Roelof Engelbrecht

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.

Validation of Generalized Delay Model for Oversaturated Conditions

With today’s ever-increasing traffic demand, more and more signalized intersections are experiencing congestion for longer periods of time. To better quantify oversaturated conditions, it is necessary to accurately estimate oversaturation delay. The generalized delay model, proposed for inclusion in the next update of the U.S. Highway Capacity Manual (HCM), is introduced here. The generalized delay model differs from the model in the 1994 edition of the HCM as it is sensitive to the duration of the analysis period and is not restricted to degrees of saturation less than 1.2. The TRAF-NETSIM microscopic simulation model was used to verify the generalized delay equation for oversaturated conditions. A simulation model was used, because it is extremely difficult to measure oversaturated delay in the field. The study was designed to cover as much of the domain of oversaturated traffic operations as possible. The variability in simulated delays was investigated, and an equation was developed to predict the standard deviation of oversaturated delay estimates. It was found that delays estimated by the proposed generalized delay model are in close agreement with those simulated by TRAF-NETSIM. On average, simulated delays are overestimated slightly, but the error is small compared with actual delays. The proposed generalized delay model is expected to provide a good estimate of actual oversaturation delays that occur in the field.

Warranting Traffic Signals on the Basis of Proximity of Railroad Grade Crossings

There were two concurrent paths in this research. Path 1 estimated the probability, pvehicle/train, that a vehicle will be in the train dynamic envelope given that a train arrives. This was done by an extensive simulation effort that included some data collection on relevant gap acceptance patterns. Path 2 identified the exposure factors, selected a risk metric, and established a target or threshold for the risk, which is expressed as an upper bound on pvehicle/train. As a result of the investigation, the warrant is based on the principle that the expected fatality rate at the crossing should not exceed rates found at unsignalized intersections. The two paths converged by expressing the constraint pvehicle/train ≤ ___ (where ___ is in terms of traffic terms in common use—e.g., volumes, composition, track location distances). This constraint led to the warrant, which was then validated with data acquired or collected for the purpose. The draft warrant was prepared as a result of the undertaking, was endorsed by the National Committee on Uniform Traffic Control Devices, and is currently in the formal review process, culminating in final edits and a decision on inclusion in the next edition of the Manual on Uniform Traffic Control Devices.