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    <title>Transport Research International Documentation (TRID)</title>
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    <atom:link href="https://trid.trb.org/Record/RSS?s=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" rel="self" type="application/rss+xml" />
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    <copyright>Copyright © 2026. National Academy of Sciences. All rights reserved.</copyright>
    <docs>http://blogs.law.harvard.edu/tech/rss</docs>
    <managingEditor>tris-trb@nas.edu (Bill McLeod)</managingEditor>
    <webMaster>tris-trb@nas.edu (Bill McLeod)</webMaster>
    <image>
      <title>Transport Research International Documentation (TRID)</title>
      <url>https://trid.trb.org/Images/PageHeader-wTitle.jpg</url>
      <link>https://trid.trb.org/</link>
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    <item>
      <title>Research on Capacity Model for Large Signalized Roundabouts</title>
      <link>https://trid.trb.org/View/1415473</link>
      <description><![CDATA[In order to calculate the capacity of signalized roundabout, this paper establishes the corresponding calculation model. Based on gap acceptance theory, the model defines the maximum number of vehicles on the approach that can be imported into the island as the roundabout capacity. At the same time, some related parameters (such as cycle, green time) are considered to impact on the model. At first, the paper analyses the difference of traffic flow characteristics between signalized roundabout and non-signalized roundabout, and according to the vehicles conflict traits, the movement process is divided into two stages. Then the new capacity model of signalized roundabout is put forward. Moreover analysis of the influence on capacity caused by different follow-up time, critical gap and green time are made, too. Finally, taking Changchun's signalized roundabout as an example, using the actual signal timing scheme and the data of the flow to verify the new capacity model, it turns out that the results match the actual situation properly. In summary, a method of calculating the capacity of signalized roundabout is put forward and proved to be practical in this paper.]]></description>
      <pubDate>Wed, 10 Aug 2016 11:05:56 GMT</pubDate>
      <guid>https://trid.trb.org/View/1415473</guid>
    </item>
    <item>
      <title>Capacity Estimation on Turboroundabouts with Gap Acceptance and Flow Level Methods</title>
      <link>https://trid.trb.org/View/1338022</link>
      <description><![CDATA[In the literature, linear models and exponential models based on gap acceptance theory are distinguished. Parameters for the linear models can be estimated only at the level of traffic flow, whereas the gap acceptance theory assumes that behavioral parameters (critical gap, follow-on time, and minimum headway) can be estimated at the vehicle level, and then capacity can be determined. However, in the latter method, measurements must also be made under saturated conditions because of so-called pseudoconflict (caused by vehicles leaving the roundabout in the opposite leg direction). An analysis of data sources (vehicle level and traffic flow level) indicates that the parameters estimated at the vehicle level correspond with those estimated at the level of traffic flow for single-lane roundabouts, but not for more complex situations. Despite this deficiency, better results can be gained with a model based on the present gap acceptance theory than with a linear model, provided that the parameters are adjusted to consider traffic flow measurements under saturated conditions. Additional research will be necessary to determine which underlying assumptions of the gap acceptance models cause these deficiencies. In Germany, capacity is higher in turboroundabouts than in compact two-lane roundabouts because of better use of the inner lane in the turboroundabout.]]></description>
      <pubDate>Sat, 28 Mar 2015 12:49:02 GMT</pubDate>
      <guid>https://trid.trb.org/View/1338022</guid>
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    <item>
      <title>The Effect of Priority Reversal on Delay and Queue Length at Roundabouts</title>
      <link>https://trid.trb.org/View/1288043</link>
      <description><![CDATA[The procedure for analyzing roundabout operations in the 2010 U.S. Highway Capacity Manual is based on the assumption of absolute priority to circulating traffic. Entering drivers have to seek for sufficient gaps in order to merge. However observations at roundabouts operating at or near capacity show reversed or shared priority between entering and circulating vehicles. This phenomenon has been reported elsewhere to influence the capacity, delay and queue length of roundabouts. To study the effects of reversed priority on the performance of roundabouts in the U.S., a roundabout in Fernley, Nevada, operating at capacity for the P.M. peak period was observed. The driver behavior characteristics were extracted together with the geometric parameters. Based on the observed driver behaviors and traffic flow characteristics, three scenarios were modeled using the micro-simulation software VISSIM. Results obtained after multiple simulation runs showed that roundabout performance improved with increasing periods of reversed priority: delay reduced by 8-16 percent and queue length reduced by 10-20 percent for every 10 percent increase in “reversed priority periods”. It was also shown that, as the priority reversal period increased, the influence of the size of inscribed diameter decreased even though an improvement in performance was observed. When the intersection experienced traffic volume increase the performance of the roundabout decreased until it reached a threshold level where it appeared to remain constant.]]></description>
      <pubDate>Mon, 10 Mar 2014 09:23:58 GMT</pubDate>
      <guid>https://trid.trb.org/View/1288043</guid>
    </item>
    <item>
      <title>Alternative Models for Roundabout Capacity</title>
      <link>https://trid.trb.org/View/1212126</link>
      <description><![CDATA[This article provides a wide perspective about roundabout capacity models. Although there are many roundabout capacity models used around the world, this article focuses on two well known models. The SIDRA model, which is based on research on roundabouts in Australia and the TRL method, which is based on research on roundabouts in the United Kingdom. A detailed table that compares the features of these two capacity models is provided. The comparison table also includes the United States (US) Highway Capacity Manual 2010 (HCM2010) model based on research on roundabouts in the US. Developed in different countries at different times, these models have common elements yet significant differences in modeling methodologies.]]></description>
      <pubDate>Tue, 28 Aug 2012 12:23:43 GMT</pubDate>
      <guid>https://trid.trb.org/View/1212126</guid>
    </item>
    <item>
      <title>Capacity of 2-Lane Roundabouts</title>
      <link>https://trid.trb.org/View/1099026</link>
      <description><![CDATA[The number of roundabouts has increased significantly in Denmark over the last two decades. The majority of roundabouts are single-lane roundabouts (used in rural and urban areas), but due to increasing traffic volumes, more and more roundabouts in rural areas are now being built as 2-lane roundabouts. In Denmark, the capacity of roundabouts is traditionally based on gap acceptance theory and a number of previous studies have measured critical gap and follow up times for varied types of roundabouts in order to set up a general capacity model to be used in Denmark. This presentation will mainly focus on 2-lane roundabouts and will be based on a new study from 2-lane roundabouts (mainly with “turbo design”) that was completed in 2008. The results from the study include new values for critical gap and follow up times to be used in 2-lane roundabouts. The study also re-estimates the values for passenger car units (pcu) for two different categories of heavy vehicles. The new pcu-values indicate that capacity of roundabouts is more influenced by heavy vehicles than previously expected. The study also estimates the effect on capacity from exiting vehicles (vehicles leaving the roundabout at the adjacent arm). It is well known, that high traffic volumes of exiting vehicles reduce the capacity of entering vehicles. The results indicate that the capacity is reduced by up to 20% in situations with high traffic volumes of exiting vehicles. Empirical data from observed capacity at 2-lane roundabouts have been used to test different theoretical and empirical capacity models. In general, the theoretical capacity models seem to have certain limitations when it comes to describing the observed data. Methods to overcome these limitations are sought and will be presented. Finally, an analysis of drivers lane use when entering roundabouts has been examined. The majority of entering drivers use the right lane (outer lane), even at high traffic volumes. It is assumed that better signing and marking might lead to a more balanced use of the entrance lanes, which could lead to higher capacity.]]></description>
      <pubDate>Thu, 28 Apr 2011 13:01:13 GMT</pubDate>
      <guid>https://trid.trb.org/View/1099026</guid>
    </item>
    <item>
      <title>Reserve Capacity for a Set of Closely-Spaced Intersections</title>
      <link>https://trid.trb.org/View/839741</link>
      <description><![CDATA[The concept of reserve capacity provides a measure of the performance of a junction, and involves determining the uniform factor α by which the existing flows into the junction can be multiplied before any of the approach flows reaches its capacity.  Previous work has been limited to cases in which it is assumed there is no “blocking back” – either because the junction is isolated or the queues are assumed to stack vertically.  However, in practice, at closely-spaced junctions (as for example on a signalized roundabout) a queue at one junction may extend and interfere with the operation of an upstream junction, so that some of the available green time at that junction is lost. In this paper the cell transmission model is used to represent the spatial extent of queues in networks with closely-spaced signalized junctions.  It is shown that the problem of estimating the reserve capacity can be formulated as an optimization problem with linear constraints.  By exploiting the problem’s structure, an efficient solution algorithm is developed that consists of solving a sequence of small linear programming problems.  The application of the algorithm is illustrated by means of a small example network consisting of two closely-spaced signalized junctions.]]></description>
      <pubDate>Mon, 26 Nov 2007 09:54:48 GMT</pubDate>
      <guid>https://trid.trb.org/View/839741</guid>
    </item>
    <item>
      <title>Including Exiting Vehicles in Capacity Estimation at Single-Lane U.S. Roundabouts</title>
      <link>https://trid.trb.org/View/777457</link>
      <description><![CDATA[The current model used in the United States to predict approach capacity at a single-lane roundabout uses information about entry driver behavior in response to the circulating traffic stream only. There is no procedure for including exiting vehicles in capacity estimation. Exiting vehicles are vehicles that leave the circulating stream of traffic by maneuvering into the exit lane at a roundabout approach. Exiting vehicles are known to have an effect on capacity at roundabout approaches in other countries, but their effect in the United States is not known. The purpose of this research is to determine whether including exiting vehicles improves capacity estimation at a roundabout approach. An exploration of the relationships of the proportion of exiting vehicles and the width of the splitter island to approach capacities is included. Capacities were estimated both with and without exiting vehicles as part of the conflicting flow and compared with measured field capacities. The findings demonstrate that capacity estimates with exiting vehicles result in improved prediction of the actual capacity of a roundabout approach over estimates without exiting vehicles. The proportion of exiting vehicles in the major stream and the width of the splitter island appear to provide some benefit in capacity prediction, but exactly how these parameters can be incorporated into the capacity prediction process must be further explored.]]></description>
      <pubDate>Fri, 03 Mar 2006 10:59:53 GMT</pubDate>
      <guid>https://trid.trb.org/View/777457</guid>
    </item>
    <item>
      <title>Analytical Analysis of Pedestrian Effects on Roundabout Exit Capacity</title>
      <link>https://trid.trb.org/View/775381</link>
      <description><![CDATA[The availability of pedestrian gaps and the queuing effects of pedestrian crossings have implications for roundabout design, particularly when considering the operations of roundabout exits and the potential for vehicle queues to spill back onto the circulatory roadway. In most jurisdictions, vehicles are required to yield to pedestrians. In practice, pedestrians often choose their crossings to coincide with gaps in the traffic stream, i.e., yielding to vehicles. This paper presents methods to analyze both conditions. In the case where pedestrians yield to vehicles, the minor-street movement capacity equations from the Highway Capacity Manual’s unsignalized intersection methodology can be adapted to determine the number of gaps in a traffic stream sufficient for a pedestrian to cross. In the case where vehicles yield to pedestrians, the effect of a vehicular queue extending into the roundabout while waiting for pedestrians to cross can be estimated, and the extent to which this queue will adversely affect capacity can be approximated. The paper concludes by discussing the implications of the interactions between pedestrians and vehicles on the design of the roundabout.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:54:06 GMT</pubDate>
      <guid>https://trid.trb.org/View/775381</guid>
    </item>
    <item>
      <title>Simulated Capacity of Roundabouts and Impact of Roundabout Within a Progressed Signalized Road</title>
      <link>https://trid.trb.org/View/775551</link>
      <description><![CDATA[Many intersections in the urban areas are signalized. As roundabouts are beginning to multiply, they are being considered adjacent to signalized intersections and for replacing some signalized intersections. Traffic simulation has been used to study the performance of both signalized and un-signalized intersections. This research uses simulation to study the traffic impacts of roundabouts. In this paper, two problems are studied. Firstly, urban single lane and dual lane roundabouts are modeled in VISSIM traffic simulation software. Simulation results are compared with the results of RODEL (empirical model) and aaSIDRA (analytical model). Comparison with real data collected from various sites in United States shows that VISSIM results are closer to the real data than the RODEL and aaSIDRA results. Secondly, the impact of signalized intersection proximity to roundabouts is studied using the developed model. More specifically, the impact of coordinated signalized arterial when a roundabout is inserted within an arterial corridor is studied. Results of average delay measures are comparable to the signalization alternative when the roundabout is operating below capacity. However, at heavy volumes, when the roundabout is operating at capacity, then the performance of signalization is slightly better.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:54:04 GMT</pubDate>
      <guid>https://trid.trb.org/View/775551</guid>
    </item>
    <item>
      <title>National Roundabout Conference: 2005 Proceedings</title>
      <link>https://trid.trb.org/View/775163</link>
      <description><![CDATA[The National Roundabout Conference took place in Vail, Colorado, May 22-25, 2005.  It was sponsored by the Transportation Research Board and the Federal Highway Administration.  The conference covered the following topics, as described in section headings: use in a range of settings, urban to rural and low speed to high speed; design elements in safety; U.S. methods of estimating safety and operational impacts and redefining design criteria; safety measures; use in an urban constrained environment; alternative analysis comparing all intersection alternatives; roundabout experience and practice; how we did it - evaluation, design and completion; making roundabouts work in a mixed environment; signs, paint, illumination, landscaping; pedestrians; modeling, evaluation, capacity; Department of Transportation policy and procedure development; and public involvement and outreach.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:54:03 GMT</pubDate>
      <guid>https://trid.trb.org/View/775163</guid>
    </item>
    <item>
      <title>Roundabouts : A State of the Art in Germany</title>
      <link>https://trid.trb.org/View/775382</link>
      <description><![CDATA[Today, Germany has 15-years of experience with different kinds of modern roundabouts. In addition to that, large conventional roundabouts 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; and larger roundabouts (40 -60 m) with 2-lane access for cars and single-lane operation for trucks. All these types turned out to be very successful regarding both traffic safety and capacity. On the other side the traditional larger 2-lane roundabouts have significant safety problems. The paper describes the German experience from a long series of research projects regarding traffic safety, capacity, and traffic performance estimation as well as geometric design.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:54:02 GMT</pubDate>
      <guid>https://trid.trb.org/View/775382</guid>
    </item>
    <item>
      <title>Incorporating Exiting Vehicles in Capacity Estimation at Single-Lane U.S. Roundabouts</title>
      <link>https://trid.trb.org/View/775543</link>
      <description><![CDATA[The current model used in the U.S. to predict approach capacity at a single-lane roundabout utilizes information about entry driver behavior in relation to the circulating stream of traffic only. No procedure is currently in place for incorporating exiting vehicles in capacity estimation. Exiting vehicles have been shown to have an effect on capacity at roundabout approaches in other countries, but it is not known what effect, if any, exiting vehicles have at roundabout approaches in the U.S. The purpose of this research effort is to determine if the incorporation of exiting vehicles improves capacity estimation at a roundabout approach, and to explain capacity prediction errors through the examination of particular geometric and flow parameters that govern entry and exiting vehicle interactions. Approach capacities were estimated using HCM Equation 17-70, with and without the incorporation of exiting vehicles, and compared to measured field capacities. The findings presented in this report demonstrate that capacity estimates with exiting vehicles result in improved prediction of the actual capacity of a roundabout approach over estimates without exiting vehicles. It was determined that the parameters proportion of exiting vehicles in the major stream and the width of the splitter island provide some explanation of capacity prediction errors, but exactly how the parameters should be incorporated into the capacity prediction process needs to be further explored.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:54:02 GMT</pubDate>
      <guid>https://trid.trb.org/View/775543</guid>
    </item>
    <item>
      <title>Capacity and Performance Analysis of Roundabout Metering Signals</title>
      <link>https://trid.trb.org/View/775172</link>
      <description><![CDATA[This paper describes a method for the analysis of capacity and performance of roundabouts operating with metering signals. When low capacity conditions occur during peak demand flow periods, for example due to unbalanced flow patterns, the use of metering signals is a cost-effective measure to avoid the need for a fully-signalized intersection treatment.  Roundabout metering signals are often installed on selected roundabout approaches and used on a part-time bases since they are required only when heavy demand conditions occur during peak periods.  Metering signals have been used in Australia, the United Kingdom and the United States to alleviate the problem of excessive delay and queuing by creating gaps in the circulating stream.  The Australian roundabout and traffic signal guides acknowledge the problem and discuss the use of metering signals.  The basic principles of the operation of roundabout metering signals are explained.  Case studies of various roundabouts where metering signals were used, or considered for use, have been presented in previous papers by the author.  This paper presents the results of analysis of one of these case studies when operating with metering signals.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:54:01 GMT</pubDate>
      <guid>https://trid.trb.org/View/775172</guid>
    </item>
    <item>
      <title>High-Capacity Roundabout Intersection Analysis: Going Around in Circles</title>
      <link>https://trid.trb.org/View/775448</link>
      <description><![CDATA[Roundabouts have become increasingly popular in recent years as an innovative operational and safety solution at both low volume and high volume intersections. And while tools are available for evaluating roundabout intersection operations, the answers provided by these tools can vary widely. This is particularly true for high-capacity roundabouts (that is, those with flared entry or double lanes). In the U. S., the benefits to installing single-lane roundabouts compared to signalized intersections have been demonstrated, but relatively few high-capacity roundabouts have been built. It is unclear how well the high-capacity roundabout will operate and under which circumstances it will perform better than a signalized intersection. This paper compares the capacity analysis suggested in the FHWA roundabout guidelines with the results of the analysis software packages RODEL, aaSIDRA, VISSIM, and Paramics. The macroscopic models RODEL and aaSIDRA apply formulas based on observed data from U. K. and Australia, respectively. These models use roadway geometry and/or driver behavior to estimate intersection capacity. The microscopic models VISSIM and Paramics simulate individual driver decisions in navigating the roadway network using a stochastic process. As a result, the microscopic model can be more closely calibrated to observed traffic conditions. The authors have found that the macroscopic models may not accurately measure multi-lane roundabout operations in all cases because these models lack sensitivity related to the effects of roadway geometry and gap acceptance. Microsimulation models were found to provide more accurate and reasonable results in this study, but required detailed calibration to accurately represent roundabouts with unique characteristics such as skewed approaches or closely-spaced intersections.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:54:00 GMT</pubDate>
      <guid>https://trid.trb.org/View/775448</guid>
    </item>
    <item>
      <title>Roundabout Model Calibration Issues and a Case Study</title>
      <link>https://trid.trb.org/View/775545</link>
      <description><![CDATA[This paper discusses issues related to calibration of models for analyzing roundabout capacity and performance.  A traffic model framework is presented to help with assessment of traffic models in a general framework, considering all aspects of models relevant to roundabout operation.  While the discussion focuses on analytical models, the issues raised are also relevant to microsimulation models.  Discussion on roundabout models should not concentrate on capacity alone, and instead, modeling requirements for estimating both capacity and performance (delay, queue length, etc.) should be considered together.  Various aspects of field observations relevant to the calibration effort are discussed.  These include issues related to the definition and measurement of capacity, delay and queue length, including the effect of unequal lane utilization.  Delay criteria for level of service definition are also discussed.  Two basic calibration methods that can be used for gap-acceptance and linear regression methods are described.  Further aspects of model calibration discussed include the environment factor, adjustment for the arrival flow / circulating flow ratio, lane utilization factor, heavy vehicle factor, driver response time and calibration of models for operating cost, emissions and fuel consumption.  A case study is presented to compare capacity estimates from the gap-acceptance and linear-regression methods, including a calibration example.]]></description>
      <pubDate>Mon, 27 Feb 2006 08:53:59 GMT</pubDate>
      <guid>https://trid.trb.org/View/775545</guid>
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