Peter T. Martin

An Investigation of Driver Distraction Near the Tipping Point of Traffic Flow Stability

Objective: The purpose of this study was to explore the interrelationship between driver distraction and characteristics of driver behavior associated with reduced highway traffic efficiency. Background: Research on the three-phase traffic theory and on behavioral driving suggests that a number of characteristics associated with efficient traffic flow may be affected by driver distraction. Previous studies have been limited, however, by the fact that researchers typically do not allow participants to change lanes, nor do they account for the impact of varying traffic states on driving performance. Methods: Participants drove in three simulated environments with differing traffic congestion while both using and not using a cell phone. Instructed only to obey the speed limit, participants were allowed to vary driving behaviors, such as those involving forward following distance, speed, and lane-changing frequency. Results: Both driver distraction and traffic congestion were found to significantly affect lane change frequency, mean speed, and the likelihood of remaining behind a slower-moving lead vehicle. Conclusions: This research suggests that the behavioral profile of “cell phone drivers,” which is often described as compensatory, may have far-reaching and unexpected consequences for traffic efficiency. Application: By considering the dynamic interplay between characteristics of traffic flow and driver behavior, this research may inform both public policy regarding in-vehicle cell phone use and future investigations of driving behavior.

VisSim-Based Genetic Algorithm Optimization of Signal Timings

Genetic algorithm optimizations of traffic signal timings have been shown to be effective, continually outperforming traditional optimization tools such as Synchro and TRANSYT-7F. However, their application has been limited to scholarly research and evaluations. Only one tool has matured to a commercial deployment: direct CorSim optimization, a feature of TRANSYT-7F. A genetic algorithm formulation, VisSim-based genetic algorithm optimization of signal timings (VISGAOST), is presented; it builds on the best of the recorded methods by extending their capabilities. It optimizes four basic signal timing parameters with VisSim microsimulation as an evaluation environment. The program brings new optimization features not available in the direct CorSim optimization, such as the optimization of phasing sequences, multiple coordinated systems and uncoordinated intersections, fully actuated isolated intersections, and multiple time periods. The formulation has two features that enhance and reduce computational time: optimization resumption and parallel computing. The program has been tested on two VisSim networks: a hypothetical grid network and a real-world arterial of actuated–coordinated intersections in Park City, Utah. The results show that timing plans optimized by the genetic algorithm outperformed the best Synchro plans in both cases, reducing delay and stops by at least 5%.

Modifying Signal Timing During Inclement Weather

Most individuals living in cold climates realize that on snowy days their commute will take longer. Although traffic volumes are often lower, the combination of reduced speeds and capacity causes severe congestion, particularly on signalized urban networks. Signal coordination that reduces traffic congestion in typical clear conditions results in an uncoordinated and suboptimal timing plan. Traffic parameters for developing signal timings during inclement weather conditions are examined. With the completion of the Utah Department of Transportation (UDOT) advanced transportation management system, there is an opportunity to change signal timing plans by communicating with each controller from the Transportation Operations Center. This operation makes feasible a library of special signal timing plans, with one allocated for inclement weather. Traffic flow data were collected over a range of seven inclement weather severity conditions at two intersections for the 1999–2000 winter season. The data indicate that the largest decrease in vehicle performance occurs when snow and slush begin to accumulate on the road surface. Saturation flows decrease by 20 percent, speeds decrease by 30 percent, and start-up lost times increase by 23 percent. UDOT is now developing and implementing modified inclement weather coordinated signal timing plans for the major signalized corridors in the Salt Lake Valley. The determination of when to implement an inclement weather signal timing plan is based on four general criteria: storm severity, projected duration, area of influence, and immediately projected running speeds. With these considerations, traffic engineers can determine whether to implement an inclement weather signal timing plan.

SCOOT REAL-TIME ADAPTIVE CONTROL IN A CORSIM SIMULATION ENVIRONMENT

The manner in which the adaptive signal control system SCOOT (Split, Cycle, Offset Optimization Technique) has been connected to the CORSIM traffic simulation model is described. To demonstrate the connection, CORSIM simulates the traffic activity of a six-node traffic network under SCOOT’s adaptive traffic signal control. CORSIM’s “virtual detectors” provide the necessary data for SCOOT optimization in real time. In a completed loop, the optimized signal timing is then communicated from SCOOT to CORSIM, which implements the timing and updates the traffic simulation. This means that SCOOT is now functioning in an entirely simulated environment. A comparison of delay and travel time is presented for a six-intersection street network under SCOOT control and under fixed-time area control optimized with TRANSYT-7F. The results show reductions in delay and numbers of stops of 20 to 30 percent. Previously, the measurement of the benefit of adaptive control has been limited to evaluations of systems after implementation. It is shown how SCOOT can now be evaluated under various network traffic conditions in a simulation environment and tested on a specific city network to evaluate the benefits before capital costs are committed.

Comparative Evaluation of Adaptive Traffic Control System Assessments Through Field and Microsimulation

Usually, benefits from deploying an adaptive traffic control system are measured by comparing data collected in the field before the system is installed with those collected after the system is fully operational. Such an approach is costly because it requires installation of a system before potential benefits can be observed. Nowadays to investigate effectiveness of many alternatives that were traditionally investigated in field, microsimulation is used. However, how much can one rely on results from microsimulation? There has not been a single study in which benefits of adaptive traffic control systems measured in the field are compared with those obtained through microsimulation. This study presents such a research effort. The authors compared performance measures from a field evaluation of Sydney Coordinated Adaptive Traffic System in Park City, Utah, to their counterparts from microsimulation. They collected a significant amount of data to assess system performance in the field and to help build a microsimulation model. A 14-intersection model of the Park City network was developed, calibrated, and validated on the basis of multiple data sources. The results show that a validated microsimulation model can accurately reflect field conditions although such an effort can be very challenging.

Comparison of Before–After Versus Off–On Adaptive Traffic Control Evaluations: Park City, Utah, Case Study

An adaptive traffic control system, the Sydney Coordinated Adaptive Traffic System (SCATS), was installed in Park City, Utah, to improve traffic performance at its network of signalized intersections. A field evaluation of the previous time-of-day actuated–coordinated signal timings was conducted before SCATS installation to compare the two systems. However, the post-SCATS field evaluation could not occur until two additional signals were installed and several other changes were made to the network. Two years after the original pre-SCATS field evaluation, the network was reevaluated with an off–on technique analogous to a before–after study. The signal timings and parameters in the off condition forced SCATS to use time-of-day actuated–coordinated control, similar to the before study but with timings readjusted for the additional signals and changed traffic conditions. The performance gains with SCATS on were measurably greater than those with SCATS off for travel time and number of stops and greater overall for stopped delay. These data provided the transportation agency with results anticipated by its traffic engineers. However, the original field evaluation data from 2 years earlier provided a less distinct conclusion. A methodology is presented to determine the relevance of an off–on study in place of a before–after study. The results show that “before” and “off ” data sets behave consistently 62.5% of the time. This value provides a basis of support for using off data, which better represent before signal timings on an after network. It also quantifies sensitivity to changes in the network, which are substantial in this case.

Prioritizing Traffic-Calming Projects Using the Analytic Hierarchy Process

The analytic hierarchy process (AHP), a multiple-criteria decision-making tool, is used to prioritize traffic-calming projects. AHP is proposed as an alternative to existing point scoring systems. Prioritization methods used in traffic-calming programs in Portland, Oregon, and Canberra, Australia, are applied; the results are compared to those of AHP. The three methods apparently produce similar rankings when applied to local streets that have speeding problems. AHP produces rankings different from those of the other two methods, however, when complex issues and qualitative factors must be taken into account. AHP may be more suitable in cases in which some factors cannot be quantified. In the examples studied, these factors included traffic diversion, the importance of the street, the impacts of existing traffic-control devices and measures adjacent to elementary schools, terrain, and neighborhood residents’ opinions. None of these factors could readily be assigned a value in a point scoring system. Some of the concerns in applying AHP are the tediousness of making pairwise comparisons between alternatives, the consistency of the comparisons, and the defensibility of the scores. The decision maker should examine the scores to ensure that they are sensible and should be adequately aware of the issues so as to defend the scores. If there are n alternatives, then n(n - 1)/2 pairwise comparisons are needed. Clearly, for expedient application of AHP, the alternatives must be limited to a reasonable number.

Development and Evaluation of Algorithm for Resolution of Conflicting Transit Signal Priority Requests

The goal of this study was the development and evaluation of an algorithm for resolving conflicting requests for transit signal priority (TSP). This algorithm was designed to work with actual traffic controllers without the need for new hardware or software installations. The algorithm was tested in VISSIM microsimulation and ASC/3 software-in-the-loop controllers on an intersection that will be upgraded to serve two conflicting bus rapid transit (BRT) lines. The ASC/3 logic processor was used to control built-in TSPs in the case of conflicting requests and to develop custom-TSP strategies that would not rely on built-in TSP. Custom TSP provides a much higher level of TSP for transit vehicles than built-in TSP, and it creates opportunities for more adaptable TSP control. The results showed that the widely used first-come, first-served policy for resolution of conflicting TSP requests was not the best solution. Such a policy could perform worse than a policy that provided no priority. For the analyzed intersection, the first-come, first-served option even increased BRT delays by 13% more than did the no-TSP option. The presented algorithm can help resolve the problem of the conflicting TSP requests. The algorithm worked best when combined with several TSP strategies. For the custom-TSP strategies, the application of the algorithm reduced BRT delays by more than 30%, with minimal impact on vehicular traffic. The algorithm shows promising results, and with small upgrades, it can be applied to any type of TSP.

Evaluation of Transit Signal Priority Options for Future Bus Rapid Transit Line in West Valley City, Utah

This paper presents an analysis of different transit signal priorities (TSPs) for a future bus rapid transit (BRT) corridor in West Valley City, Utah. The goal was to find the optimal TSP strategy for estimated and planned traffic and transit operations. The study used VISSIM microsimulation software in combination with ASC/3 software-in-the-loop simulation. Four models were used in the analysis: no TSP, TSP, TSP with phase rotation, and custom TSP. The results showed that TSP with phase rotation and custom TSP could both be considered for implementation. TSP with phase rotation would provide significant benefits for BRT, with minimum impacts on vehicular traffic. Custom TSP would provide major benefits for BRT in travel times, delays, and stops. However, this strategy has more impact on vehicular traffic. Custom TSP is an advanced strategy that still needs examination and improvement. The study provides a set of instructions on how the described strategies can be implemented in field traffic controllers.

Predictive Priority for Light Rail Transit: University Light Rail Line in Salt Lake County, Utah

The goal of this paper is to assess the operational implementation of strategies for predictive light rail priority through microsimulation. A 2-mi corridor in Salt Lake County, Utah, where the University Line of light rail line operates, was studied. The study used VISSIM microsimulation models to analyze light rail operations and the effects that light rail priority has on transit and vehicular traffic. Results showed that although the existing priority strategies had no effects on vehicular traffic along the corridor, they reduced train travel times by 20% to 30%. Left turns along the main corridor were slightly affected by the priority. Although the priority strategies could have minor to major effects on vehicular traffic along side streets through increased delays, they reduced train delays by 2.5 min along the corridor. Enabling priority at the 700 E intersection (where the priority was currently not active) would help reduce delays for trains by an additional 10%, with a small increase in vehicle delays. However, the coordinated north–south through movements would experience minimum impacts. Three recommendations emerged from the study: enable priority at 700 E to improve transit without major effects on vehicular traffic; reset priority parameters at intersections adjacent to light rail stations so that the priority call encompasses station dwell times; and consider removing the queue jump strategies, so as to reduce delays for the corridor through movements and help preserve coordination patterns.