Single-point Urban Interchange Research Articles

Traffic Evaluation of SPUI and TUDI Interchanges Based on Traffic Conditions in Tehran with Aimsun and Synchro Software

There are various interchanges with specific features and applications. The single-point urban interchange (SPUI) and tight urban diamond interchange (TUDI) are diamond and used in areas with limited right of way, and they have not been used in Iran so far. The choice between these two very similar interchanges is challenging. It is difficult for transportation engineers to choose between them without being aware of their right performance. This study aimed at measuring the efficiency of these interchanges using five measures of effectiveness for having more efficient interchanges in specific traffic conditions. The Synchro 9, Sim Traffic 9 and AIMSUN 6 software and calibrated values have been used for simulation for Tehran. Parameters such as vehicle specifications, give way time, lost time and saturation flow rates were calibrated using the calibration values based on the conditions in Tehran. The traffic data of NCHRP 3-60 report were used. Twelve different traffic scenarios which are covering different volumes were used for simulation. First, the least time of scenario cycle was calculated using Webster’s formula, and then, each scenario phase was optimized using Synchro software, and optimized scenarios were simulated using AIMSUN software. The results indicated that the SPUI interchange had lower delay, stop time, travel time and density while its average speed was higher. In general, by increasing traffic volume, the performance differences increase between the SPUI and TUDI interchanges in different parameters.

A Framework for Prioritizing Urban Arterial/Freeway Interchange Types using Multi-Criteria Analysis

An interchange is a road junction that typically uses grade separation to accommodate different volumes of traffic safely and efficiently through interconnecting roads. There are many types of interchanges, each of which has different characteristics and applications. In this paper, a prioritization framework is proposed for selecting a suitable interchange type based on a variety of performance measures, including operational performance, socio-environment, safety, and cost. The assessment is based on a multi-criteria analysis (MCA) approach. The relative importance of the criteria is obtained using the Analytic Hierarchy Process (AHP) whereby the weights of the criteria are derived using multilevel hierarchic structures. Consequently, using a linear additive model, each alternative is evaluated on a set of criteria derived from several objectives. This paper illustrates the implementation of the proposed framework in a case study of an urban interchange in the city of Riyadh. This research study considers three interchanges in the analysis to demonstrate the procedure; other interchange forms may also be appropriate at the site used in the study. This research helps in defining the criteria that play a significant role in determining the preferred design alternative, and the extent to which each of these criteria has an effect on the overall priority of a certain interchange configuration. The results of the study showed that the Diverging Diamond Interchange (DDI) outperformed its interchange counterparts and was chosen as the most preferred interchange configuration. The Single Point Urban Interchange (SPUI) ranked second while the Tight Urban Diamond (TUDI) was least preferred. Sensitivity analysis was undertaken to determine the impact of changes in criteria scoring or weights on the overall results, and revealed the robustness of the proposed model.

Maintenance of Traffic for Innovative Geometric Design Work Zones

In an effort to improve the safety and capacity of existing roads, many transportation practitioners are implementing innovative designs at intersections and interchanges. The development of construction phasing plans for these projects is a critical component for maintaining safety and mobility on the facility during construction. The goal of this study was to address the gaps in existing knowledge by presenting the state of the practice and providing guidance for transportation practitioners in developing construction phasing and maintenance of traffic (MOT) plans for projects with innovative geometric designs. Several types of innovative geometric designs were studied, including the round about, single-point urban interchange, diverging diamond interchange, restricted-crossing left turn, median U-turn, and displaced left turn. Examples of MOT phasing diagrams showing phasing sequencing and construction work areas were developed on the basis of a review of the literature, a survey of practitioners, interviews with industry experts, and a review of actual project plans. The examples of MOT phasing diagrams are intended to serve as a starting point for transportation practitioners, but project-specific factors such as driver experience, availability of detours, traffic counts, adjacent land use, elevation differences, barrier offsets, number of lanes, and anticipated impacts of a possible closure should be considered when deciding on the best MOT methods for a given project.

A heterogeneity-in-means count model for evaluating the effects of interchange type on heterogeneous influences of interstate geometrics on crash frequencies

This paper presents a heterogeneity-in-means, random-parameter negative binomial (HMRPNB) model of interchange type effects on interchange and non-interchange segment crash frequencies. For non-interchange segments, upstream and downstream type combinations were evaluated. Eight interchange types, namely, directional, semidirectional, clover, partclover, diamond, part diamond, single-point-urban-interchange (SPUI), and other were studied on the Washington State interstate system. A total of 575 interchange and 578 non-interchange segments were analyzed for the period 1999–2007. In interchange segments, semidirectional, partclover and other (excluding directional, diamond, SPUI, or fullclover) types significantly contributed to heterogeneity in the random parameter effects of average daily traffic, median continuous lighting proportion, minimum and maximum vertical gradients. Full and partial diamond types contributed to heterogeneity in the random parameter effects of median continuous lighting proportion, maximum horizontal degree of curvature, minimum and maximum vertical gradients. In non-interchange segments, the upstream type set including directional, semidirectional, and clover/collector-distributor type, and downstream set of directional, semidirectional, diamond, partclover, partdiamond and other type significantly contributed to heterogeneity in the random parameter means of total length of adjacent interchanges, two foot left shoulder width and two foot right shoulder width proportions. Statistically significant fixed parametric effects included urban/rural location, right continuous lighting proportion, proportion by length of three lane cross section, four lane cross section, three-to-four foot left shoulder, five-to-nine foot left shoulder, ten foot left shoulder, three-to-four foot right shoulder, five-to-nine foot right shoulder, and ten foot right shoulder, as well as number of horizontal curves in segment, and shortest horizontal curve length.

Evaluation of Traditional and Nontraditional Interchange Treatments to Preserve Service Life of Narrow Over-and Underpass Roadways

This paper presents key findings from an evaluation of the potential of traditional and nontraditional interchange treatments to postpone or eliminate the need for overpass or underpass widening. The study compares the relative performance of five traditional and five nontraditional interchange treatments at two common sites. The treatments included traditional widening, single-point urban interchange, tight urban diamond, Texas three-phase (TX-3), Texas four-phase (TX-4), tight urban diamond with reversing lanes, double roundabout, diverging diamond, median U-turn, and superstreet. Although the performance of each treatment will always vary with site-specific geometric and volume characteristics, some general trends were apparent across sites. The TX-3 and TX-4 treatments were found to be excellent short- to medium-term performers, especially considering their low cost for implementation. The tight urban diamond with reversing lanes and the diverging diamond treatments were found to be reasonable medium- to long-term performers, although each was better suited to particular traffic patterns. The traditional widening and single-point urban interchange treatments were the best long-term performers, with the single-point urban interchange providing the best performance overall.

Two-Level Signalized Intersection

Severe traffic congestion often occurs at intersections of high-volume traffic arteries. To reduce this congestion, innovative intersection designs such as the single-point urban interchange (SPUI), center-turn overpass (CTO), and echelon interchange (EI) have been implemented. This paper introduces a new type of intersection, the two-level signalized intersection (TLSI), which improves the efficiency of intersection operations under a wide variety of traffic conditions. The paper compares the TLSI with other innovative intersection types. Unlike the SPUI, CTO, or EI, the TLSI provides for complete separation of east–west and north–south traffic. The TLSI also enables the use of directional separation and leading, lagging, or overlapping lefts on both upper and lower levels. Simulation results indicate that, compared with these other innovative intersection types, the TLSI has the shortest delay times in most evaluation scenarios as well as the least sensitivity to variations in traffic volume. However, the TLSI shows significant delay when traffic volumes on the major and minor roads are vastly different.

Comparison of the New W-lnterchange with Conventional Interchanges

Congestion is a problem at many urban interchanges, particularly at diamond interchanges. A new interchange design, called the W-interchange, seeks to reduce congestion at a reasonable cost. Similar to the Michigan urban diamond interchange (MUDI), which uses U-turn structures, the W-interchange attempts to reduce delay by eliminating the left-turn movements onto and off of the cross street. The new W-interchange was compared with other, more commonly used designs for freeway-to-arterial interchanges, specifically the diamond interchange, the single point urban interchange (SPUI), and the MUDI. A major portion of the effort was a comparison of the travel times and stops across a range of traffic demands using a microscopic traffic simulation package. The results from the simulations indicated that the W-interchange minimized the number of stops and produced an overall travel time that was lower than the diamond interchange and comparable with the MUDI. The W-interchange had lower travel times than the SPUI for a few of the cases simulated, but generally the SPUI had lower travel times. A design of the W-interchange was created to estimate its construction and right-of-way cost. At about $14 million, the W-interchange costs about $4 million more than the SPUI and about $6 million more than the diamond interchange. Overall, the W-interchange performed well enough in the simulations and has enough other advantages to justify further research in the areas of cost, signing and marking, continuous frontage road operation, and a possible freeway-to-freeway design.

Technique for Comparing Operation of Alternative Interchange Types

Interchanges have been the focus of several recent studies that attempted to identify the conditions in which one interchange type is more efficient than another. However, the guidance offered is of such a general nature that it is difficult to apply with confidence in a specific situation. The objective of this study was to describe an interchange evaluation technique that is of sufficient precision to facilitate interchange-type selection or planning-level evaluations of interchange performance. This technique was based on the use of characteristic curves that define the relationship between interchange delay and the sum-of-critical-flow-ratios. This sum represents a unique parameter that combines the effect of signal phase sequence, traffic volume, number of lanes, and saturation flow rate. A family of characteristic curves was developed and used to illustrate the interchange evaluation technique. The curves describe the relationship between delay and the sum-of-critical-flow-ratios for the single-point urban interchange (SPUI) and the tight urban diamond interchange (TUDI), both serving frontage-road systems. For the range of conditions considered, it was concluded that the TUDI will generally operate with less delay than the SPUI.

Guidelines for Selecting Single-Point Urban and Diamond Interchanges: Nationwide Survey and Literature Review

Metropolitan areas in the United States are experiencing a steep increase in traffic growth and traffic congestion on their urban freeways. In the past, the most prevalent type of urban interchange has been the diamond interchange (DI). In more recent years, however, to overcome the need for significant additional right-of-way that is generally required for this type of interchange and still respond to increased traffic demand, a new type of interchange has emerged: the single-point urban interchange (SPUI). Traffic engineers do not agree about the relative operational efficiency of the SPUI compared with the DI. Also, the unfamiliar geometry and operation of the SPUI and its large, uncontrolled conflict area have raised several concerns about the safety of motorists who travel through it. The purpose of this study was to evaluate and compare the safety and operational characteristics of the SPUI and DI and to develop guidelines that identify traffic and geometric conditions that favor one over the other. We surveyed state traffic engineers across the nation about their experiences and opinions of the two interchanges in terms of operational performance and safety. A complete literature review on the subject revealed significant findings as well. We used both of these sources of information to develop guidelines to aid traffic engineers in selecting and designing the appropriate type of interchange.

Field Analysis of Operation and Design of Single-Point Urban Interchanges

Michigan is considering the much needed rehabilitation and upgrading of many of the freeway interchanges located in urban environments. The single-point urban interchange (SPUI) has been suggested as an alternative to designs presently used, in particular the Michigan urban diamond interchange. To evaluate the appropriateness of the SPUI, a literature review was conducted, along with an e-mail and telephone survey of other state departments of transportation experience with the SPUI. Then, a field review of selected SPUIs was conducted to collect observations about the geometric design, signal operation, pedestrian control, pavement markings, and land use and landscaping of SPUIs. A SPUI with the crossroad going over the freeway was found to be a preferred design. In addition, the smaller designs were observed to function the best. However, the use of continuous frontage roads is believed to counteract the advantages of the design. Currently, progression of the crossroad is not a problem, although the impact of the SPUI on intersections downstream must be considered. Additionally, the need for pavement markings in large SPUIs is paramount. However, these pavement markings overlap and cause driver confusion. Several other observations were also made: most SPUIs reviewed were not able to adequately provide for pedestrians, controlled access near the SPUI is suggested, and landscaping dramatically increases the aesthetics of the interchange. The SPUI is a good design and has a place. However, some of the newer and enhanced designs with the resulting increase in size may be counterproductive.

Calibrating and Validating TRAF-NETSIM Model of Single-Point Urban Interchange

A model of a single-point urban interchange (SPUI) was calibrated and validated using the traffic simulation program (TRAF-NETSIM). The model was calibrated on the basis of field observations at several existing SPUIs, and the model was validated on the basis of measures of effectiveness at a prototype interchange. Statistical analyses of several parameters were performed to determine confidence intervals and decile distributions and to compare values and distributions with defaults provided in TRAF-NETSIM. Other parameters were verified by informal field observations at the prototype interchange. The validation of the model demonstrated that TRAF-NETSIM can adequately represent traffic operations at a SPUI. Observations regarding the SPUI will provide additional insight into SPUI traffic operational characteristics. The methodology used to calibrate and validate the model of the SPUI should also be applicable to other modeling applications, ensuring a realistic and accurate representation of traffic operational conditions in a transportation system.

Bridge Size and Clearance Time of Single‐Point Urban Interchange

The single-point urban interchange (SPUI) is a relatively new type of diamond interchange. Its reported advantages are improved operational efficiency, improved safety, and reduced right-of-way requirements as compared to other interchange forms. This paper focuses on the efficiency and right-of-way requirement issues by examining the relationships among the various SPUI design elements. In particular, constraints placed on the SPUI geometry by the bridge-abutment locations and offset left-turn paths made it possible to develop a geometric model relating turn radius, cross-section width, number and width of turn lanes, and skew angle to bridge length and clearance path length. A sensitivity analysis using the geometric model developed in this paper indicates that skew angle, cross-road median width, and ramp-major road clearance distance have a significant effect on the length of the bridge and the duration of the all-red clearance interval. An illustrative application of the model indicates that it can be used to develop relationships among SPUI design elements that yield the most cost-effective design.

Change Intervals and Lost Time at Single‐Point Urban Interchanges

The single‐point urban interchange (SPUI) is a relatively new type of diamond interchange. It is claimed to offer improved operational efficiency, improved safety, and reduced right‐of‐way requirements over other interchange forms. This paper addresses the area of operational efficiency by focusing on the SPUI's change interval requirements and the implications of these requirements on the amount of lost time incurred at the end of the signal phase. Values of end lost time are calculated for the SPUI and compared to those of a typical, high‐type at‐grade intersection (AGI). End lost time calculations for the SPUIs and AGIs indicated that the SPUI signal phases have longer end lost times than do the AGI phases. In particular, the SPUI through phase was found to have 1.5‐2.5 sec more end lost time than the AGI through phase. Similarly, the SPUI left‐turn phase was found to have about 1.0‐2.0 sec more end lost time than the AGI left‐turn phase.