Zong Z Tian

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.

Driver Behavior and Gap-Acceptance Characteristics at Roundabouts in California

Critical headway and follow-up headway are two important parameters to perform design and operational analyses at a roundabout. This paper addresses drivers’ gap-acceptance behavior characteristics at roundabouts and presents the results of critical headway and follow-up headway measurements at seven single-lane and three multilane roundabouts in California. Results indicate that the mean critical headway is consistent with the values recommended by the newly completed research NCHRP 3-65; however, the follow-up headway is significantly lower than that recommended by NCHRP 3-65. This paper also reveals that circulating flow rate and speed are two major factors affecting critical headway and follow-up headway. Results from this study enhance the existing database related to drivers’ gap-acceptance behavior at U.S. roundabouts.

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.

Green Extension and Traffic Detection Schemes at Signalized Intersections

This paper provides analyses of green extensions associated with two vehicle detection schemes for actuated signal control: the current single-channel detection and the emerging lane-by-lane detection. The current single-channel detection has all detectors across all lanes on a particular approach providing a single input to a signal phase. Lane-by-lane detection, however, monitors headways and gaps on a lane-by-lane basis. A simulation model was developed to analyze both detection schemes. With the simulation model, green extensions by the two detection schemes were compared over a wide range of traffic scenarios. On the basis of study results, it was found that the two detection schemes do not produce significantly different green extensions under normal traffic flow conditions. For the various factors examined, maximum allowable headway (also passage time) is found to be more sensitive compared with other factors such as arrival headway patterns and lane volume distribution. Although the difference between the two detection schemes in average green extension is generally minimal, large differences do exist among certain cycles, and the actual impact on signal operations could be more significant; this would need further evaluation with other standard traffic simulation models.

Evaluating the Operational Impact of Left-Turn Waiting Areas at Signalized Intersections in China

Transportation professionals in China have started using left-turn waiting areas and lagging left-turn phasing to improve traffic efficiency at signalized intersections. Left-turn waiting areas are usually set up beyond the stop bar at an exclusive left-turn lane or dual left-turn lanes. This design allows left-turning vehicles to enter and wait at the left-turn waiting area after the initiation of the through phase. The operational impact of left-turn waiting areas at signalized intersections was evaluated, and the arrival and departure processes of left-turning vehicles at exclusive left-turn lanes with and without waiting areas were compared. A procedure for estimating the capacity of an exclusive left-turn lane with a waiting area was proposed. Cross-sectional analysis used data collected from 12 approaches at nine signalized intersections to compare the start-up lost time and saturation headways of left-turn passenger cars for four scenarios. The capacities of exclusive left-turn lanes with differently sized left-turn waiting areas were compared. The results indicate that left-turn waiting areas increase the capacity of exclusive left-turn lanes and that the capacity gains would increase with an increase in the storage capacity of the left-turn waiting area.

Effectiveness of Lead–Lag Phasing on Progression Bandwidth

Quantitative assessments are provided on two signal timing issues related to progression bandwidth maximization: the effectiveness of using lead–lag phasing and the effect of the number of signals on progression bandwidth. A computer program was developed to generate multiple signal system scenarios randomly and to provide maximum bandwidth solutions. The randomly generated signal system scenarios represented various signal systems likely to be seen in the real world. On the basis of these randomly generated system scenarios and their associated maximum bandwidth solutions, conclusions were drawn regarding the two issues. Lead–lag phasing had a significant advantage over the leading-left-turn and lagging-left-turn phasing schemes to provide maximum bandwidth solutions. At any signal in a system, lead–lag phasing was used in more than 70% of the cases compared, with about 20% for leading-left-turn phasing and 10% for lagging-left-turn phasing. The number of signals had a profound impact on bandwidth attainability, exhibiting a nonlinear decline in attainability and bandwidth with an increasing number of signals in a system.

SIMULATION-BASED STUDY OF TRAFFIC OPERATIONAL CHARACTERISTICS AT ALL-WAY-STOP-CONTROLLED INTERSECTIONS

Traffic operational characteristics at all-way-stop-controlled intersections were investigated by using AWSIM, a microscopic simulation model. The effects of vehicle arrival distribution and traffic volume split on intersection operations were studied. Traffic operations were analyzed from delay, capacity, and queue length perspectives. Empirical models were developed on the basis of simulation results for delay and queue length estimations. It was found that longer delays and queues resulted from platoon arrivals. Higher intersection capacity or lower control delay can be achieved with even volume splits on all the approaches. A generalized form of the delay model showed a better correlation compared with those for exponential-form models used by other studies. A nonlinear relationship was found to exist between the 95th percentile queue length and the average queue length. An empirical model was developed on the basis of the simulation results for estimation of the 95th percentile queue lengths. The model proves to be reliable and easy to use on the basis of field studies, and it fulfills one of the major shortcomings in queue estimation, which is currently unavailable in the Highway Capacity Manual.

Effects of Left-Turn Waiting Areas on Capacity and Level of Service of Signalized Intersections

Transportation professionals in China have started using an unconventional left-turn waiting area design as an innovative approach to mitigate traffic congestion at signalized intersections. The waiting area is set up beyond the stop bar at single or dual left-turn lanes at signalized intersections with lagging protected left-turn phases. This paper evaluated the effects of left-turn waiting areas on capacity and level of service of exclusive left-turn lanes at signalized intersections using empirical data. Capacity and delay models were developed by analyzing the arrival and discharge patterns of left-turning vehicles at single and dual left-turn lanes with differently sized left-turn waiting areas. The saturation headway, start-up lost time, and clearance time for left-turning passenger cars were estimated with field data measured from 22 sites. The capacity and delay models were calibrated and validated against both field-measured and simulated data. The results of sensitivity analyses showed that left-turn waiting areas increase the capacity of left-turn lanes at signalized intersections, and the capacity gains will increase with an increase in the storage capacity of the left-turn waiting areas. A procedure was proposed to determine the level of service of left-turn lanes with left-turn waiting areas at signalized intersections.

Capacities of Unsignalized Intersections Under Mixed Vehicular and Nonmotorized Traffic Conditions

Unsignalized intersections consist of three types–-two-way stop-controlled, all-way stop-controlled, and uncontrolled intersections–- all with different priority relationships between traffic movements according to traffic laws. A conflict technique method was used to develop capacity models for the three types of unsignalized intersections under mixed traffic conditions involving vehicular, bicycle, and pedestrian movements. With field data collected from several unsignalized intersections, the model parameters were calibrated by a comparison analysis of traffic conditions in China and were modified on the basis of actual traffic conditions. The capacities obtained by the proposed models matched well with the observed capacities and the capacities calculated by conventional methods, both of which verified the effectiveness of the proposed models. The models proved to be valuable tools for determining capacities of vehicular movements at unsignalized intersections.

Models for Quantitative Assessment of Video Detection System Impacts on Signalized Intersection Operations

Various models are presented for quantitative assessment of the impacts of video detection system applications at signalized intersections. The models are developed mainly to address the occlusion issue, one of the unavoidable phenomena associated with video detection systems. Two types of occlusion scenarios and their potential impacts on intersection operations are analyzed on the basis of typical parameter values and detection setup. Also addressed are the limitations of video detection systems in providing advance detection. Occlusion in video detection systems can result in missing detections, false detections, and increased detector presence time and thus may affect intersection operations under actuated control. It is found that missing detections due to occlusion to the following vehicles are generally less than 5% when the approach volume is less than 600 vphpl and the proportion of trucks is less than 5%. At this traffic volume level, additional phase extension time caused by occlusion is generally less than 3 s. To minimize false detections due to occlusion to adjacent lanes, the horizontal offset between the camera and the travel lane should be at the minimum, with an ideal mast-arm mounting and positioning to the division line between the lanes. Because of limitations on the achievable camera height and mounting angle, it is found to be difficult to use one camera to satisfy the required advance detection for speeds over 50 mph. The study does not address the impacts of physical limits of video detection systems such as pixel size, grayscale depth, lighting, and shadows.