Kevin Balke

Development and Evaluation of Intelligent Bus Priority Concept

Development and laboratory testing of an intelligent concept for providing priority to buses at signalized intersections without disrupting progression are discussed. The concept used bus position information to predict when in the cycle a bus would arrive at the bus stop and stop line of a signalized intersection and to determine whether a bus needs priority. The strategy used to provide priority was selected on the basis of the estimated arrival time of the bus at the stop line. Priority was provided by using phase extension, phase insertion, and early return strategies without causing the controller to drop from coordination. Implementation of the strategies was accomplished through normal traffic-signal controller commands (such as Ring Force Offs and Phase Holds). Hardware-in-the-loop simulation studies were performed to evaluate the effectiveness of the concept with real traffic-signal controllers. The performance of the intelligent bus priority approach was examined at three volume-to-capacity levels: 0.5, 0.8, and 0.95. Significant reductions in bus travel times were achieved at all three volume-to-capacity levels by using the intelligent bus priority approach. Use of the intelligent bus priority approach resulted in only minor increases in total system stop delay and individual approach stop delays at volume-to-capacity levels of 0.5 and 0.8. The results of the simulation studies performed as part of this study suggested to the researchers that the intelligent bus priority approach could be used at moderate traffic levels (up to volume-to-capacity levels of 0.9 or less) without significantly affecting cross-street delays.

VARIATIONS IN CAPACITY AND DELAY ESTIMATES FROM MICROSCOPIC TRAFFIC SIMULATION MODELS

One of the issues involved in using microscopic simulation models is the variation in the simulation results. This study examined some of the more popular microscopic traffic simulation models, CORSIM, SimTraffic, and VISSIM, and investigated the variations in the performance measures generated by these models. The study focused on the capacity and delay estimates at a signalized intersection. The effects of link length, speed, and vehicle headway generation distribution were also investigated. With regard to variations in performance measures, the study found that CORSIM yields the lowest variations, whereas SimTraffic yields the highest. The highest variation in each simulation model normally occurs when the traffic demand approaches capacity. It was also found that delays are affected by the link length and speed in simulation models. Such an impact on delays is closely related to the range of speed variations. In general, shorter links and higher link speeds result in lower delays. There is no strong evidence that the headway distribution used to generate vehicles in the simulated network has any effect on capacity and delay estimates. Multiple simulation runs are necessary to achieve an accurate estimate on the true system performance measures. With a 10% error range in estimated delay, two to five runs may be enough for under-capacity conditions, but more than 40 multiple runs may be necessary to accurately estimate delay at, near, or over capacity.

INTEGRATED CONTROL STRATEGIES FOR SURFACE STREET AND FREEWAY SYSTEMS

Freeway ramp metering often exists in the vicinity of a signal-controlled diamond interchange, at which the surface street system and the freeway system intersect. Even though both systems are controlled by traffic signals, they primarily operate independently of one another. The integrated operations of a surface street system and a freeway system were studied with VISSIM, a microscopic simulation model. A traffic network consisting of a surface street and a freeway segment was constructed in VISSIM. The surface street and freeway are connected through a diamond interchange with on-ramps and off-ramps. The objective of the study was to develop an integrated control algorithm for both the diamond interchange signal and the ramp-metering signal. The proposed control algorithm, including an adaptive diamond interchange control and a traffic-responsive ramp-metering control were programmed with VISSIM’s vehicle-actuated programming, which serves as an external control function for the simulation model. Preliminary tests of the proposed control algorithm indicated improved operations for both the surface street system and the freeway system. Traffic operations were significantly improved compared with nonadaptive diamond control and static ramp-metering control. Although similar performance measures were found (with the particular scenarios evaluated) with no-metering control, with the existing traffic-responsive ramp-metering control, and with the proposed traffic-responsive rampmetering control, the proposed control algorithm has more flexibility and may be adapted over a broader range of traffic flow conditions.

Pedestrian Timing Alternatives and Impacts on Coordinated Signal Systems Under Split-Phasing Operations

Split phasing can sometimes be more efficient in serving vehicular traffic under certain geometric and traffic flow conditions, such as the case in which a high volume of left-turning traffic is served from a sharedlane configuration. However, pedestrian crossing-time requirements can have a significant impact on intersection operations, especially in coordinated signal systems. Various alternatives for providing pedestrian timings under split-phasing operations are presented. The advantages and disadvantages, implementation strategies, and potential impact on intersection operations, especially on coordinated signal systems, are addressed with regard to each timing alternative. Further, the concept of the two-stage crossing design and the use of an exclusive pedestrian phase under split-phasing operations are investigated. The proposed model can be used to determine when exclusive pedestrian phasing can actually improve operational efficiency.

Modeling Impact of Ramp Metering Queues on Diamond Interchange Operations

Methodologies to model the impact of ramp metering queues on diamond interchange operations were developed in this study. The methodologies are part of an overall research effort to address the integrated operations of a diamond interchange and a ramp metering system. The methodologies were implemented in DRIVE, a computer model characterized as a mesoscopic simulation and analytical model. A meso-scopic model has the advantages of both macroscopic and microscopic models with less computing time while still considering stochastic traffic flows. DRIVE can be used to perform system analysis and evaluation of an integrated diamond interchange and ramp metering system over multiple cycles. The component concerning the diamond interchange operations is documented here, with a particular focus on the impact of potential queue spillback to the diamond interchange signals from ramp metering. The study focused on the two common diamond phasing schemes: basic three-phase and Texas Transportation Institute (TTI) four-phase. DRIVE was validated against the VISSIM microscopic traffic simulation model, and general agreement was found in the results between the two models. For oversaturated ramp conditions with queue spillback, both DRIVE and VISSIM indicated that TTI four-phase favors the frontage road approach, whereas the basic three-phase favors the arterial left-turn movement. DRIVE provides a new analysis tool for analyzing diamond interchange operations taking ramp metering into consideration, which would facilitate studies on developing operational strategies to better manage such systems.

Assessing Weather, Environment, and Loop Data for Real-Time Freeway Incident Prediction

Weather, environment, and loop data conditions are promising indicators for real-time freeway incident prediction. The ability to predict the likelihood of selected incident types by using weather and environment data was examined. Loop detector data were analyzed for conditions useful for in-lane incident prediction. Nonnested and nested multinomial logit models were estimated with data from selected freeways in Austin, Texas. The estimation results revealed that factors such as visibility, time of day, and lighting condition are significant determinants of incident type, whereas 5-min average occupancy and coefficient of variation in speed are strong predictors of in-lane freeway accidents.

Integration of Diamond Interchange and Ramp Metering Operations

The aim of this research was to develop operational strategies for integrating the operations between diamond interchange traffic signals and ramp metering signals. Integrated operational strategies were developed on the basis of the two commonly used diamond interchange phasing schemes: the basic three-phase scheme and the Texas Transportation Institute four-phase scheme. The key elements of the integration system and its operations include a proposed enhanced detection system and an operational algorithm. Through implementation of special signal timings at the diamond interchange, the traffic flows feeding the ramp meters can be effectively controlled and thus minimize ramp queues and the resulting need to flush the queues, which would possibly lead to freeway breakdown. The operational strategies were evaluated with VISSIM microscopic simulation under three general traffic demand scenarios–-low, medium, and high–-as characterized by the volume-to-capacity ratios at the metered ramps. The results of the evaluation indicated that the integrated operations were most effective under a medium traffic demand scenario in preventing or delaying the onset of ramp metering queue flush, thereby reducing freeway breakdown and system delays.

Part 2: Traffic Signal Systems: Integration of Diamond Interchange and Ramp Metering Operations

The aim of this research was to develop operational strategies for integrating the operations between diamond interchange traffic signals and ramp metering signals. Integrated operational strategies were developed on the basis of the two commonly used diamond interchange phasing schemes: the basic three-phase scheme and the Texas Transportation Institute four-phase scheme. The key elements of the integration system and its operations include a proposed enhanced detection system and an operational algorithm. Through implementation of special signal timings at the diamond interchange, the traffic flows feeding the ramp meters can be effectively controlled and thus minimize ramp queues and the resulting need to flush the queues, which would possibly lead to freeway breakdown. The operational strategies were evaluated with VISSIM microscopic simulation under three general traffic demand scenarios—low, medium, and high—as characterized by the volume-to-capacity ratios at the metered ramps. The results of the e...