Diverging diamond interchanges (DDIs) are widely recognized to be capable of reducing conflict points, number of stops, and consequently average traffic delay. However, the design of their crossover spacing and signal offsets, which is critical to the capacity and efficient operations of DDIs, have not been addressed in most design guidelines. These two critical design components are actually interdependent in nature, because the estimated travel time between a DDI’s two subintersections for all movement paths is essential for the design of signal offsets. Also, the crossover spacing should be designed to accommodate queues comprised mostly of those vehicles not moving within the signal progression band, which is often designed with a given crossover spacing. Considering such an interdependent relation between signal offsets and the crossover spacing, this study presents a model that can concurrently optimize these two vital DDI design elements at the planning level. A case study at a DDI site with the proposed model has also been conducted to justify the necessity to perform the concurrent optimization under different operational conditions. The results of extensive numerical experiments confirm that the design with the optimized crossover spacing and offset can yield the shortest total delay and the least number of stops for vehicles over the entire network, especially under near-saturated conditions. The optimized crossover spacing can also prevent the formation of the queue spillover over the crossovers in a DDI.

Design of signal progression for arterials accommodating significant transit and passenger car flows is a challenging task, as its effectiveness with state-of-the-art methodologies often degrades with the increasing percentage of bus flows due to the inevitable mutual impedance in the mixed traffic. Conceivably, bus flows, prior to reaching a link’s downstream intersection, may first experience excessive delays incurred by passenger-car queues before arriving or dwelling at a roadside stop. Those dwelling bus flows will in turn constitute a temporary lane-blockage, causing passenger cars, especially those in the rightmost lane, either to perform lane changes to interfere with neighboring traffic or to endure the excessive delay. The recently-emerged innovative design methodologies producing a bus-based signal progression system or dual-modal progression band, despite their effectiveness at the experimental level, are difficult to be supported by responsible agencies in practice unless with the policy mandate of promoting transit usage. Hence, this study proposes an alternative to facilitate bus flows by incorporating the bus delay minimization in the progression design for passenger-car flows, using the optimized offsets, phase sequences, and link progression speed. The proposed model explicitly accounts for all types of bus delays under various traffic scenarios constituting the bus entry time, bus stop location, and the patterns of passenger-car queues. The results from the extensive numerical experiments confirm that the proposed model can generate a bus-friendly signal progression plan for a wide range of mixed bus and passenger car traffic scenarios.

On-ramp merging areas are congestion-prone segments of freeways. Depending on the aggressiveness of the driving population and the congestion level, the speed variance among travel lanes due to lane changes and ramp-merging flows may be so significant as to affect the optimal settings of deployed traffic control systems, such as metering rates or advisory speed limits. Extending from METANET, this study presents a lane group–based (LGB) traffic model to reflect the temporal and spatial distributions of traffic conditions among lane groups. The proposed model would allow traffic engineers to reliably assess the impacts of lane-changing activities in both upstream and downstream segments of an on-ramp area and better design their coordinated control strategies. To assess the effectiveness of the proposed model, this study has compared its performance with METANET under various traffic scenarios. The comparison results show that the proposed model can yield up to 26.9% improvement on the accuracy of predicting the temporal and spatial evolution of a freeway’s speed at the interchange area where freeway segments often experience extensive lane-changing activities due to on-ramp merging flows.

Despite the abundance of studies on signal progression for arterial roads, most existing models for bandwidth maximization cannot concurrently ensure that the resulting delays will be at a desirable level, especially for urban arterials accommodating high turning volume at some major intersections or constrained by limited turning bay length. Extending from those models that aim to address delay minimization in the progression design, this study provides two enhanced progression maximization models for arterials with high turning volumes. The first model aims to select the signal plan that can produce the lowest total signal delays for all movements from the set of non-inferior offsets produced by MAXBAND. Failing to address the impact of potential turning bay spillback at some critical intersections under such a design may significantly degrade the quality of through progression and increase the overall delay. For this reason, the second model proposed in this study offers the flexibility to trade the progression bandwidths within a pre-specified level for the target delay reduction, especially for turning traffic. The evaluation results from both numerical analyses and simulation experiments have shown that both proposed models can produce the desirable level of performance when compared with the two benchmark models, MAXBAND and TRANSYT 16. The second model yielded the lowest average network delay of 117.2 seconds per vehicle (s/veh), compared with 121.7 s/veh with TRANSYT. Moreover, even its average delay of 141.8 s/veh for through vehicles is comparable with that of 141.2 s/veh by MAXBAND, which is designed mainly to benefit through-traffic flows.