Browsing by Author "Liu, Henry"

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Automatic Generation of Traffic Signal Timing Plan
(Minnesota Department of Transportation, 2014-09) Liu, Henry; Zheng, Jianfeng
Due to budget constraints, most of the traffic signals in the US are retimed once every 2-5 years. Despite that, traffic delay increases 3-5% per year with outdated timing plans. It would be desirable to reduce the signal retiming costs by automating all or a portion of the manual process. This research takes one step forward in this direction. In this project, we develop a performance visualization and fine-tuning tool for arterial traffic signal systems, aimed at reducing the labor costs for signal retiming. Using high-resolution event-based data from the SMART-Signal system, a set of easy-to-use algorithms are developed to refine traffic signal systems. Specifically, a framework is developed to diagnose operational problems regarding cycle lengths, green splits and offsets. Then, algorithms for offsets and green splits fine-tuning are proposed. To fine-tune offsets, a practical procedure to construct time space diagram (TS-Diagram) to visualize the progression quality on arterials is proposed and validated. For green splits, an adjusted measure of effectiveness (MOE), the utilized green time (UGT), is proposed for performance evaluation. Moreover, a practical procedure for time of day (TOD) transitions is also developed to generate optimal timing plan schedules. Field case studies and simulation experiments are carried out to illustrate and validate the proposed algorithms. The algorithms could be used during the retiming process to help agencies reduce labor costs, or to periodically refine traffic signal systems for coordinated arterials.
Benefit and Cost Analysis of the I-394 MnPASS Program
(Intelligent Transportation Systems Institute, Center for Transportation Studies, University of Minnesota, 2012-02) Cao, Xinyu (Jason); Munnich, Lee; Liu, Henry; He, Xiaozheng (Sean); Xu, Zhiyi; Huang, Yan (Arthur)
In this report, we explored the benefits and costs associated with converting the I-394 High Occupancy Vehicle (HOV) lanes to High Occupancy Toll (HOT) lanes. The study focused on the I-394 corridor, with a 10-year timeframe from 2006 to 2015. The benefits included travel time savings, safety benefits, and vehicle operating cost savings, and the costs consisted of capital costs and annual operating costs. Where applicable, the implementation of this study followed the benefit-cost analysis guidance of MnDOT. This study considered the benefits of both travel time savings and travel time reliability and the valuations of travel time savings and reliability were derived from econometric models for individual drivers’ behavior. HOT lane users choose the lanes because of travel time savings and/or the reliability of the lanes whereas previous studies considered only travel time savings and exclusively relied on standardized economic value of travel time. This study estimated safety benefits from crash reduction using the Empirical Bayes method. Previous studies scarcely considered the benefits resulting from the conversion of HOV lanes to HOT lanes. This study also showed that “naïve” approaches tended to overstate safety benefits, which highlighted the importance of using a sound methodology.
Development of a Platoon-Priority Control Strategy with/out Smart Advance Warning Flashers for Isolated Intersections with High-Speed Approaches
(Minnesota Department of Transportation, 2009-07) Liu, Henry; Bhimireddy, Sundeep
Most of the rural or suburban high-speed isolated intersections have higher traffic volumes on the major approach compared to the minor approach. The vehicle platoons on major approach are often forced to stop frequently due to conflicting calls placed by few vehicles on the minor approach. To overcome this issue, platoon-priority signal control systems have been developed to progress platoons efficiently at these intersections in a number of previous studies. In addition, Advance Warning Flashers (AWF) are used at these intersections to provide advance warning of end of green to the motorists. The conventional method uses trailing overlap green which holds the green for a fixed time after gap-out. This trailing overlap green replaces the existing dilemma-zone protection provided by loop detectors and also increases delay on the minor approach. Recently, Advance Warning of End of Green System (AWEGS) has been developed to provide advance warning at these intersections without having to hold green after gap-out. The purpose of this research study is to develop and evaluate the benefits of an integrated system which provides platoon-priority, advance warning of end-of-green, and also dilemma-zone protection at the end-of-green for rural high-speed isolated intersections. Cabinet-in the-loop tests were performed using a real world scenario. These study results showed 50 percent reduction in delay and stops on the major approach with platoons. It was found that the total intersection delay and stops were reduced by as much as 20 percent. The system was also successful in providing advance warning to the motorists by predicting gap-outs 7 to 8 seconds earlier in the majority of the cases.
Employment of the Traffic Management Lab for the Evaluation and Improvement of Stratified Metering Algorithm - Phase III
(Minnesota Department of Transportation, Research Services Section, 2007-05) Liu, Henry; Wu, Xinkai; Michalopoulos, Panos; Hourdos, John
The evaluation results (done in Phase II) demonstrated that the SZM strategy was generally beneficial. However, they also revealed that freeway performance degraded by reducing the ramp delays. Therefore, it is desired to improve the effectiveness of the current SZM control. There are two objectives in this study. One objective is to improve the control logic of current SZM strategy. This is accomplished through an estimation algorithm for the refined minimum release rate. The simulation results indicate that the improved SZM strategy is very effective in postponing and decreasing freeway congestion while resulting in smoother freeway traffic flow compared to the SZM strategy. The second objective of this project is to improve the current queue size estimation. Depending on the counting error of queue and passage detectors, freeway ramps are classified into three different categories, and different methods are applied respectively for improved queue size estimation. The surveillance video data were recorded and used to verify the improvement of the proposed methods. The results indicate that the proposed methods can greatly improve the accuracy of queue size estimation compared with the current methodology. Also, the proposed method was evaluated by the micro-simulation. The simulation results indicate the performance of freeway mainline is significantly improved. And the total system performance is better than the original SZM control.
Employment of the Traffic Management Lab for the Evaluation and Improvement of Stratified Metering Algorithm - Phase IV
(Minnesota Department of Transportation, 2007-12) Liu, Henry; Wu, Xinkai; Michalopoulos, Panos; Hourdos, John
Freeway ramp control has been successfully implemented since mid 60's, as an efficient and viable freeway management strategy. However, the effectiveness of any ramp control strategy is largely dependent on optimum parameter values which are preferably determined prior to deployment. This is certainly the case happening to the current Stratified Zone Metering (SZM) strategy deployed in the 260 miles freeway network of Minneapolis - St. Paul metropolitan area. In order to improve the performance of the SZM, which highly depends on the values of more than 20 parameters, this research first proposed a general methodology for site-specific performance optimization of ramp control strategies using a microscopic simulation environment, as an alternative to trial and error field experimentation, and implemented the methodology to the SZM. The testing results show that the new SZM control with site-specific optimum parameter values significantly improves the performance of freeway system compared with the original SZM strategy. Secondly, this research proposed a methodology to explore the common optimum parameter values for the current SZM strategy for the whole Twin Cities freeway system, in order to replace the site-specific optimum values which have minor practical value because of the difficulties in implementation and numerous time-consumption to search the site-specific optimum values for all the freeway sections. The common parameter values are identified applying the Response Surface Methodology (RSM) based on 4 specifically selected freeway sections which can represent all types of freeway sections in Minneapolis-St. Paul metropolitan area.
Engaging Undergraduate Students in Transportation Studies through Simulating Transportation for Realistic Engineering Education and Training (STREET).
(Transportation Research Board, 2009) Liao, Chen-fu; Liu, Henry; Levinson, David M
The practice of transportation engineering and planning has evolved substantially over the past several decades. A new paradigm for transportation engineering education is required to better engage students and deliver knowledge. Simulation tools have been used by transportation professionals to evaluate and analyze the potential impact of design or control strategy changes. Conveying complex transportation concepts can be effectively achieved by exploring them through simulation. Simulation is particularly valuable in transportation education because most transportation policies and strategies in the real world take years to implement with a prohibitively high cost. Transportation simulation allows learners to apply different control strategies in a risk-free environment and to expose themselves to transportation engineering methodologies that are currently in practice. Despite the advantages, simulation, however, has not been widely adopted in the education of transportation engineering. Using simulation in undergraduate transportation courses is sporadic and reported efforts have been focused on the upper-level technical elective courses. A suite of web-based simulation modules was developed and incorporated in the undergraduate transportation courses at University of Minnesota. The STREET (Simulating Transportation for Realistic Engineering Education and Training) research project was recently awarded by NSF (National Science Foundation) to develop web-based simulation modules to improve instruction in transportation engineering courses and evaluate their effectiveness. Our ultimate goal is to become the epicenter for developing simulation-based teaching materials, an active textbook, which offers an interactive learning environment to undergraduate students. With the hand-on nature of simulation, we hope to improve student understanding of critical concepts in transportation engineering and student motivation toward transportation engineering, and improve student retention in the field. We also would like to disseminate the results and teaching materials to other colleges to integrate the simulation modules in their curricula.
Improving Freeway Traffic Speed Estimation Using High-Resolution Loop Detector Data
(Intelligent Transportation Systems Institute, Center for Transportation Studies, University of Minnesota, 2013-04) Liu, Henry; Sun, Jie
In this project, we developed an innovative methodology to solve a long-standing traffic engineering problem, i.e. measuring traffic speed using data from single inductive loop detectors. Traditionally, traffic speeds are estimated using aggregated detector data with a manually calibrated effective vehicle length. The calibration effort (usually through running probe vehicles), however, is time consuming and costly. Instead of using aggregated data, in this project, our data collection system records every vehicle-detector actuation "event" so that for each vehicle we can identify the time gap and the detector occupation time. With such high-resolution "event-based" data, we devised a method to differentiate regular cars with longer vehicles. The proposed method is based on the observation that longer vehicles will have longer detector occupation time. Therefore, we can identify longer vehicles by detecting the changes of occupation time in a vehicle platoon. The "event-based" detector data can be obtained through the implementation of the SMART-Signal (Systematic Monitoring of Arterial Road Traffic Signals) system, which was developed by the principal investigator and his students in the last five years. The method is tested using the data from Trunk Highway 55, which is a high-speed arterial corridor controlled by coordinated traffic signals. The result shows that the proposed method can correctly identify most of the vehicles passing by inductive loop detectors. The identification of long vehicles will improve the estimation of effective vehicle length on roads. Consequently, speed estimation from the inductive loop detector is improved.
Indifference Bands for Route Switching
(2016) Di, Xuan; Liu, Henry; Zhu, Shanjiang; Levinson, David M
The replacement I-35W bridge in Minneapolis saw less traffic than the original bridge though it provided substantial travel time saving for many travelers. This observation cannot be explained by the classical route choice assumption that travelers always take the shortest path. Accordingly, a boundedly rational route switching model is proposed assuming that travelers will not switch to the new bridge unless travel time saving goes beyond a threshold or indifference band. To validate the boundedly rational route switching assumption, route choices of 78 subjects from a GPS travel behavior study were analyzed before and after the addition of the new I-35W bridge. Indifference bands are estimated for both commuters who were previously bridge users and those who never had the experience of using the old bridge. This study offers the first empirical estimation of bounded rationality parameters from GPS data and provides guidelines for traffic assignment.
Multi-agent Route Choice Game for Transportation Engineering
(Transportation Research Board, 2015) Di, Xuan; Liu, Henry; Levinson, David M
In undergraduate transportation engineering courses, traffic assignment is a difficult concept for both instructors to teach and for students to learn, because it involves many mathematical derivations and computations. We have designed a multiplayer game to engage students in the process of learning route choice, so that students can visualize how the traffic gradually reach user equilibrium (UE). For one scenario, we employ a Braess' Paradox, and explore the phenomenon during the game-play. We have done the case-control and before-after comparisons. The statistical results show that, students who played the game improve their understanding of the Braess' Paradox more than those who did not play. Among game players, younger students benefit more in their learning; while those who are not comfortable with exploring a phenomenon on their own think this game not as effective as those who prefer hands-on learning experiences.
Perception of Waiting Time at Signalized Intersections
(Transportation Research Board, 2009) Wu, Xinkai; Levinson, David M; Liu, Henry
Perceived waiting time at signalized intersections differs from the real value, and varies with signal design. The onerousness of delay depends on the conditions under which it is experienced. Using weighted travel time time may contribute to optimal signal control if its use can improve upon assuming that all time is weighted equally by users. This research explores the perception of waiting time at signalized intersections based on the results of an online survey, which directly collected the perceived waiting time and the user ratings of the signal designs of each intersection on an arterial including 3 intersections. Statistically analyzing the survey data suggests the perception of waiting time is a function of the real time; and a quadratic model better can describes relationship. The survey also indicates that there exists a tradeoff between the total waiting time and the individual waiting time of each intersection. It turns out that drivers prefer to split the total waiting time at different intersections at the price of a longer total wait if the difference of the total waiting time of two signal designs is within 30 seconds. The survey data shows that the perceived waiting time, instead of the real waiting time, better explains how users will rate the individual signal designs for both intersections and arterials including multiple intersections.
SMART-Signal Phase II: Arterial Offset Optimization Using Archived High-Resolution Traffic Signal Data
(Intelligent Transportation Systems Institute, Center for Transportation Studies, University of Minnesota, 2013-04) Liu, Henry; Hu, Heng
Traditionally, offset optimization for coordinated traffic signals is based on average travel times between intersections and average traffic volumes at each intersection, without consideration of the stochastic nature of field traffic. Using the archived high-resolution traffic signal data, in this project, we developed a data-driven arterial offset optimization model that will address two well-known problems with vehicle-actuated signal coordination: the early return to green problem and the uncertain intersection queue length problem. To account for the early return to green problem, we introduce the concept of conditional distribution of the green start times for the coordinated phase. To handle the uncertainty of intersection queue length, we adopt a scenario-based approach that generates optimization results using a series of traffic-demand scenarios as the input to the offset optimization model. Both the conditional distributions of the green start times and traffic demand scenarios can be obtained from the archived high-resolution traffic signal data. Under different traffic conditions, queues formed by side-street and main-street traffic are explicitly considered in the derivation of intersection delay. The objective of this model is to minimize total delay for the main coordinated direction and at the same time it considers the performance of the opposite direction. Due to model complexity, a genetic algorithm is adopted to obtain the optimal solution. We test the performance of the optimized offsets not only in a simulated environment but also in the field. Results from both experiments show that the proposed model can reduce travel delay of coordinated direction significantly without compromising the performance of the opposite approach.
The Traffic and Behavioral Effects of the I-35W Mississippi River Bridge collapse
(Elsevier, 2010) Zhu, Shanjiang; Levinson, David M; Liu, Henry; Harder, Kathleen
The collapse, on August 1, 2007, of the I-35W bridge over the Mississippi River in Minneapolis, abruptly interrupted the usual route of about 140,000 daily vehicle trips and substantially disturbed the ﬂow pattern on the network. It took several weeks for the network to re-equilibrate, during which period, travelers continued to learn and adjust their travel decisions. A good understanding of this process is crucial for traffic management and designing mitigation schemes. A survey collected behavioral responses to the bridge collapse. Traffic data were also collected to understand the traffic conditions experienced by road users. Data from both resources are analyzed and compared. Findings of behavioral effects of capacity changes could have significant implications for travel demand modeling, especially of day-to-day travel demand
Traffic Flow and Road User Impacts of the Collapse of the I- 35W Bridge over the Mississippi River
(Minnesota Department of Transportation, Research Services Section, 2010-07) Zhu, Shanjiang; Levinson, David; Liu, Henry; Harder, Kathleen; Dancyzk, Adam
Major network disruptions have significant impacts on local travelers. A good understanding of behavioral reactions to such incidents is crucial for traffic mitigation, management, and planning. Existing research on such topics is limited. The collapse of the I-35W Mississippi River Bridge (August 1, 2007) abruptly disrupted habitual routes of about 14,000 daily trips and forced even more travelers to adapt their travel pattern to evolving network conditions. The opening of the replacement bridge on November 18, 2008 generated another disturbance (this time predictable) on the network. Such “natural” experiments provide unique opportunities for behavioral studies. This study focuses on the traffic and behavioral reactions to both bridge collapse and bridge reopening and contributes to general knowledge by identifying unique patterns following different events. Three types of data collection efforts have been conducted during the appropriate frame of reference (i.e. before vs. after bridge reconstruction): 1) GPS tracking data and associated user surveys, 2) paper and internet-based survey data gauging travel behavior in the post-bridge reconstruction phase, and 3) aggregate data relating to freeway and arterial traffic flows, traffic control, and transit ridership. Differences in reactions to planned versus unplanned events were revealed. Changes in travel cost were evaluated and their temporal and spatial patterns were analyzed. This report concludes with thorough discussions of findings from this study and policy implications.
Zhu, Shanjiang, David Levinson, and Henry Liu (2012) Measuring Winners and Losers from the new I-35W Mississippi River Bridge. Transportation.
(2016) Zhu, Shanjiang; Levinson, David M; Liu, Henry
The opening of the replacement for the I-35W Mississippi River Bridge on September 18th, 2008 provides a unique opportunity to evaluate the impacts generated by this additional link on network performance. Using detailed GPS data to estimate travel times on links and for origin-destination pairs, this research finds that while on average travel time improved with the reopening of the bridge, the subsequent restoration of parts of the rest of the network to their pre-collapse configuration worsened travel times significantly on average. In all cases, the distribution of winners and losers indicates clear spatial patterns associated with these network changes.

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