Aimee Flannery

Geometric Design and Safety Aspects of Roundabouts

The findings of a 3-year study conducted at Pennsylvania State University for FHWA on the safety and operational performance and geometric characteristics of roundabouts are presented. The study focused on geometric characteristics that influence the safety and operational performance of roundabouts. Accident data (including driver testimonials) were reviewed along with videotapes developed at several roundabouts in Maryland, Florida, and Nevada. Several conclusions were drawn, including the need to improve geometric design on approaches to rural roundabouts to reduce loss-of-control accidents; the need for adequate right-of-way to properly deflect vehicles around the center island; and the need for guidance regarding operating-volume-to-capacity ratios. The findings from the study will assist planners, designers, and engineers in avoiding unnecessary safety hazards and operational failures.

Multimodal Level of Service Analysis for Urban Streets

This report will be of interest to public agencies responsible for the planning, design, and operation of urban streets. This report provides a method for assessing how well an urban street serves the needs of all of its users: auto drivers, transit passengers, bicycle riders, and pedestrians. The project developed and calibrated a method for evaluating the multimodal level of service (MMLOS) provided by different urban street designs and operations.

SAFETY, DELAY, AND CAPACITY OF SINGLE-LANE ROUNDABOUTS IN THE UNITED STATES

Roundabouts are a form of at-grade intersection control that is used frequently around the world and is becoming popular in the United States. Roundabouts are being used to replace two-way and all-way stopcontrolled intersections and traffic signals in the United States. Roundabouts have also been used recently in Vail, Colorado, to improve an existing stop-controlled freeway interchange system. In this study, five single-lane roundabouts are studied to assess their safety and operational performance. All five sites were stop-controlled before roundabouts were installed, and overall the sites experienced a reduction in accident frequencies, rates, and also control delay. Aside from a review of safety and delay data before and after installation of the roundabouts, this study includes a comparison of field-measured control delay with that predicted by SIDRA, an analytically based software package that can analyze at-grade sign- and signal-controlled intersections as well as roundabouts. This study will help agencies better understand their ability to predict delay at American roundabouts. Finally, findings are presented regarding the accuracy of the roundabout capacity model contained in the 1997 update to Chapter 10 of the Highway Capacity Manual. Because of the lack of roundabout entries that are operating at capacity in the United States, an approximation of potential capacity based on available gaps in the circulating stream was made. These findings indicate that the manual may be optimistic in its prediction of capacity for single-lane roundabouts in the United States; however, it should be noted that the lack of roundabout entries operating under capacity in the United States only allows for an approximation of field capacity to be made at this time.

Multimodal Level of Service for Urban Streets

This paper presents the results of research to develop a national method for the multimodal assessment of the quality of service provided by an urban street. The method considers the level of service from the point of view of four types of travelers typically using the urban street: auto driver, transit passenger, bicycle rider, and pedestrian. Video laboratories and transit onboard surveys were used to develop data sets of the quality of service perceived by the general public for each of the modes of travel on the urban street. The laboratories were conducted in several different metropolitan areas of the United States. Four level-of-service (LOS) models were developed, one for each mode. The four LOS models share a common measure, user satisfaction. The models assign a letter grade LOS (A–F) based on the street cross section, intersection controls, and traffic characteristics (auto, transit, bicycle, and pedestrian volumes on the street). The models can be used in combination to compare the trade-offs of different street cross sections from the unique perspectives of each mode. The models are particularly useful for testing the impacts of converting auto through lanes to bicycle lanes, wider sidewalks, and wider planter strips.

Analysis and Modeling of Automobile Users’ Perceptions of Quality of Service on Urban Streets

Modeling travelers’ perception of service quality is an emerging research area in surface transportation. The first national effort to study automobile drivers’ perceptions is NCHRP Project 3-70. The study was developed in response to agencies’ needs to understand which factors enter into users’ perception of quality of service and the interaction between modes on urban streets. This paper describes the research efforts taken to analyze and model automobile level of service (LOS) from the drivers perspective. A data collection approach using video laboratories and a modeling approach suitable for the discrete and ordered nature of the response variable were selected. The approach enabled the estimation of the distribution of LOS responses, given a set of explanatory variables that describe the geometry and operational effectiveness of the urban street facility. Analysis indicated that participants’ responses tended to be highly variable and that models that characterize the distribution would be preferable to point estimates. A model that uses the number of stops per mile experienced on the facility and the presence of exclusive left-turn lanes at intersections gave the best fit to the data and matched the mean video clip LOS rating 71% of the time. Although direct links between various modes to the automobile users’ perceptions of service quality were not identified, the variable “number of stops” incorporates interruptions experienced by automobile drivers when interacting with buses, bicycles, or pedestrians on urban streets and when encountering traffic signal stops. Although the proposed model was found to improve LOS prediction as compared with the current Highway Capacity Manual method, there are implementation issues that have yet to be addressed.

Queuing delay models for single-lane roundabouts†

This article presents an analytical model for a few important operational characteristics of single-lane roundabouts in the United States. In particular, we obtain renewal-based analytical expressions for the mean and variance of the time required for entry into the circulating stream for an arbitrary vehicle occupying the first position of the approach, regardless of the distribution of time headways for the circulating stream. These analytical models are subsequently applied in a M/G/1 queuing model to compute the steady-state average delay and length of the queue at the approach under stable conditions. The analytical models are validated by comparing numerical results for average delay with field observations obtained at six single-lane roundabout sites in the United States. The models are shown to perform well under a range of circulating stream flow rates. †The views expressed in this paper are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the US Government.

Risk and Resilience Analysis for Highway Assets

Transportation agencies own tens of thousands of assets, providing essential mobility and economic services to the communities they serve. Moving Ahead for Progress in the 21st Century and subsequent legislation require asset managers to implement risk-based asset management. A discussion is presented on the application of one quantitatively based framework—the American Society of Mechanical Engineers Innovative Technology Institutes Risk and Resilience Analysis and Management for Critical Asset Protection—for analyzing risks posed by physical threats to highway transportation systems and assets. The application of this particular risk analysis framework by the Colorado Department of Transportation following the 2013 floods is recounted; the analysis was used to support requests for federal emergency response funding. Finally, the potential benefits of such analysis for highway transportation project planning and strategic planning are also examined.

Incorporating Truck Analysis into the Highway Capacity Manual

Transportation decisions should facilitate and account for freight flows, but analysts lack the tools needed to evaluate them. Incorporation of truck analysis into the Highway Capacity Manual (HCM) would help transportation agencies address the freight and highway needs of their community, region, state, and nation. This report presents capacity and level-of-service techniques that (1) improve transportation agencies’ abilities to plan, design, manage, and operate streets and highways to serve trucks and (2) better evaluate the effects of trucks on other modes of transportation and vice versa. The research team for National Cooperative Freight Research Program (NCFRP) Project 41 took a comprehensive approach to addressing the issue. In addition to a literature review, federal, state, regional, and local agencies were contacted to document the state of the practice. Carriers and shippers were interviewed to determine the critical factors that affect logistical decisions. Based on the insights from these activities, the research team developed a truck level-of- service framework. This framework was refined through two workshops with a wide variety of public transportation agency staff to ensure that it would be useful in their work, particularly in evaluating the impacts of system improvements on goods movements. The utility of this framework was demonstrated through the development of three case studies. The research team then collected field data and calibrated simulation models on freeways and arterials. These models were used to develop improved methods of estimating performance measures for trucks and other vehicles. The report includes several recommendations for improvements to the HCM, and these are being considered in National Cooperative Highway Research Program (NCHRP) Project 03-115, which is updating the HCM for expected publication in 2015.

HIGHWAY CAPACITY MANUAL AND HIGHWAY CAPACITY SOFTWARE 2000 AND ADVANCED TRANSPORTATION MODELING TOOLS: FOCUS GROUP FINDINGS

Focus groups were conducted in several cities throughout the United States and Canada between June 2002 and July 2003 to gain insight into the use of the newly released Highway Capacity Manual (HCM) 2000 and to understand better how professionals choose advanced modeling tools. Part I of the HCM 2000 was noted as being a good reference for both laypersons and those working with the HCM on a daily basis. Part V was noted as needing improvement to be useful to professionals, including more specific information regarding model strengths and weaknesses. In regard to the use of computer-based models, more than 80% of the participants use the HCM and the Highway Capacity Software to analyze operational performance and estimate capacity of highway systems. For advanced modeling techniques, 70% of those surveyed turn to SYNCHRO, but few participants could say why they had chosen a particular software package other than being directed by a sponsor agency to use it. It is noted that focus groups are a useful means to obtain qualitative and quantitative information regarding the use of the HCM 2000 and to identify areas of the manual that need improvement and further research. In addition, the information gleaned from this study points to the need for guidance regarding the selection and application of advanced transportation modeling tools. Finally, it is noted that additional focus groups should be held to include opinions from other user groups including planners, decision makers, and designers.