2013 Session: 405

• Free-Flow Speed Model Based on Portuguese Roadway Design Features for Two-Lane Highways
  Abstract: Speed is a key performance measure in economic and environmental analyses of two-lane highways. Speed, combined with the percent time-spent-following, is also used in the assessment of level of service. Under free-flow conditions, the circulation of a given vehicle is not constrained by the presence of other vehicles, and the driver’s speed choice reflects his response to the geometric features of the road and roadside interference, as well as his perception of risk. Many studies concerned with the effects of road characteristics, design features in particular, on vehicle speed, have been conducted in several countries in recent decades, providing useful tools for modeling speed and evaluating alignment consistency.This paper presents an exponential free-flow speed model, applicable to both curves and tangents, that was developed for two-lane highways in Portugal. The variables included in the model are representative not only of the road element under consideration (curve or tangent) but also of the preceding road section and of the visual field downstream from the element. The results from this model are compared with other authors’ results and with the guidelines in the Highway Capacity Manual 2010. In addition to the primary influence of the horizontal curvature on speed, the results show that other factors, such as the cross-sectional width, the density of access points and the downstream visibility, are also important.
  Authors: Lobo, António; Rodrigues, Carlos; Couto, António Fidalgo
  Authors: Lobo, António; Rodrigues, Carlos; Couto, António Fidalgo
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0512
• Operational Effects of CFI Geometrics: A Deterministic Model for Continuous Flow Intersections
  Abstract: The Continuous Flow Intersection (CFI), also known as a Displaced Left Turn (DLT) intersection, increases capacity and reduces delays by eliminating the left turn traffic signal phase. This is achieved by changing the geometric layout of the intersection so that left turns cross opposing traffic lanes at a location upstream of the main intersection. Although the installation of CFIs has become more prevalent in the United States (US), insights to CFI geometrics gained through their application have not yet been dispersed to industry or academia. This paper considers the impacts that geometric design has on the operations of CFIs. It presents a deterministic model that considers the effects of the DLT length and other CFI geometrics on traffic operations. Although “academic” in nature, this model evolved through practical application. The model takes functional and operational insights gained through the analysis, design, and observation of multiple operational CFIs in Utah and articulates them in mathematical terms. The quantitative relationships presented provide transportation professionals and officials more accurate and precise guidance to facilitate better decisions when considering, evaluating, and designing a CFI.
  Authors: Carroll, Diego H.; Lahusen, Derek
  Authors: Carroll, Diego H.; Lahusen, Derek
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0428
• Calibrating Road Design Guides Using Risk-Based Reliability Analysis
  Abstract: Existing geometric design guides provide deterministic design standards for highway elements which ignores the uncertainty associated with many design parameters. Reliability analysis has been advocated as an approach to account for this uncertainty and to evaluate the risk associated with a particular design feature. This paper discusses one important application of reliability analysis: the calibration of geometric design models to yield consistent safety (risk) levels. The paper provides calibrated design charts for the middle ordinate (M), defined as the lateral distance between edge of median barriers and centerline of the adjacent traffic lane, at different probability of non-compliance levels. The results show that the calibrated values of middle ordinate (M) are generally lower than those derived from the AASHTO design guide. The calibrated design charts can offer designers dealing with highways with constricted right-of-way an option to use lower middle ordinate values and enable them to estimate the safety consequences of their decisions. Overall, the calibrated charts can aid the decision maker in determining the safety implications of deviating from geometric design standards and quantifying the safety level built in design values that are deemed acceptable.
  Authors: Hussein, Mohamed Hussein Azab; Sayed, Tarek; Ismail, Karim; Van Espen, Adinda
  Authors: Hussein, Mohamed Hussein Azab; Sayed, Tarek; Ismail, Karim; Van Espen, Adinda
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0877
• Selecting Speed Limits for Multilane Highways with Curbs and Gutters
  Abstract: Most states discouraged the use of curb on roadways with design or operating speed over 45 mph. The research herein is to assess current speed limit practices when widening two-lane highways with speed limits of 55 mph to multilane highways with curb and gutter cross-sections and to recommend, if appropriate, enhanced design standards and speed limit guidelines. The research included a number of related tasks. The speed studies at 32 sites, which had recently been upgraded from 2 lane roadways to 4 or 5 lane roadways with curb and gutter profiles was conducted. The speed variances and speeding rates at these sites were examined and compared to corresponding data from similar sites on two-lane roadways. A series of human factors studies, utilizing the state-of-the-art DS-600c driving simulator system at the University of Tennessee, was conducted to assess the effects of curb and gutter profiles and posted speed limits on driver speed selection. In addition, a study of the effects of speed limits, in combination with cross-sectional elements, on crash rates was performed utilizing crash data from the Highway Safety Information System (HSIS). Based on the collective research findings, it is recommended that traffic agencies develop an alternative design standard for use on ¡°transitional roadways¡± as defined in this research. The alternative standard would allow the use of design speeds and posted speed limits in excess of 45 mph, along with the use of appropriate sloping curbs, on this new category of roadways.
  Authors: Richards, Stephen H.; Dong, Chunjiao; Yang, Qiang; Jiang, Ximiao
  Authors: Richards, Stephen H.; Dong, Chunjiao; Yang, Qiang; Jiang, Ximiao
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-3105
• Development of Planning Framework for Geometric Design of Continuous-Flow Intersections
  Abstract: Despite the increasing use of continuous flow intersections (CFI) to contend with the congestion caused by heavy through and left-turn traffic flows, a reliable and convenient tool for the traffic community to identify potential deficiencies for CFI¡¯s geometry design are not available yet. This is due to the unique geometric feature of CFI which comprises one primary intersection and several crossover intersections. The interdependent relation between the traffic delays and queues at a CFI¡¯s five closely-spaced intersections cannot be fully captured with the existing analysis models developed for conventional intersections. In response to such a need, this study presents a comprehensive analysis for the overall CFI delay, identifies the potential queue spillback locations, and develops a planning framework for CFI¡¯s geometry designs. To facilitate the application of our proposed models, this paper also includes a case study of a CFI at intersection MD 4 and MD 235 conducted by Maryland State Highway Administration.
  Authors: Yang, Xianfeng; Chang, Gang-Len; Lu, Yang; Rahwanji, Saed
  Authors: Yang, Xianfeng; Chang, Gang-Len; Lu, Yang; Rahwanji, Saed
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-3809
• Empirical Study and Assessment of Operational Performance of Double Crossover Diamond Interchanges
  Abstract: Operational and safety issues at many freeway-to-surface street interchanges have increased in recent years, as they are unable to handle larger traffic volumes, especially the oftentimes heavy left-turning demands at many interchanges. Limitations in right-of-way availability, high right of way costs, and expensive bridge structures have pushed engineers to find innovative solutions to solve some of these issues. Examples of these innovative designs include the roundabout interchange, the single point urban interchange, and the double crossover diamond (DCD) interchange (also known as the diverging diamond interchange, DDI), among others. This paper provides results from the first 1.5 years of a research study commissioned by the Federal Highway Administration to evaluate the first few DCD installations in the US (FHWA Project Number DTFH61-10-C-00029). This paper includes summary findings on operational performance at DCD interchanges built in recent years at I-44 and MO-13 in Springfield, MO; US-60 and National Avenue in Springfield; I-270 and Dorsett Road in Maryland Heights, MO; and US-129 and Bessemer Street in Alcoa, TN. The paper focuses mostly on the operational conditions of these interchanges, although safety assessment, simulation analysis, and design guidance are all part of the overall project. Performance measures presented in this paper include peak-hour queue lengths, spillback and blockage, origin-destination travel times, signal compliance, erratic maneuvers, field-observed conflicts, and vehicle speed profiles. The paper also includes assessments of walkability and bikeability, as well as user perceptions. The paper will provide tentative conclusions on the operational performance of DCD interchanges to this point and will describe the program of future work planned to fully investigate the operational and safety impacts of DCDs under aforementioned project.
  Authors: Vaughan, Chris; Cunningham, Christopher M.; Schroeder, Bastian J.; Hummer, Joseph E.
  Authors: Vaughan, Chris; Cunningham, Christopher M.; Schroeder, Bastian J.; Hummer, Joseph E.
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-4946
• Estimation of Operational Benefits of Slow Vehicle Turnouts on Rural Highways in Alaska
  Abstract: This paper draws on the results from previous research regarding two-lane highway operations and platooning characteristics to develop an estimate of the operational benefit that will be realized when slow vehicle turnouts are constructed on a 70-mile section of the Sterling Highway on the Kenai Peninsula in Alaska. Speed and volume data were collected at three sites in the study area to quantify the existing conditions. Three measures of effectiveness are presented: the distribution of platoon sizes, the percent following (defined as the percent of all vehicles that are following another vehicle at time headways of 3 seconds or less), and the percent impeded. Percent impeded is calculated as a modification of the percent following measurement to account for vehicles that are following, but are traveling at their desired speed, and are therefore not being impeded.Future operations after the installation of the proposed slow vehicle turnouts are estimated by modifying the percent following to account for lead vehicles pulling into the slow vehicle turnouts and allowing others to pass. The percent impeded was then calculated for this modified value for percent following. A sensitivity analysis was performed to show how percent impeded was affected by differences in the percentage of lead vehicles who use the slow vehicle turnouts to let others pass. It is concluded that the slow vehicle turnouts will provide a measurable benefit to operations on the subject highway.
  Authors: Dunham, Connor; Bowie, Jeanne; Kinney, James Randall
  Authors: Dunham, Connor; Bowie, Jeanne; Kinney, James Randall
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0555
  Practice-Ready: Yes
• Evaluation of Continuous Green T-intersections on Isolated Undersaturated Four-Lane Highways
  Abstract: The research presented in this paper analyzes the merging version of the Continuous Green T-Intersection (CGT), an alternative intersection design/control that allows certain lanes along the main street to bypass three-way intersections, with side-street traffic merging onto the main road. A comprehensive model encompassing 2,445 unique intersection condition combinations was run, comparing the merging CGT to the traditional three-way signalized intersection. The study demonstrated significant intersection improvements compared to conventional traffic signal timing. Specifically, significant benefits were observed for the merging CGT in terms of total delay, fuel usage, hydrocarbon, carbon monoxide, oxides of nitrogen, and carbon dioxide emissions. Additionally, an economic analysis showed significant user savings associated with CGT control. Because of the significantly higher volumes along the main- versus the side-street, savings for main-street vehicles outweighed any dis-benefits associated with side street traffic merging into the main street flow. These findings strongly support the decision to implement the merging CGT over traditional three-way signalized intersection control. This work introduces a comprehensive model or case study that systematically evaluates the merging CGT and compares it to conventional signalized intersection control.
  Authors: Litsas, Stephen; Rakha, Hesham
  Authors: Litsas, Stephen; Rakha, Hesham
  Year: 2013
  Document Type: Paper
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0591
  Practice-Ready: Yes
• Selecting Speed Limits for Multilane Highways with Curbs and Gutters
  Authors: Dong, Chunjiao
  Authors: Dong, Chunjiao
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-3105
• Development of Planning Framework for Geometric Design of Continuous-Flow Intersections
  Authors: Yang, Xianfeng
  Authors: Yang, Xianfeng
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-3809
• Operational Effects of CFI Geometrics: A Deterministic Model for Continuous Flow Intersections
  Authors: Carroll, Diego
  Authors: Carroll, Diego
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0428
• Free-Flow Speed Model Based on Portuguese Roadway Design Features for Two-Lane Highways
  Authors: Couto, António
  Authors: Couto, António
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0512
• Estimation of Operational Benefits of Slow Vehicle Turnouts on Rural Highways in Alaska
  Authors: Bowie, Jeanne
  Authors: Bowie, Jeanne
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0555
• Evaluation of Continuous Green T-intersections on Isolated Undersaturated Four-Lane Highways
  Authors: Litsas, Stephen
  Authors: Litsas, Stephen
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Operations and Traffic Management
  Session: 405
  Paper Number: 13-0591