Issues for Trucks Using Roundabouts: General Roundabout Design

Benekohal, Alturi, et al., 2009. This study surveyed Illinois Department of Transportation (IDOT) staff about roundabouts, evaluated two popular roundabout software programs (aaSIDRA and RODEL), developed a multi-criteria site selection procedure, and used it to identify 10 potential intersections in Illinois that may reduce fatalities and severe injuries by using a roundabout. IDOT jurisdiction includes three roundabouts built or under construction and four roundabouts in planning stages. One third of IDOT districts were familiar with the RODEL and aaSIDRA, but they did not have working knowledge of the software. Some districts were concerned about proper roundabout usage by unfamiliar, younger, or elderly drivers. For the same traffic conditions, aaSIDRA yielded higher delay than RODEL for most of the reasonable volume combinations. Capacity values from RODEL are very similar to FHWA’s 2000 Guide and both are higher than the new NCHRP Report 572 model, but aaSIDRA’s capacity curves are similar to that of NCHRP Report 572. This study used aaSIDRA for developing the proposed site selection procedure. A multi-criteria site selection procedure was developed and its application is presented. The factors considered in the site selection process included crash history, intersection delay (LOS), roundabout capacity, distribution of traffic volume among approaches, location of intersection, and input from “local” engineers. The site selection process was used to identify the top 10 potential roundabout locations in Illinois. Further modification to this procedure is needed to cover other types of roads (multi lane) and traffic conditions. It is recommended that, for the time being, IDOT follow the FHWA Roundabout Guide (2000) in conjunction with the findings of recently published NHCRP Report 572 as a guideline for the design of roundabouts. The upcoming FHWA Roundabout Guide and a new roundabout chapter in the 2010 Highway Capacity Manual will include significantly new information that could be used in developing a roundabout design guide for IDOT.

IIHS, 2019. Roundabouts are a traffic calming device used to improve safety over traditional intersections, however, the safety of two-lane roundabouts is less well-known. This article reports on a study of the safety of two-lane roundabouts, potentially a more complex strategy to manage traffic intersections. Researchers from the Insurance Institute for Highway Safety (IIHS) investigated how crash trends at both single-lane (n = 98) and two-lane (n = 29) roundabouts change over time, with an eye to including driver familiarity as a variable. The study was conducted in Washington State, which has more than 300 roundabouts. The number of crashes at two-lane roundabouts decreased on average 9% per year, with more serious crashes decreasing by nearly 35% annually. The odds of a crash caused by failure to yield right-of-way fell 11% annually at the two-lane roundabouts. The authors conclude with a brief discussion of strategies to increase roundabout safety, regardless of driver experience, by better design and better signage (including pavement markings). Full report by Hu and Cicchino under "Crash Predictions" tab.

Porter, Gooch, et al. 2020. This article describes the design, performance, and enhanced safety opportunities of turbo roundabouts. The turbo roundabout is a roundabout alternative that eliminates conflict points that are associated with lower-severity crashes that sometimes occur in modern 2x2 multilane roundabouts, roundabouts characterized by two entry lanes approaching two circulating lanes. The turbo roundabout was first designed and implemented in the Netherlands in the 1990s. Now there are more than 500 turbo roundabouts worldwide, with more than 370 in the Netherlands. This article would need to be requested.

Pochowski, Paul, Rodegerdts, 2016. As of 2013, there were an estimated 3,200 roundabouts in use throughout the country. This synthesis documents and summarizes roundabout policies, guidance, and practices in place within state departments of transportation (DOTs) as of 2015. The primary focus of the synthesis is on roundabout selection and design with a secondary focus on performance analysis. Information used in this study was gathered through a literature review and a survey of state DOTs. Personnel from seven states were interviewed to create case examples and more specific information. Among the findings, the primary reason cited for the selection of roundabouts is improved safety performance compared with other intersection options, followed by shorter vehicular delays and higher capacity. Planning-level cost estimates (screening or feasibility level estimates) for mini-, single-lane, and multilane roundabouts ranged from $249,000 to $2.05 million. The majority of reporting states have developed some public education materials to support roundabout projects, such as websites, videos, and flyers and/or pamphlets. To analyze roundabout performance, about three-quarters of the reporting states use some form of the Highway Capacity Manual 2010 model and SIDRA’s Standard Model. Most of the reporting states analyze safety performance at roundabouts, using crash modification factors, crash reduction factors, and/or the Highway Safety Manual predictive methodology. Most states use a design life of 20 years when developing the ultimate roundabout design. However, the majority uses a phased implementation approach.

Rodegerdts, Blogg, et al., 2007. NCHRP 572. Based on a comprehensive evaluation of roundabouts in the United States, this report presents methods of estimating the safety and operational impacts of roundabouts and updates design criteria for them. The report will be useful to geometric designers and traffic engineers who are considering improvements to an intersection.

Rodegerdts, Blogg, et al., 2006. Based on a comprehensive evaluation of roundabouts in the United States, NCHRP Report 572 presents methods of estimating the safety and operational impacts of roundabouts and updates design criteria for them. This web document contains the appendices to NCHRP Report 572, as follows: (A) Literature Review on Safety Performance; (B) Literature Review of Operational Models; (C) Site Inventory; (D) Pedestrian and bicycle Analysis Details; (E) Summary of Goodness-of-Fit Measures and Statistical Terms; (F) Statistical Testing of Intersection-Level Safety Models; (G) Definitions for Estimating Fastest Vehicle Paths; (H) Statistical Testing of Approach-Level Safety Models; (I) Statistical Testing of Speed-Based Safety Models; (J) Operations Appendix; (K) Pedestrian Analysis Tables; (L) Pedestrian and Bicycle Images; (M) Draft Highway Capacity Manual Chapter 17; and (N) Research Problem Statement.

Soltykevych, Mah, Kenny, 2014. In Alberta, there are concerns that current roundabout construction costs are high in comparison to alternate options. Numerous commercial loads, including oversize and overweight (OSOW) vehicles, use Alberta’s highway corridors on a daily basis. The department’s intent is to maintain a highway system that will have no blockages for high loads or oversized loads. Roundabouts may present a bottleneck if OSOW loads are not considered.
The department aims to determine best design practices for roundabouts with the most cost-effective solution. For example, one good design practice is to build a roundabout that works for current volumes plus a reasonable time period (i.e. 10 years), and then be prepared to add additional lanes if required. Aside from saving cost, this practice is safer and allows drivers to adjust to a simpler roundabout. Cost-sharing formulas have also been proposed to finance the roundabout.
The paper includes discussion of existing roundabouts on Alberta highways, compares advantages and disadvantages of roundabouts on their highway system, and presents recommendation/cost-reducing strategies.

Wallwork, 2014. Ensuring proper design of roundabouts is essential to their expansion and attainment of their benefits. Yet recent press reports have highlighted significant increases in crashes when roundabouts replaced signalized intersections. This paper seeks to highlight the most common design issues that are degrading the potential benefits of roundabouts in the U.S. and what could be done to reduce the number of poorly designed roundabouts. Material for this paper is drawn from the Author’s experiences designing roundabouts, undertaking formal and informal peer reviews, design reviews that predicted specific problems that unfortunately came true and 55 post-construction reviews of poorly designed and built roundabouts over a 37 years in four countries. This paper will briefly discuss basic design philosophy, identify the most common design issues and provide some ideas as to improve roundabout design quality.

Brilon, 2011. Germany has now 25‐years of experience with different kinds of modern roundabouts. In addition to that, large conventional rotaries have been used for 70 years. Modern roundabouts include
• compact single‐lane roundabouts with diameters between 26 and 40 m
• mini‐roundabouts with a traversable island and diameters between 13 and 25 m
• larger roundabouts (40 –60 m) with 2‐lane access for cars and single‐lane operation for
trucks
• turbo‐roundabouts.
All these types have emerged as very successful regarding traffic safety as well as traffic performance. On the other side the traditional larger 2‐lane roundabouts have significant safety problems. The paper describes the German experience from practice and from a long series of research projects regarding traffic safety, capacity, and traffic performance estimation as well as geometric design.

Elhassy, Abou-Senna, et al., 2020. With the growing number of vehicles utilizing roads in the city of Doha, Qatar, most intersections, particularly multilane roundabouts, have been facing traffic congestion dilemma, where traffic demand exceeds capacity. A new design for multilane roundabouts, known as a rotor turbo roundabout, was considered as an alternative to an existing highly congested multilane roundabout. The new design features spiral roadway markings and raised lane dividers which prevent maneuvering within the roundabout and eliminate cutting-offs and weavings. This design has achieved high capacity and low delay in many European countries. In this study, a traffic simulation program, VISSIM, is used to model the complex traffic operation of both the existing and proposed multilane roundabouts and to replicate the high traffic conditions and aggressive driving behavior prevalent among the Middle East countries. Three different rotor designs were examined in an attempt to have a valid comparison between the two types of roundabouts and to adhere to the standard design of the rotor roundabout without violating its essential features. The proposed designs performed slightly better on the minor approaches and managed to deliver an overall improved LOS compared to the conventional design. Major approaches, however, exhibited an increase in vehicle delay and queue lengths. The results showed that the capacity of the conventional three-lane roundabout was always superior to the capacity of the rotor roundabouts. It was concluded that rotor roundabouts may not be suitable for intersections with high demand volumes exceeding 4500 vehicles per hour, and whenever the traffic flow condition is oversaturated.

Gallelli, Vaiana, 2019. In 2011, the United Nations Road Safety Collaboration (UNRSC) developed a Global Plan for the Decade of Action for Road Safety 2011–2020. Among the categories or “pillars” of activities, is the improvement of road safety for infrastructures. Furthermore, this plan is aligned by the UN Sustainable Development Goals that included even traffic safety. In this regard, this study estimates safety improvements achieved by converting a standard roundabout into an egg turbo roundabout. In particular, turbo roundabouts have become very popular in Northern Europe for both their safety and their capacity. Many studies have shown these advantages thanks to their features: preventive separation of entering flows, limited lane changing and low speeds due to curbs. Given the absence of existing turbo roundabouts in Italy, this research studied and compared a “virtual” roundabout with spiraling circular carriageways to an existing multi-lane roundabout in order to assess its significant reduction in terms of potential collisions. This study relied on traffic conflicts in micro-simulation by using VISSIM software and then Surrogate Safety Assessment Model (SSAM). The research is based on the traffic process observed at a standard roundabout in Cosenza (Italy) marked by a high level of congestion and safety problems. Speeds, critical gaps, queue lengths, and floating car data, obtained from video observations, have been used as input data for the calibration procedure of the first scenario (case study roundabout). Then, the turbo roundabout solution was built and simulated by using the previously derived parameters. Finally, the two roundabout scenarios were compared in terms of spatial distribution of the potential conflicts determined by SSAM. The results could help to measure the performance and safety impact of these two roundabout configurations

Silva, Santos, Gaspar, 2013. Conventional roundabouts are excellent solutions for:
• Traffic regulation;
• Traffic calming;
• Urban regeneration and landscaping;
• Etc…
However, the international experience over the last
years has been showing some functional problems
in double-lane roundabouts:
• Lane changing on the roundabout disregarding
lane markings;
• Cut the trajectory curvature;
• Achieve higher speeds.