Interchange Ramp Terminals Research Articles

Developing Commercial Motor Vehicle Crash-Specific Safety Performance Functions at Interchange Ramp Terminals in Kentucky

Commercial motor vehicle (CMV) drivers often experience difficulties in turning and crossing at ramp terminals. CMV-involved crashes could cause queue spillback at ramp terminals and possibly nearby freeway mainline and crossroads. The safety and mobility of ramp terminals for accommodating CMVs is thus of significant importance. However, interchange ramp terminals have rarely been investigated in previous safety studies, especially with regard to CMV-involved crashes. This study aimed at examining CMV-related crashes that occurred at ramp terminals. Heterogeneous negative binomial (HTNB) and traditional negative binomial (NB) models were fitted and compared while developing a safety performance function (SPF) for predicting CMV crashes and identifying high-risk ramp terminals. Information on crash history and site-specific characteristics was collected at 285 ramp terminals in the state of Kentucky between 2015 and 2019. The results showed that the HTNB model outperformed the NB model in relation to various goodness-of-fit measures (e.g., likelihood ratio test “LRT”, Akaike information criterion “AIC” and McFadden Pseudo R-squared statistic). The predicted crash frequencies while applying the HTNB model were then used to identify and rank high-risk ramp terminals. Ramp terminals with signalized traffic control type, with greater average daily traffic on the exit ramp, with two or more traffic lanes on the exit ramp, and adjacent to commercial or industrial areas were found to experience more CMV crashes. Several safety countermeasures were proposed to alleviate CMV-involved crashes at ramp terminals. One example is ensuring the presence of physical medians on crossroads (or major roads) ahead of exits ramps.

Are Roundabouts Safe and Economically Viable Replacing Conventional Diamond Interchange Ramp Terminals?

Roundabout implementations at traditional intersections have been shown to be effective at reducing severe crashes. Roundabouts have also been implemented at interchange ramp terminals; however, limited research is available. In this study, 25 roundabout ramp terminal implementations were evaluated. The methodological approach consisted of Empirical Bayes for safety effectiveness and crash cost changes, crash type weighted distribution, crash rate analysis of bypass configuration, and cost of implementation. Roundabouts were effective at reducing fatal and injury crashes when replacing existing interchange diamond ramp terminals: 65% reduction for roundabouts replacing stop-controlled ramp terminals and 41% reduction for roundabouts replacing signal-controlled ramp terminals. Observed crash type weighted distributions are provided to visualize the frequency and location of crashes within roundabout ramp terminals for design considerations. Exit ramp and outside crossroad approaches with right-turn bypass showed significantly lower crash rates than designs without bypass. The crash cost analysis showed that roundabouts replacing diamond ramp terminals yielded crash cost savings of between $95,000 and $253,000 per site per year (69% to 54% decrease in crash costs). Considering crash costs savings only, the cost of implementation should be less than $1.9 million for a roundabout replacing a stop-controlled ramp terminal and less than $5.1 million for a roundabout replacing a signal-controlled ramp terminal to accomplish benefit-cost ratios greater than one for a service life cycle of 20 years. Costs are in 2019 dollars.

Experimental investigation of the effect of roundabouts on noise emission level from motor vehicles

Roundabouts can provide benefits in many ways. They are safer, more efficient, less costly and more aesthetically appealing than standard at-grade intersections (controlled and signalized). The administrations have often identified roundabouts as a proven safety countermeasure for the following reasons: a) reduce overall conflict points and remove left-turn conflicts; b) reduce the vehicle speeds; and c) reduce accident severity for all users, allow safer merges into circulating traffic, and provide more time for all users to detect and correct their mistakes or the mistakes of others due to lower vehicle speeds. Also, from an operational point of view, roundabouts offer the following significant advantages: a) may reduce delays and queues than other forms of controlled intersections; b) can reduce lane requirements between intersections, including bridges between interchange ramp terminals; and c) create possibility for adjacent signals to operate with more efficient cycle lengths where the roundabout replaces a signal that is setting the controlling cycle length. Furthermore, roundabouts can provide important environmental benefits reducing delays and the number and duration of stops. Even in heavy traffic, vehicles keep moving in the queue rather than coming to a complete stop. This may reduce significantly noise and air quality impact as well as fuel consumption by reducing the number of acceleration/deceleration cycles and the time spent idling. In this article, the environmental aspects associated with roundabouts are treated with reference to issues related to noise production. The experimental investigation will refer to a heterogeneous succession of intersections (standard intersections, two-way stop-controlled and roundabouts) characterized by homogeneous traffic flows. The results of the research are interesting and contribute to consolidate the awareness, already highlighted by other international studies, of the role of roundabouts as effective measures to mitigate traffic noise. © 2019 Institute of Noise Control Engineering

When driving on the left side is safe: Safety of the diverging diamond interchange ramp terminals

How safe are the ramp terminals of a diverging diamond interchange (DDI)? This paper answered this question using data from DDI sites in Missouri. First, crash prediction models for ramp terminals for different crash severities were developed. These models were then utilized in the Empirical Bayes (EB) evaluation of DDI ramp terminals. Due to inconsistencies in crash reporting for freeways in Missouri, individual crash reports were reviewed to properly identify ramp terminal crashes. A total of 13,000 crash reports were reviewed for model development and EB evaluation. The study found that the DDI ramp terminals were safer than the conventional diamond signalized terminals. The DDI ramp terminals experienced 55% fewer fatal and injury crashes, 31.4% fewer property damage only crashes, and 37.5% fewer total crashes.

Site-Specific Safety Analysis of Diverging Diamond Interchange Ramp Terminals

A site-specific analysis of the safety of diverging diamond interchange (DDI) ramp terminals was conducted for the first time. Crash modification factors (CMFs) were developed for DDI ramp terminals for different crash severity levels. The ramp terminal CMFs complement the project-level DDI CMFs developed in previous research. The study used data from 20 ramp terminals in Missouri. The safety evaluation was conducted with two before-and-after observational methods: comparison group (CG) and empirical Bayes (EB). Because of inaccurate crash locations, a systematic crash location correction process was developed. An extensive review of 8,400 individual crash reports was conducted for the calibration and safety evaluation. Both before-and-after safety evaluation methods produced consistent results. The DDI design replacing a conventional diamond decreased the numbers of ramp terminal–related crashes for all severity levels. The most significant crash reduction by method was observed for fatal and injury (FI) crashes: 73.3% (CG) and 63.4% (EB). Property damage only (PDO) crashes were reduced by 21.0% (CG) and 51.2% (EB). The total crash frequency also decreased by 42.7% (CG) and 54.0% (EB). The EB CMF values for the DDI ramp terminal were 0.366 for FI crashes, 0.488 for PDO crashes, and 0.460 for total crashes. This study serves as a case study for conducting a site-specific analysis of ramp terminals or other interchange facilities. The methodology that was used is transferable and can be used to quantify the safety effects of other innovative intersection and interchange designs.

Field Data for Evaluating 2010 Highway Capacity Manual Operational Analysis Methodology for Interchange Ramp Terminals

Interchange ramp terminals provide connections between various highway facilities (freeway–arterial, arterial–arterial, freeway–freeway, etc.), so their efficient operation is essential. NCHRP Project 3–60 developed analysis methods for evaluating the operational performance of signalized interchange ramp terminals. Those methods were based primarily on simulated data. Further research used collected field data to evaluate the interchange analysis methodology developed under NCHRP Project 3–60 to identify application limitations and to suggest improvements. Data were collected at eight sites representing various types of interchanges with different geometric and traffic characteristics. Video cameras, mounted on existing poles on the side of the road and on nearby buildings, were used to gather traffic operation data, including demand, lane utilization, and queue length at each site. The research evaluated both individual analytic model components (such as lane utilization factors and internal queue length) and the performance of the interchange (reflected in average queue lengths and control delay). Overall, the models performed well. However, large discrepancies were observed when operating conditions of the interchange were close to capacity. The queue length prediction at the beginning of the upstream arterial or ramp phase was calibrated to modify the effective green time estimation. A new lane utilization model was based on field data for a four- or five-lane external arterial approach where the two leftmost lanes were extensions of the downstream dual left turn lanes. The findings of the work were incorporated into the 2010 Highway Capacity Manual.

Some Guidelines for Selecting Microsimulation Models for Interchange Traffic Operational Analysis

There are several commercially available traffic simulation packages that are able to model the full range of interchange ramp terminals. Each of these packages has pros and cons for simulating various types of interchanges and traffic control plans. Previous research has studied specific simulators in terms of their capabilities for simulating single urban point interchanges (SPUIs) and diamond interchanges. There is no guidance provided however on how to generally select appropriate simulators based on their capabilities and internal algorithms for analyzing specific interchange design and control scenarios. This paper focuses on identifying the elements that should be available in a simulator in order to evaluate a specific interchange scenario (including type, geometry, and traffic control characteristics). The paper does not identify all the specific packages that are appropriate for a specific scenario because these evolve constantly; even though the current version of a package may not support a particular function, future versions of that package may incorporate it. Thus, identifying the most appropriate package would very quickly become obsolete. It is up to the analyst to examine whether a particular characteristic or algorithm is present in a specific package. To accomplish this, three simulators were selected and studied: AIMSUN, CORSIM, and VISSIM. Data from two interchanges (one SPUI and one diamond interchange) at Arizona were obtained and used in the assessment of these packages. The model parameters and assumptions are examined for each model. Simulation is conducted first by using default parameter values followed by a calibration process to adjust parameters related to driver behavior, vehicle performance, and others. In conclusion, several elements are identified as critical in simulating interchanges, which fall in the following categories: (1) the capability of representation of specific geometric characteristics; (2) the capability of simulating specific signal control plans; (3) calibration needs and accuracy in comparison to field conditions; (4) the extraction of specific performance measures from the simulator; and (5) other observations from the research.

Methodology for Evaluating the Operational Performance of Interchange Ramp Terminals

Interchange ramp terminals are critical components of the highway network. They provide the connection between various highway facilities (e.g., freeway–arterial and arterial–arterial), and their safe and efficient operation is essential. The objective of this research was to develop improved methods for capacity and quality-of-service analysis of interchange ramp terminals. The research focuses on at-grade intersections but not on the freeway proper. All geometric configurations and interchange types except trumpet interchanges are considered, and the scope of the research includes only signalized interchanges and not “Stop” sign–controlled interchanges and roundabouts. The development of the analytical methodology is primarily based on simulation. The reason for using simulation is that adequate samples of field data are not available, and it is prohibitively expensive to collect them for all types of interchange configurations. The research team assessed several simulation models that were identified as capable of simulating all types of interchange ramp terminals and selected the most appropriate one for model development. Once a simulation model was selected, a variety of interchange configurations were simulated, and selected measures of effectiveness were obtained. Analytical models were developed on the basis of the results of simulation to predict different measures, such as average control delay, volume-to-capacity ratio, and queue-to-storage ratio, for a variety of design and traffic control characteristics.

Methodology for Evaluating the Operational Performance of Interchange Ramp Terminals

Interchange ramp terminals are critical components of the highway network. They provide the connection between various highway facilities (e.g., freeway-arterial and arterial-arterial), and their safe and efficient operation is essential. The objective of this research was to develop improved methods for capacity and quality-of-service analysis of interchange ramp terminals. The research focuses on at-grade intersections but not on the freeway proper. All geometric configurations and interchange types except trumpet interchanges are considered, and the scope of the research includes only signalized interchanges and not Stop sign-controlled interchanges and roundabouts. The development of the analytical methodology is primarily based on simulation. The reason for using simulation is that adequate samples of field data are not available, and it is prohibitively expensive to collect them for all types of interchange configurations. The research team assessed several simulation models that were identified a...

Distribution of Roadway Geometric Design Features Critical to Accommodation of Large Trucks

The ability of the roadway system to accommodate large trucks is constrained by the geometric design of key features, including horizontal curves, interchange ramps, interchange ramp terminals, at-grade intersections, and steep grades. The distribution of the dimensions of roadway elements that are critical to accommodation of larger trucks on the highway is shown, including horizontal curves and grades on mainline roadways, horizontal curves on interchange ramps, and curb return radii for at-grade ramp terminals and intersections. Frequency of mainline and ramp curves with very sharp radii and of very steep mainline grades was found to be very limited. For example, only about 5 percent of interchange ramps have horizontal curves with radii of 30 m (100 ft) or less, and approximately 20 percent of rural ramps and 30 percent of urban ramps have radii of 75 m (250 ft) or less. Curb return radii less than or equal to 12 m (40 ft) on which trucks would frequently encroach are more frequent. Curb returns with sharp radii are more prevalent in urban than in rural areas and are more prevalent at intersections than at ramp terminals.

Phase Capacity Characteristics for Signalized Interchange and Intersection Approaches

Described in this paper are the development, calibration, and application of models that collectively can be used to predict the saturation flow rate and start-up lost time of through movements at signalized interchange ramp terminals and other closely spaced intersections. These models were calibrated with data collected at 12 interchanges. It is concluded that saturation flow rate decreases as the distance to the downstream queue decreases. This queue is formed by the signal at a downstream intersection. Saturation flow rate increases with traffic pressure, as quantified by traffic volume per cycle per lane. It is recommended that an ideal saturation flow rate of 2,000 passenger-car units per hour of green per lane be used for signalized ramp terminals and other high-volume intersections in urban areas. The data collected for this research indicate that start-up lost time increases with saturation flow rate.