Alternative Intersection Designs Research Articles

Models for quick evaluation of displaced right turn intersection performance

ABSTRACT Alternative intersection designs can provide cost-effective solutions to overcome the proven inadequacy of conventional approaches. Several studies have assessed the performance of alternative designs against a range of traffic volumes and geometric design aspects, each in isolation, but a model which can factor in multiple variables into the analysis is the identified research gap. The displaced left-turn – DLT intersection design was found to be the most versatile, efficient, and transferable to locations elsewhere in the world. In this paper, a displaced right-turn intersection – a variant of DLT, was modelled for a range of traffic flows and design conditions. Regression models were developed for Practical Reserve Capacity and Delay as dependent variables with traffic flow, proportion of right-turning traffic, signal cycle time and length of displaced turn as explanatory variables. These models can provide relatively quick preliminary estimates of the performance indicators before committing to resource-consuming junction remodelling works.

Can turbo-roundabouts and restricted crossing U-Turn be effective solutions for urban three-leg intersections?

This research explored the potential of replacing conventional single-lane roundabouts by turbo-roundabouts and yield- and stop-controlled restricted crossing U-Turn (RCUT) at intersections located in urban corridors. A simulation approach based on a well-integrated assessment of traffic performance, emissions and driving volatility indicators of alternative intersection designs (AIDs) is therefore a major contribution of the study. The paper also addressed the impacts that several locations of the U-Turn crossovers have on travel and idling times, as well as on carbon dioxide and nitrogen oxides emissions. Traffic, pedestrian, and cyclist flow data were collected from two urban three-leg single-lane roundabouts. A microscopic simulation platform of traffic and emissions (respectively, VISSIM and vehicle specific power) was used to evaluate intersection-specific design and operations. The results indicated that turbo-roundabout and yield-controlled RCUT outperformed the existing single-lane roundabout at the two sites. Turbo-roundabout generally yielded the lowest travel times and emissions compared to the other intersections. The Yield-RCUT also performed better than the single-lane roundabout for U-Turn crossover located 100–170 m from the main intersection. Our findings bring a solid basis for academic research and transportation planners to promptly consider the implementation of AIDs and contribute to sustainable mobility in cities.

Safety Performance of Unsignalized Median U-Turn Intersections

Alternative intersection designs can offer safety and operational benefits with potentially lower costs than conventional intersections when implemented in the proper setting. The Federal Highway Administration has previously identified a subset of alternative designs called reduced left-turn conflict intersections as a proven safety countermeasure. Median U-turn intersections (also known as “Michigan lefts” or “boulevard turnarounds”) are one such design that accommodates all left-turn movements via directional U-turn crossovers within the median. Prior work has consistently shown that median U-turn intersections can provide superior safety performance when used in the appropriate conditions. However, research that is specific to unsignalized reduced left-turn conflict intersections continues to be limited to work conducted before the Highway Safety Manual, or which includes restricted crossing U-turn intersections. This study included the evaluation of historical traffic crashes and volume data at 95 unsignalized intersections in the state of Michigan. This included the collection of data for 39 median U-turn sites and 56 reference group sites to estimate safety performance functions and crash modification factors that can be used when considering future conversions. Ultimately, crash modification factors for fatal and injury crashes of 0.438 and 0.686 are recommended when converting intersections with undivided two-lane two-way major approaches and four-lane divided boulevard major approaches, respectively. Although there was no significant difference in property damage only crashes associated with converting intersections with undivided, two-lane, two-way major approaches, a crash modification factor of 1.325 is recommended for property damage only crashes specific to conversions with four-lane, divided boulevard major approaches.

Platooning-based trajectory planning of connected and autonomous vehicles at superstreets

ABSTRACT Alternative intersection designs are popular existing strategies that can be used for handling heavy traffic in the U.S., and research on the performance of CAVs in the alternative intersection design can complement our knowledge of the impact of CAVs. Hence, this research attempts to mitigate this research gap through a simulation-based study on a superstreet, one of the popular alternative intersection designs. A real-world superstreet is selected for the simulation-based study with collected traffic volumes and average speeds. HDVs are modeled using Wiedemann 99 with calibrated parameters while CAVs are modeled using the intelligent driver model (IDM). Platooning and trajectory planning capabilities of CAVs are modeled in the designed simulation. The simulation results show that the proposed framework for CAVs can successfully reduce fuel consumption in different market penetration rates and traffic scales.

Operational Performance Comparison between Conventional Intersections and Two Unconventional Alternative Intersection Designs (UAIDs) Under Heterogeneous Traffic Conditions in Cairo, Egypt

Although the wave of valuable research works and the numerous efforts done by researchers focused on studying, analyzing and evaluating the different types of UAIDs relying on the homogenous traffic conditions, the heterogeneous traffic conditions still underestimated. Thus, the driving force of this study is to investigate the UAIDs applicability under the heterogeneous traffic complexities. These complexities are characterized by the diversity of some static and dynamic properties of vehicles, and the aggressive driving behavior which results in non-lane based traffic systems. Hence, the purpose of this article is to compare the operational efficiency of existing conventional signalized intersections with two proposed UAIDs schemes namely; the Displaced Left-Turn (DLT) intersection and Superstreet Median (SSM) intersection. As a realistic case study of such traffic conditions, three existing intersections in an arterial corridor in Cairo, Egypt were selected. The microsimulation software VISSIM, as a time-step, stochastic and behavior based model was utilized to accomplish this study objective. The simulation results emphasized the outperformance of the proposed UAIDs over the conventional counterparts. The results indicated to the obvious improvement of the level of service (LOS) of the studied intersections. It was found that the proposed UAIDs schemes reduced the overall delay and the total travel time while the average speed was increased. On the other hand, it was concluded that the heterogeneous traffic influenced the proposed UAIDs efficiency. The main findings of this research may provide a guidance on presenting UAIDs in the developing cities.

Computation and Assessment of Environmental Emissions Resulting from Traffic Operations at Roundabouts

Now recognised as a dangerous side effect of increasing human activity and particularly the use of fossil fuels, is the impact on air quality, which has deteriorated considerably in recent times. In particular, the dramatic rise in road traffic has driven up the levels of air pollution in our towns and cities, leading to serious problems for both public health and the environment. To redress this, Environment Policies developed by the World Health Organisation (WHO) and European Commission (EC) have identified clean air as an essential requirement to human health and for the environment – a human right. Therefore, the need for managing emissions resulting from road traffic has seen significant attention. Coincidentally, the increases in the last decades of computational ability has enabled rapid development of transport modelling tools that can be integrated with environmental emissions packages. This has led to the increased ability to test the impact of road emissions on public health and environment. The computational ability to analyse the impact of traffic conditions for both current and forecast operations has enabled engineers to identify improved management and operation techniques for the road intersections with a view to reduce the emissions’ impact. This study presents a new and innovative approach to inform such forecast emissions, representing an approach to develop designs that can reduce emissions by linking various software packages as demonstrated at a trial site at a road intersection at Bath, United Kingdom. The technique linked the transport micro-simulation tool VISSIM with the environmental software EnViVer for emissions analysis. The models are also linked to Geographic Information System software, QGIS, to display changing levels of NOx, CO 2 and PM (Particulate Matter) under alternative intersection operational management regimes. This approach has enabled three important outcomes: 1) development of a methodology to directly test emissions and inform air quality thresholds using a series of software tools in an integrated manner, 2) enable the investigation of the air quality outcomes for alternative intersection designs and 3) to produce functions for intersection operational parameters that can predict NO X , CO 2 and PM savings. The results have demonstrated that the linking of the computational ability of micro-simulation modelling of road intersections with environmental package has been successful in analysis of operations. The ‘combined’ modelling approach has enabled design development of future intersection operation to with a view to minimising NO X , CO 2 and PM. This approach has brought together the crucial traffic parameters of road speed, delay and queuing and related them directly to change in emissions formulation, with the evidenced based prospect of adoption for similar studies in future. To put numbers to the achievement, the results have shown significant achievements for both the NO X level, which is projected to reduce by 14-34%, CO 2 by over 13-32% and PM by over 14-26% compared with the base conditions.

A Capacity Estimation Model for a Contraflow Left-Turn Pocket Lane at Signalized Intersections

Alternative intersection designs are increasingly proposed and adopted by different agencies to meet the needs of growing traffic demand and constrained transportation resources. The left turn (LT) is one of the most critical movements at signalized intersections from both a safety and operations perspective. Heavy LT volumes are especially impactful to the operational efficiency of a signalized intersection and often result in queue spillback. A contraflow left-turn pocket lane (CLPL) is proposed to mitigate congestion caused by heavy LT demand and has been shown in simulation to greatly mitigate the impact of queue spillback. The CLPL dynamically uses the opposing through lane (OTL) as an additional LT lane within the signal cycle on a temporary basis when the OTL is not occupied by through traffic. While geometric design schematics and analytical procedures for estimating delay have been proposed and discussed in existing literature, methodologies for estimating capacity benefits and traffic operations are not yet well defined. This paper has three primary contributions to the literature: development of a probabilistic capacity estimation model, exploration of the impact of key characteristics (e.g., cycle length, LT demand, lane selection preference) on estimated intersection capacity, and recommendations for the real-world implementation of a CLPL. The simulation results indicate that the CLPL treatment can increase a signalized intersection’s throughput up to 25% and decrease the intersection’s average delay by 35%.

Impact of Including Exiting Vehicles in Single-Lane Roundabout Capacity Models

Roundabouts are classified as alternative intersection forms because they provide operational conditions not found at conventional intersections (e.g., a low-speed environment and fewer possible vehicle conflicts). Such conditions provide a combination of safety and operational benefit that normally is not achievable at a conventional intersection. The ability to predict capacity accurately is important when alternative intersection designs are examined. Highway Capacity Manual 2010 provides a current method for determining the capacity of a roundabout approach in which vehicles that exit on the same approach are considered not to influence capacity. However, the inclusion of exiting vehicles in the model creates a different circulating vehicle gap distribution (dividing larger gaps into smaller gaps) that reduces the number of opportunities to measure follow-up headway. Also, most critical headway values for the analysis are smaller than the critical headway values found when exiting vehicles are excluded. Capacity equations were developed from video imagery recorded for 28 approaches at 13 roundabouts, and the impact of including exiting vehicles in the analysis of single-lane roundabout capacity was evaluated. Vehicles exiting the roundabout were found to have a measurable impact on the estimated capacity of the roundabout approach. Both follow-up and critical headway values decreased when exiting vehicles were included. Also, the estimated capacity including exiting vehicles was found to increase or decrease (relative to capacity estimates that excluded exiting vehicles) depending on the percentage of conflicting vehicles that were exiting vehicles.

An exploration of alternative intersection designs in the context of Safe System

Fatal and serious injury crashes persist at intersections despite current efforts to address this. Little research specifically investigates the role played by existing intersection design in perpetuating serious intersection crash outcomes despite an increasing move to incorporate Safe System design on to roads. This paper identifies design principles deemed important to align intersection design with Safe System approaches, including exploring the impact of speed and angle on overall kinetic energy of a crash. Existing as well as new intersection designs are presented that are believed to incorporate the identified principles. An assessment is made of the alignment of the new and existing designs with the identified principles.

Reduced-Conflict Intersection

The selection of the appropriate geometry and traffic control for intersections on high-speed arterial highways often requires balancing increased highway user costs with attempts to control construction costs. Providing a safe design that is cost-effective is always a primary consideration in deciding on a solution. Another issue is that one form of intersection and traffic control may be optimal during peak traffic times on an average day but may not have sufficient capacity for extreme peak times such as those for events or on recreational routes. Alternatively, those same controls can cause needless delay when traffic is lighter. Examples include traffic signals that operate 24 h/day and roundabouts that require all traffic to slow dramatically, regardless of the amount of conflicting traffic. This study presents a group of new alternative intersection designs designated as reduced-conflict intersections. This study describes those designs and their associated traffic control, which consists of a demand-based traffic signal system. This study then presents the results of a comparative analysis of this new type of intersection along with other designs, including a conventional intersection with signal control, two forms of roundabouts, and the super street intersection. The results of the study show that the reduced-conflict intersection may be an effective addition to the toolbox of intersection designs.

Development of safety-based level-of-service criteria for isolated-signalized intersections: Ha, Tae-Jun, Ph.D. The University of Wisconsin-Madison, 1994. 359 pp. Supervisor: William D. Berg

The Highway Capacity Manual specifies procedures for evaluating intersection performance in terms of delay per vehicle. What is lacking in the current methodology is a comparable quantitative procedure for assessing the safety-based level of service (LOS) provided to motorists. The objective of the research was to develop a computational procedure for evaluating the safety-based LOS of signalized intersections based on the relative hazard of alternative intersection designs and signal timing plans. Conflict opportunity models were derived for those crossing, diverging, and stopping maneuvers associated with left-turn and rear-end accidents. Safety-based LOS criteria were then defined on the basis of distribution of conflict opportunities computed from the developed models. A case study evaluation of the LOS analysis methodology revealed that the developed safety-based criteria were not as sensitive to changes in prevailing traffic, roadway, and signal timing conditions as the traditional delay-based measure. However, the methodology did permit a quantitative assessment of the tradeoff between delay reduction and safety improvement. The results of the research are considered to be of an exploratory nature.