new sensor technologies that will make tomorrow’s roads smarter.

Introduced in the 1960s, pavement-embedded loop detectors gave engineers a powerful new tool to measure traffic flows. At that time, urban freeway systems were becoming increasingly important to metropolitan traffic, carrying high volumes of commuter traffic between suburban communities and central cities. Over the years, metropolitan areas have installed thousands of loop detectors. More than 6,000 loop detectors are now in place around the Minneapolis-St. Paul region.

The design of loop detectors, however, creates several disadvantages. One significant problem is cost—a single loop detector unit may cost several hundred dollars. Because they are embedded directly in the pavement, loop detectors are difficult to maintain. They require an external power source to operate, so electrical service must be installed along the roadway. When this service fails, detectors are difficult to repair or replace.

Batteryless wireless traffic sensors

A new type of traffic sensor that overcomes many of the limitations of loop detectors is taking shape in the mechanical engineering department laboratory of Rajesh Rajamani. Working with research fellow Lee Alexander and graduate student Krishna Vijayaraghavan, Rajamani is developing a self-powered wireless sensor for highway and arterial use that is more flexible and potentially less expensive than inductive loop systems.

The sensor consists of a beam embedded in the road surface and a data processing unit installed within a few hundred feet. Piezoelectric materials in the beam convert mechanical energy (in the form of pressure and vibration from cars passing over the beam) into electrical energy, providing enough voltage to power an onboard electronic system that wirelessly transmits a signal to the wireless processing unit.
The shape of the piezoelectric beam sensor—a thin bar rather than a large loop—makes it easier to install than a loop detector. There is also no need to connect the beam sensor to power supply lines or data transmission cables.

The narrow shape also unlocks sensing options beyond the capabilities of loop detectors. A piezoelectric unit can detect not only the presence of a vehicle, but can also count each axle that passes over it and determine the length of the vehicle. This information will give traffic managers important new data on what kind of vehicles are using roads.

Wireless mesh sensing

On the University’s Duluth campus, the advantages of wireless sensing are also key to research by Taek Mu Kwon of the electrical and computer engineering department. Kwon directs the Transportation Data Research Laboratory, managing data from the Twin Cities’ freeway loop detector system. But in a recent research project, he turned his attention to the challenge of monitoring complex traffic patterns in arterial intersections.

Engineers analyzing intersection traffic are often forced to rely on manual data collection—workers record the movements of vehicles through the intersection using handheld data loggers, an approach that is both tedious and error-prone. Kwon’s research aims to automate the process with small wireless sensor nodes that are easy to install temporarily on the road surface.

Kwon’s sensor nodes are designed to be installed in groups, with each sensor responsible for detecting vehicles in a single lane of traffic. Once in place, the sensor nodes automatically configure themselves as a “mesh” network, moving the raw data to a processing unit that extracts vehicle trajectories. The sensors are particularly suitable for short-term installations because each one is powered by its own battery and mounted on the road surface with an adhesive, but they can also be connected to an external power source for longer-term applications.

A mesh network is defined by multiple links between nodes. In a mesh topology, data can hop from node to node to reach a destination, rather than being transmitted directly to and from a central point. In a full mesh network, the number of links increases rapidly as more nodes are added. Kwon’s sensor network strikes a balance between flexibility and complexity via a partial-mesh topology, in which each sensor is connected to at least two other nodes in order to provide alternative data routes, but not to every other node in the system.

In operation, each sensor node in an intersection registers the magnetic disturbance caused by a vehicle passing directly over it and transmits the exact time of that event through the mesh network to a data logger positioned nearby. The nodes’ communication protocol ensures that their internal clocks are synchronized, so the timing of every vehicle detection event is recorded accurately.

To turn raw sensor output into usable information, the collected data are processed using a tracking algorithm that reconstructs vehicle trajectories from vehicle detection events. Because individual sensor nodes are responsible for each lane of traffic,  individual nodes can be designated as “entrance” and “exit” nodes. Taking into account the geometry of the intersection, the tracking algorithm matches vehicle detection events recorded by entrance nodes with events recorded by exit nodes. The result is a set of node-to-node trajectories representing the movements of individual vehicles.

Carbon nanotube pavements

A very different approach to sensor design is being taken by Xun Yu in the mechanical and industrial engineering department of the University of Minnesota Duluth. Yu’s current research seeks to turn the pavement itself into a sensor by exploiting the electromechanical properties of carbon nanotubes.
Since their discovery nearly 20 years ago, carbon nanotubes—cylindrical carbon molecules in which atoms are organized into hollow cylinders that are only a few atoms in diameter but up to millions of atoms long—have attracted the interest of researchers in many fields due to their unusual properties. In addition to being extremely strong, carbon nanotubes are electrical semiconductors that exhibit linear changes in electrical resistance in response to mechanical stress, a quality known as piezoresistance.

Yu is attempting to put piezoresistance to work by mixing carbon nanotubes (CNTs) with cement. In a well-formulated CNT/cement composite, evenly distributed nanotubes would form a web of carbon filaments spanning the entire paved area. Installing a simple set of electrodes to measure electrical resistance would turn the pavement into a single large pressure sensor.

Though such a sensor design is mechanically simple, with no complex circuits or moving parts, fabricating CNT/cement composites that can perform effectively as sensors is far more challenging than pouring a cupful of nanotubes into a cement mixer. CNTs have an unfortunate tendency to clump together when placed in solution, forming discrete blobs rather than the even, continuous network that piezoresistive sensing requires. To overcome this tendency, Yu is studying different chemical methods of encouraging CNTs to disperse, with an eye toward identifying methods that can be incorporated into commercial concrete mixing processes.

In addition to developing a manufacturing process for CNT/cement composites, Yu’s sensor concept also depends on developing a thorough understanding of their electrical and mechanical properties. In the laboratory, Yu is currently investigating composites’ piezoresistive response to dynamic and static stresses, as well as the effects of temperature, humidity, and other environmental factors

Transit, Bicyling, and Walking

Seminar surveys transit fare collection technologies, policy issues

Visiting professor Nigel Wilson presented an overview of technological and policy issues related to transit fare collection at an ITS Institute seminar April 21, 2009. A streaming webcast of the seminar is available online.

Wilson, professor of civil and environmental engineering at the Massachusetts Institute of Technology, is the director of major research and education collaborations between MIT and transit agencies in Chicago and London. He has advised Twin Cities Metro Transit on fare collection policy, technology, and data collection. 

Wilson highlighted several emerging technologies that are likely to become more widespread in the coming years. New “contactless” fare cards that can be read when they are briefly held in front of a fare card reader are poised to replace current card-reader technologies that require riders to insert their cards in a slot and wait for them to be returned, a change that will speed up boarding.

The use of bank cards for direct fare payment is also being explored by several major metropolitan transit providers, Wilson said, especially in Europe where chip-based bank cards are the norm. The magnetic-strip cards that dominate in the United States, Wilson said, are less suitable for this task because they must be inserted into a reader like current fare cards.

In the long term, Wilson predicted that smart cellular phones would be widely used for transit fare payment. However, technological issues are likely to keep this technology on the drawing board for at least several more years.

Implementing new technologies can also raise important policy issues, Wilson noted. Among the most significant issues are those related to equity—for example, payment systems that require riders to purchase a large-capacity prepaid fare card in order to take advantage of low overall costs can effectively exclude low-income transit users who cannot afford the initial cost. Likewise, if reusable smart cards, discounted passes, or other payment options are only available in a small number of retail locations, residents of other areas may not be able to take advantage of them. 

Wilson also discussed a variety of business models applicable to transit agencies, including the conventional merchant model (transportation as an item) and other models including loyalty or rewards programs or multiple travel products. Subsequent discussion with audience members explored the possibility of free public transit as a public good, and transit service based on a “club goods” model in which service is sponsored by a coalition of private and public organizations.

Watch the webcast.

Transportation Infrastructure

Mechanistic-empirical pavement design guide: A Minnesota perspective

The introduction of mechanistic-empirical pavement design, which combines empirical data from pavement testing with the results of mechanistic models of pavement materials, is an important development in transportation engineering. Since the introduction of the AASHTO interim Mechanistic-Empirical Pavement Design Guide (MEPDG) in 2008, many transportation agencies—including the Minnesota Department of Transportation—have begun to evaluate its prediction models and attempt to calibrate its guidelines for local conditions.

University of Minnesota pavement researchers Mihai Marasteanu and Lev Khazanovich, working with graduate students Raul Velasquez, Kyle Hoegh, Iliya Yut, and Nova Funk, and former Mn/DOT pavement engineer George Cochran, looked at the MEPDG as it relates to Minnesota conditions with support from Mn/DOT and the Minnesota Local Road Research Board. Their final report is now available from CTS.

The researchers used pavement performance date from the MnROAD pavement testing facility, where a wide variety of pavement materials and construction techniques are exposed to freeway traffic while data is recorded in a database to facilitate research.

The researchers examined the MEPDG cracking models for rigid and flexible pavements, as well as the models of rutting for the base and subgrade of flexible pavements. Correct calibration of these models for local conditions is essential if they are to yield usable results in the pavement design process.

With mechanistic-empirical pavement design in its early days, much more research will be required before engineers are able to fully apply mechanistic-empirical principles to the wide variety of local environmental and project conditions. The Minnesota research represents a step in that direction, and will contribute to a better understanding of mechanistic-empirical design techniques.

Implementation of the MEPDG for New and Rehabilitated Pavement Structures for Design of Concrete and Asphalt Pavements in Minnesota (Mn/DOT  2009-06) is available from the CTS Web site.

More Upcoming Events

June 1–3
ITS America's 2009 Annual Meeting & Exposition, National Harbor, MD.

June 10
ITS Minnesota Luncheon and Training Event.

June 15–16
International Transport Economics Conference: Incorporating the International Conference on Funding Transport Infrastructure, Minneapolis, Minnesota.

June 18–20
1st Transatlantic NECTAR (Network on Communications and Transport Activities Research) Conference, Arlington, Virginia.

July 8
Toward Zero Deaths Stakeholder Breakfast, Shoreview, MN

August 23–27
National Rural ITS Conference, Seaside, Ore.

August 30–September 2
ASCE 14th Conference on Cold Regions Engineering, Duluth, Minn.