Articles

This presentation highlights various aspects of roundabouts in AUSTRALIA and NEW ZEALAND in response to the following objective of the International Roundabout Design and Capacity Seminar: Present an overview of typical roundabout designs in urban and rural environment and treatments of bicycles and pedestrians, capacity and level-of-service methods used, and recent research on capacity and level-of-service. For more detailed information, see the publications referred to in the presentation and refer to related guidelines used in Australia and New Zealand.

Roundabout metering signals help to create gaps in the circulating stream to solve the problem of excessive queuing and delays caused by unbalanced flow patterns and high demand flow levels. This paper gives a brief summary of the control of roundabouts using metering signals and describes the basic concepts of an analytical model of the operation of roundabouts with metering signals. The model estimates capacities and performance measures (delay, queue length, stop rate, and so on) of the metered and controlling approaches of the roundabout as well as other approaches which operate as normal roundabout entries. Timing of roundabout metering signals is discussed and a case study is presented demonstrating the application of the model to a real-life roundabout in Melbourne, Australia, including consideration of alternative timing strategies.

This paper presents an assessment of the roundabout capacity model given in the new Highway Capacity Manual 2010 (HCM 2010) with a focus in its use in the SIDRA INTERSECTION software. The model can be viewed as a non-linear empirical (regression) model with a theoretical basis in gap-acceptance methodology. The general importance of some fundamental aspects of the model is discussed. Model extensions provided by SIDRA INTERSECTION are discussed. Using a multilane roundabout example given in HCM 2010, capacity and the resulting degree of saturation (v/c ratio), delay, level of service and queue length estimates from the HCM 2010 model are compared with those from the SIDRA Standard capacity model for roundabouts. Discussions are presented in relation to the finding of lower capacity of roundabouts in the USA compared with Australian and UK roundabouts and the issue of possible increases in roundabout capacities in the USA over time due to changes in driver behavior.

This paper considers three well-known analytical models of roundabout capacity, and discusses some common and differing aspects of these models. The models considered are the US Highway Capacity Manual 2010 (HCM 2010) model, Australian SIDRA INTERSECTION model and the UK TRL (linear regression) model. These models have some common features as well as significant differences. A detailed table comparing the features of these capacity models is presented. The UK TRL and SIDRA models are compared in relation to several geometric parameters (entry radius, entry angle, inscribed diameter and flaring). Detailed comparison of estimates of capacity and degree of saturation (v/c ratio) produced by these models are presented for a multi-lane roundabout example. The aim of the paper is to enhance understanding of the fundamental aspects of different roundabout capacity models available around the world.

There has been some controversy in relation to the lane-based gap-acceptance model used in SIDRA INTERSECTION software vs the approach-based linear regression (UK TRL "empirical") model used in RODEL and ARCADY software for estimating the capacity of roundabouts. This is relevant to the US scene where roundabouts are relatively new. This detailed discussion note presents Dr Akçelik's views on the subject. A section on lessons learned from the US research on roundabouts leading to the HCM 2010 roundabout capacity model is included, and reference is made to the HCM 2010 model implemented in SIDRA INTERSECTION where relevant.

This short document presents a table comparing main features of three roundabout capacity models, namely the Australian model as implemented in the SIDRA INTERSECTION software, the HCM 2010 model described in the 2010 Highway Capacity Manual, and the UK TRL model implemented in the RODEL and ARCADY software. The HCM 2010 model has been implemented in the SIDRA INTERSECTION software. The terms SIDRA Standard and HCM 2010 models are used to distinguish between the two model options in SIDRA INTERSECTION. The features compared include methodology, model level of detail (lane-based or approach-based), parameters used in the model to represent driver behavior and roundabout geometry, and model calibration methods.

This paper proposes a method for speed control at roundabouts based on setting maximum values of entry path radii for roundabouts, in lieu of the deflection criteria used in the current Austroads (1993) roundabout guide. The proposed method uses the results of a Queensland study of roundabouts. The aim of the Austroads (1993) method is to control speeds through roundabouts by the provision of 'deflection', which is measured as a maximum vehicle path radius of 100m through the circulating carriageway. This method of speed control is based on 1975 design recommendations by the UK Department of the Environment. The current practice in the UK Department of Transport 'Design Manual for Roads and Bridges' provides speed control on the entry curve in lieu of the circulating carriageway. USA and Queensland also have criteria for speed control through roundabouts. This includes setting maximum values of the vehicle speed on the entry curve (and subsequent entry path radii). No mandatory speed control criterion is given for the circulating carriageway.

This paper presents a collation of comments received from Australian traffic and transport professionals in response to a survey about the current status of roundabouts in Australia. The survey posed questions related to the extent to which roundabouts are being built, replacement of roundabouts with signals or signals with roundabouts, use of roundabouts with metering signals and fully-signalised roundabouts, educating road users (drivers, cyclists, pedestrians) in using roundabouts correctly, public's view on roundabouts, current experience with roundabouts in terms of efficiency (delays, congestion), environmental aspects and safety, and future of roundabouts in Australia.

A major project was undertaken for VicRoads, the state transport authority in Victoria, to investigate the performance of roundabouts with metering signals in Melbourne, Australia. Five multi-lane roundabout sites were chosen for comprehensive surveys of traffic and driver behaviour at roundabouts with metering signals. The survey data included video recordings of driver gap acceptance behaviour, intersection turning movement volume counts, automated counting of circulating traffic, GPS-equipped floating car surveys, and metering signal timings. Using the survey data, the entering and circulating traffic characteristics were investigated at the controlling and metered roundabout approaches. These included queue lengths, delays and queue spacing on approach roads, critical gap and follow-up headways of entering drivers, bunching and headway distribution of circulating traffic as well as circulating vehicle speeds. This paper presents various aspects of data collection, analysis, and findings of the project.

Analytical models use traffic stream capacity as a basic parameter in traffic performance estimation. Traffic characteristics that affect capacity are not often clearly explained or understood. This causes difficulties in practice when professional judgement has to be used in interpreting output and calibrating models for specific applications. There is also a need to establish relationships between traffic parameters used in analytical models and microsimulation models. This paper presents a general analytical model that provides a common formulation relating key variables in intersection analysis to various driver behaviour (driver-vehicle) characteristics. These key variables are follow-up headway for gap-acceptance situations (roundabouts, sign control, and filter turns at signals) and saturation flow rate for signalised intersections. This provides a direct relationship between capacity and parameters representing driver behaviour, namely driver response time during queue discharge, spacing between vehicles in the queue (jam spacing) and saturation (queue discharge) speed.

Gap-acceptance capacity models apply to the analysis of minor movements at two-way stop and give-way (yield) sign-controlled intersections, entry streams at roundabouts and opposed (permitted) turns at signalised intersections. The same modelling principles apply to all these cases with different model parameters representing the intersection geometry, control and driver behaviour at different traffic facilities. This paper presents a review of some well-known analytical models that use bunched exponential and simple negative exponential distribution of headways in the opposing stream. Different bunching models are considered including the latest model used in SIDRA INTERSECTION. The capacity estimates from different models are compared. Research recommendations are included. The paper includes a survey method for follow-up headway and critical gap parameters.

This presentation discusses relationships between microsimulation and analytical models.

The following paper is recommended in relation to this presentation:
Akçelik, R., and Besley M. (2001). Microsimulation and analytical methods for modelling urban traffic. Paper presented at the Conference on Advance Modeling Techniques and Quality of Service in Highway Capacity Analysis, Truckee, California, USA.

Estimation of operating cost, fuel consumption and pollutant emissions for evaluating intersection traffic conditions is useful for design, operations and planning purposes in traffic management. A four-mode elemental (drive cycle) model is used for estimating fuel consumption, emissions and operating cost. The drive cycles vary significantly for different intersection types (roundabout, signalised, sign-controlled), for different signal phasings and timings, and for different congestion levels. A case study is presented comparing a roundabout with and without metering signals in terms of operating cost, fuel consumption and pollutant emissions as well as delay and degree of saturation.

Akçelik's time-dependent speed - flow model based on queuing theory concepts is used to develop alternative versions of the HCM speed - flow models for basic freeway segments, multilane highways and urban streets. The corresponding travel time - flow models show that higher-quality facilities have lower levels of bunching delays. A version of the speed - flow model that describes in-stream vehicle interactions and resulting queuing in terms of traffic bunching characteristics is introduced. Speed - flow and headway distribution models for uninterrupted traffic streams are integrated using a common traffic delay parameter. A new model of the proportion of bunched vehicles is proposed for the bunched exponential model of headway distribution. The driver response time parameter at capacity flow is discussed. A model for forced flow conditions is developed, and unsaturated and forced flow conditions are contrasted in relation to determining headway distributions.

This paper discusses issues related to calibration of models for analyzing roundabout capacity and performance. Two basic calibration methods that can be used for gap-acceptance and linear regression methods are described. A case study is presented to compare capacity estimates from the gap-acceptance and linear-regression methods, including a calibration example. 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. 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.

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 basis since they are required only when heavy demand conditions occur during peak periods. Metering signals have been used to alleviate the problem of excessive delay and queuing by creating gaps in the circulating stream. 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.

This paper presents a summary of the differences between the AUSTROADS (1993) Roundabout Guide and aaSIDRA methods for roundabout capacity and performance analysis, and discusses some important aspects of the analysis method where significant differences exist.

Transfund New Zealand funded the development of a signal audit methodology in order to examine the safety and efficiency of traffic signals. A representative number of signal installations across nine territorial local authorities (TLAs) were audited. Stops & Goes of Traffic Signals presents findings of this study.  Targeted at engineers who design, construct, install, manage and maintain traffic signals, the booklet examines the key factors required for safe and efficient operation of signalised intersections. These include technical requirements of the signals such as phasing and operational issues, intersection and lane layout, placement of signal posts, road marking, conspicuity of signals and other factors influencing the environment at intersections.

An improved understanding of the effect of origin-destination demand pattern of traffic on roundabout capacity and level of service will help towards designing new roundabouts that will cope with future increases in demand levels and solving any problems resulting from unbalanced flow patterns at existing roundabouts. Case studies are presented to show that roundabout capacity and level of service depend not only on the circulating flow rate but also the characteristics of approach flows contributing to the circulating flow. The amount of queuing on the approach road, circulating lane use, priority sharing and priority emphasis are the factors that need to be taken into account. Dominant circulating flows that originate mostly from a single approach with high levels of queuing and unequal lane use (with most vehicles in one lane), cause priority emphasis and reduce the entry capacity significantly. This is evident from the use of part-time metering signals under peak demand conditions in order 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.

A single-lane roundabout case study from the United States is presented to compare capacity estimates from various analytical models. Some contradictory results that can be obtained from these models are highlighted and reasons for differences are discussed. Such systematic differences have important design implications. The paper discusses why the TRL Linear Regression model may underestimate capacity for low circulating flows and overestimate capacity for high circulating flows. The case study displays an unbalanced flow pattern which contributes to significant differences between models.

A time-dependent speed-flow model based on queuing theory concepts (referred to as Akçelik's function in the literature) was previously used to develop alternative versions of the HCM speed-flow models for basic freeway segments, multilane highways and urban streets. This paper introduces an explicit model that describes in-stream vehicle interactions and resulting queuing in terms of traffic bunching characteristics. For this purpose, the bunched exponential model of the distribution of vehicle headways is used. A new model of the proportion of bunched vehicles is proposed. The model uses the delay parameter of Akçelik's speed - flow model as a bunching parameter, thus linking the bunching and speed-flow models. The paper also discusses the driver response time parameter at capacity flow. A model for forced flow conditions is then developed, and unsaturated and forced flow conditions are contrasted for the purpose of determining headway distributions. The paper also discusses application of the new bunching model and the associated speed-flow model to roundabout circulating streams.

Estimation of operating cost, fuel consumption and pollutant emissions for evaluating intersection and mid-block traffic conditions is useful for design, operations and planning purposes in traffic management. This paper describes the method to model operating cost, fuel consumption and emissions (CO2, CO, HC, NOx) in the aaSIDRA intersection analysis and aaMotion trip / drive-cycle simulator software packages developed by Akcelik & Associates.

aaMotion was released as the SIDRA TRIP software. {http://www.sidrasolutions.com/software_trip_overview.aspx}

The report explains the uninterrupted travel speed concept, and describes the HCM speed-flow models for basic freeway segments and multilane highways as well as for the running time component of the HCM travel speed model for urban streets. The HCM speed-flow models are given for undersaturated conditions on these uninterrupted-flow facilities. Multilane highway analysis is qualified as relevant to "rural and suburban" highways. Urban roads with signalized intersections spaced at 3 km or more fall into this category, otherwise they are classified as urban streets. The aaSIDRA speed-flow relationships for uninterrupted movements were calibrated against the US Highway Capacity Manual speed-flow models. This effort indicated various unexpected characteristics of the HCM 2000 models. The report presents the results of an investigation of issues of concern with the HCM models. A proposed solution based on the use of "Akcelik's" speed-flow function is presented. Akçelik's function, which applies to both undersaturated and oversaturated conditions is described in detail.

New models for estimating negotiation radius, distance and speed values of through and turning vehicles at roundabouts were introduced in aaSIDRA version 2. The intersection negotiation parameters are important in determining geometric delays, fuel consumption, pollutant emission and operating cost values for traffic using roundabouts. The paper gives figures showing simplified constructions of vehicle paths for through, left-turning and right-turning vehicles. The method allows for path smoothing by drivers. The safe negotiation speed formula uses a side friction factor that is a function of vehicle mass. Graphs showing the side friction factor as a function of the vehicle mass, and negotiation speed as a function of the turn radius are presented. The negotiation radius, distance and speed values as a function of the roundabouts size, and graphs showing the sensitivity of geometric delay to roundabout size and approach and exit cruise speeds are presented.

This paper describes the exponential queue discharge flow rate and speed models, and presents a summary of model calibration results based on data from 18 intersections in Melbourne and Sydney, Australia. Relationships among saturation headway and speed, jam spacing, queue departure response time, queue departure wave speed and acceleration characteristics are presented. Implications of the queue discharge flow and speed models for adaptive signal control practice, namely the SCATS control parameters, optimum detector loop (detection zone) length and the gap setting parameter are discussed.

This website presents lots of information about which side of the road people drive on around the world, and some reasons why. The file below lists the countries driving on the Left-Hand and Right-Hand side of the road. CountriesLH&RH (75KB)

Which side of the road do they drive on?

Please let us know your comments to help us improve this glossary.

Modelling of acceleration and deceleration distances and times associated with speed change cycles under normal driving conditions is essential for the analysis of operating cost, fuel consumption and pollutant emissions, as well as for determining geometric, stopped and queuing components of overall delay. Similarly, modelling of acceleration and deceleration characteristics of individual vehicles is a key issue in relation to the accuracy of microsimulation models. This paper describes the acceleration and deceleration models used in the aaSIDRA software for intersection analysis, and emphasises the research needs for better model calibration considering different vehicle types, specific traffic facilities, different traffic demand levels, different road types, and a wide range of initial and final speeds.

This report presents findings of an investigation pedestrian movement characteristics at three pedestrian actuated mid-block signalised crossings in Melbourne, Australia. Two surveys were carried out at each site, one on a weekday and one on the weekend. A survey method was developed and used as part of this project. The main objective of the study was to obtain information on crossing speeds of pedestrians for signal timing purposes, and pedestrian movement start loss and clearance time gain parameters for pedestrian delay and queue calculations. Other information obtained from surveys included the proportions of pedestrians using different signal intervals (Walk, Flashing Don't Walk). The study also investigated characteristics of pedestrians with walking difficulties. The 15th percentile speed for all sites combined is very close to the general design speed of 1.2 m/s recommended by the Australian and US design guides, and the 15th percentile speed for pedestrians with walking difficulty is very close to the design speed of 1.0 m/s recommended by the Australian and US design guides for sites with higher populations of slower pedestrians.

This paper presents findings of an investigation of pedestrian movement characteristics at intersection signalised crossings. The main objective of the study was to obtain information on pedestrian crossing speeds used for signal timing purposes, and pedestrian movement start loss and clearance time gain parameters used in pedestrian delay calculations. Surveys were conducted at four signalised intersections of four-lane roads in busy suburban shopping strips. Two surveys were conducted at each site, one on the weekend and one on a weekday. Pedestrian crossing speeds for different sites, weekdays and weekends, queued and unqueued pedestrians, and pedestrians with and without walking difficulty are presented. The results of this study are compared with the findings from the study of pedestrians at mid-block signalised crossings.

A major improvement in the latest edition of the US Highway Capacity Manual is the introduction of a back of queue model for signalised intersections. The model was derived by simplification of the more general form used in the aaSIDRA software. The report describes the HCM 2000 queue model fully, and presents comparisons with microscopic simulation data, real-life data and aaSIDRA estimates. The theoretical background to the development of the model is discussed, and a method for determining model parameters in the case of unequal lane utilisation is given. The model uses progression factors for platooned arrivals consistent with the HCM 2000 delay model. The model can estimate the average back of queue, as well as 70th, 85th, 90th, 95th and 98th percentile queue values. A model to predict the queue clearance time is also given for use in the opposed (permitted) turn model and the actuated signal timing method. The back of queue measure is useful for identifying spillback conditions (i.e. the blockage of available queue storage distance). The queue storage ratio measure is presented for this purpose. The queue model allows for an initial queued demand at the start of the flow period.

This Technical Note specifies various conditions for use in implementing the HCM 2000 progression factors for delay and back of queue. The progression factor equations are given, the implementation conditions (constraints) are explained, and the issue of large values of progression factors that may result under certain circumstances is discussed. The method has been approved by the TRB Highway Capacity and Quality and Service Committee.

Microsimulation models can help with the analysis of complex traffic problems in urban areas, alongside the analytical techniques that are in use. However, concerns are often expressed regarding misuse of microsimulation. This paper focuses on compatibility between microsimulation methods and established analytical techniques that are used in traffic engineering. Several key components of traffic models are discussed, and various recommendations are made, with a view to improving the practical usefulness of microsimulation models. These include (i) the use of simulation for capacity analysis, (ii) modelling of queue discharge (saturation) flow rate, queue discharge speed and other queue discharge parameters at signalised intersections, and relating them to the general queuing, acceleration and car-following models used in microsimulation; (iii) modelling of gap-acceptance situations at all types of traffic facilities, and (iv) estimation of lane flows at intersection approaches, and relating this to lane changing models used in microsimulation. The consistency of definitions and measurement methods for traffic performance variables such as delay (stopped, geometric, etc) and queue length (cycle average and back of queue) is also discussed. It is suggested that comparison of specific microsimulation and analytical model components is useful towards model benchmarking for evaluation of new and existing models. Towards this end, a simple signalised intersection case is specified in sufficient detail to enable assessment of two basic traffic model components, namely queue discharge flow rate and lane flow distribution

This paper presents commentary on selected points from an earlier article titled "Traffic Engineering Folklore" which presented "a summary of quick and easy approximations to traffic related problems" based on an Australia-wide survey of traffic professionals in the 1980s. The comments given here are based on information from recent research and detailed analytical and computer-based traffic modelling. The points covered relate to vehicle dimensions, capacities of uninterrupted and interrupted facilities, traffic flow characteristics, vehicle and pedestrian speeds, signalised intersections and roundabouts. It is shown that most points considered are generally valid as approximations. Suggestions are made for revising the statements analysed, and additional statements are presented for consideration.

Free download is available from AUSTROADS Publications website.

North Carolina State University carried out a comprehensive study and evaluation of alternative software packages for signalised intersections (SIDRA, TRANSYT, HCS/HCM, Signal 94, HCM/Cinema, EVIPAS) for the North Carolina Department of Transportation.  SIDRA was rated best on all accounts (model usability , quality of signal optimization and delay model evaluation) and recommended for use as the standard model.  The report concluded: “SIDRA’s powerful combination of high model usability, superior matching of field delays at the lane group level, versatile optimization features, and graphical user interface makes it the model of choice from this study.”.

This report presents findings of a study of fundamental characteristics of freeway traffic flows based on analysis of individual vehicle data collected on the Eastern Freeway in Melbourne under both saturated and unsaturated conditions using a two-loop presence detection system. The aim of the study was to assess data collection and analysis methods, and to develop analytical models to describe the relationships between traffic flow parameters. The traffic flow parameters considered include flow rate, speed, density, spacing, gap length, vehicle length, headway, occupancy and space time, gap time, vehicle passage time and occupancy ratios. Travel speeds were also measured using an instrumented car. Six analytical traffic flow models are presented including various single-regime and two-regime models. Model calibration results are summarised and the parameters representing the traffic characteristics at the survey site are given. Graphs showing fundamental relationships with measured data and model predictions are presented.

This report presents findings of a study of fundamental traffic characteristics at signalised intersections based on surveys of queue discharge headways and speeds for individual vehicles and jam spacings at eighteen intersections in Sydney and Melbourne. Exponential queue discharge flow, headway and speed models are given. Other traffic parameters considered are spacing, gap length, density, gap time, occupancy time, space time, occupancy ratios, queue clearance wave speed, departure response time, saturation flow rate, start loss and end gain times. The report presents basic material on fundamental traffic flow relationships, describes the survey methodology, survey site characteristics, data processing, analysis method, calibration method, and presents calibration results. Results on uninterrupted flow models and downstream queue interference at paired intersections are presented. Implications of findings on capacity and performance modelling and adaptive signal control practice are discussed. Relationships for use in practice are given.

The well-known SCATS wide-area traffic control system has a unique feature for the control of a single non-coordinated signalised intersection known as SCATS Master Isolated control. The effectiveness of this control was evaluated using microsimulation and field surveys. Various SCATS-like adaptive control algorithms were developed and tested through extensive simulation tests with a wide range of demand flow patterns and a large number of control parameter combinations. Traditional vehicle-actuated and fixed-time control were also evaluated. The results for an algorithm that emulated the SCATS Master Isolated control, and the traditional vehicle-actuated control method are presented in this paper. A new survey method was developed, trialed and used for field surveys. The survey method produced a large number of timing, capacity and performance statistics. The surveys produced results that were in line with the results of simulation studies. The overall conclusion was that SCATS Master Isolated control gives better intersection performance than traditional actuated signal control as indicated by lower delays and shorter queue lengths achieved with shorter cycle times. Evaluation of the effects of detector failure indicated substantial benefits from the SCATS Master Isolated control in terms of all performance measures considered.

Using aaSIDRA, VicRoads engineers redesigned a highly congested two-lane roundabout in Melbourne, Australia as a three-lane roundabout eliminating persistent congestion. Detailed field surveys were conducted before and after the reconstruction. This article presents detailed information about this study.

Results of extensive research on actuated signal control methods are presented. A method for predicting actuated signal timings, and analytical models for predicting various performance statistics (delay, queue length, queue clearance time, proportion queued), that are implemented in the SIDRA software package are described. With actuated signal timings, equal degrees of saturation do not necessarily result, and the minor movements may get lower degrees of saturation than the major movements, indicating inefficient operation with long cycle times. Comparisons of SIDRA estimates of delay, back of queue and queue clearance time for actuated signals with those predicted by microsimulation as well as those observed at a real-life intersection are presented.

This paper was prepared in response to two articles, "ARCADY Health Warning: Account for unequal lane usage or risk damaging the Public Purse!" by B. Chard, and "Modelling flares at traffic signal-controlled junctions" B.F. Simmonite and P. Moore. These articles address problems associated with the "approach" method of traffic modelling which lumps traffic in individual lanes of an intersection approach together. Chard demonstrates by means of case studies that "(the ARCADY model) can take no account of either unused or unequally used lanes or flared sections on roundabout entry approaches. Simmonite and Moore state that "the art of modelling (flared approaches) is difficult and, as such, often overlooked by practitioners". This paper discusses important aspects of the two articles from the perspective of the lane-by-lane method and short lane model used in aaSIDRA for all types of intersection (signalised, roundabout, sign control). SIDRA computations are heavily based on estimating lane flows, modelling traffic in shared lanes including any lane blockages, establishing any de facto (effective) exclusive lanes, determining reduced short lane capacities and any excess flows from short lanes into adjacent lanes.

New capacity and performance (delay, queue length, proportion queued and queue move-up rate) models are presented for unsignalised intersections (roundabouts and give-way and stop-sign control). The new models were developed by converting the block and unblock periods in traditional gap acceptance modelling to effective red and green periods by analogy to traffic signal operations. This enabled the modelling of performance statistics in a manner consistent with models for signalised intersections. The models are based on the bunched exponential model of arrival headway distributions for all traffic streams, and are also applicable to simple negative exponential and shifted negative exponential distributions. Data generated by a modified version of the microscopic simulation model MODELC were used for model calibration. The new capacity model is compared with various existing formulae based on gap-acceptance modelling.

The estimation of arrival headways is fundamental to the modelling of gap acceptance processes for estimating capacities of sign-controlled traffic streams, roundabout entry streams and filter turns at signalised intersections. It is also essential in modelling both vehicle-actuated signal timings and queuing at all types of intersections for performance prediction. This paper considers a class of arrival headway distributions known as negative exponential, shifted negative exponential and bunched exponential. A description of the bunched exponential arrival headway distribution is presented, and the results of its calibration using real-life data for single-lane traffic streams and simulation data for multi-lane streams are given. Examples of gap-acceptance capacities and delays predicted by different exponential headway distributions are also presented.

A travel time function proposed by Davidson for transport planning purposes has been subject to much discussion and efforts of calibration including some controversy over the meaning of its parameters. This paper presents a time-dependent form of the original Davidson function, derived using the coordinate transformation technique. The derivation of the original Davidson function is discussed. A new travel time function is proposed as an alternative to Davidson's function. In the proposed function, the delay parameter takes a meaning consistent with the formulae used for estimating intersection delays. Both the steady-state and time-dependent forms of the new function are given.

Excel application that includes calculations and graphs for various travel time functions discussed in the 1991 paper (copy above).

This paper presents information about the HCM option in the SIDRA software and discusses various aspects of the 1985 HCM / HCS methodology as well as the proposals to modify it.  An application of SIDRA to an example from the HCM is presented.  The paper also presents a discussion of the generalised delay formula used in SIDRA.

This article responds to the question "Which principle should be used as a basis for estimating lane flows at the approaches to an intersection, and how does this principle relate to the lane choice decisions of individual drivers? ". Equal degree of saturation, equal average delay, minimum travel time, equal queue length principles are discussed. Relationship between lane choice and route choice principles, interaction with signal timings, and relation to the Level of Service criteria are also considered.

An important element of capacity estimation at signalised intersections is the estimation of opposed (permissive) turn saturation flows. Although the basic modelling philosophies of the Highway Capacity Manual and Australian methods are similar, there are significant differences in the procedures used and therefore, in the results from the two methods. In particular, the method used in the SIDRA INTERSECTION software has eliminated the use of opposed turn adjustment factors for lane groups and adopted an explicit and direct method of modelling individual lanes. The paper discusses these new methods

The traditional adjustment factor and the new lane interaction methods for estimating the capacity of a shared lane at a signalised intersection are discussed. The adjustment factor method is needed when the shared lane is combined with adjacent lanes into a lane group with the same departure characteristics (Highway Capacity manual method). The lane interaction method as implemented in the SIDRA INTERSECTION software employs a direct method to predict individual lane capacities without using adjustment factors. The method allows for differences in the departure characteristics of shared lanes (e.g. opposed turns and through traffic) and the adjacent lanes (e.g. through traffic only) by treating the intervals of lane blockage as effective red (lost time). This improves the prediction of not only the queue lengths and delays but also the short lane and opposed turn capacities. A generalised model of lane blockage is employed for predicting the number of departures before being blocked.

Three new models of acceleration profile (a two-term sinusoidal, a three-term sinusoidal and a polynomial model) are described.  A comparative evaluation of these three models and the previously known constant and linear-decreasing acceleration models is reported.  The polynomial model has been found to be the best overall for predicting acceleration distance and fuel consumption.  Similar results have been found for deceleration profiles.

A substantial set of analysis techniques exist for the consideration of fuel consumption in urban traffic management.  This report provides a guide to assist the traffic manager in selecting techniques which are appropriate to the various traffic management contexts, and presents a comprehensive guide to the use of techniques for fuel consumption analysis in urban traffic systems.  Fuel consumption models of four levels of detail are described and numerical examples are given to illustrate their use.  These are an energy-related instantaneous fuel consumption model, a four-mode elemental (modal) model, a running speed model and an average speed model.  These fuel consumption models are inter-related, forming part of the same modelling framework and the vehicle parameters are explicit at all model levels.

SIDRA-2 implements the techniques described in the ARRB Research Report ARR No. 123 (Traffic Signals: Capacity and Timing Analysis). Although the fundamental principles and methods are the same, there are some important differences between ARR No. 123 and SIDRA-2. The most important is the lane-by-lane calculation of capacities and operating characteristics such as delay, number of stops and queue length. Lane-by-lane calculations allow for the use of different lost times, hence different effective green and red times, for different lanes, which improve the results for operating characteristics.

Research Report ARR No. 124 collects together four papers presented at an ARRB Seminar on Fuel Consumption Modelling in 1981, as well as two subsequent papers. The objective of the Seminar was to focus on the issue of fuel consumption modelling and data needs for urban traffic management purposes.  The first paper (Part 1) specifies the general requirements of a fuel consumption model which is compatible with other elements of the traffic system analysis process.  It then discusses an elemental model of fuel consumption. In Parts 2 and 3, the elemental model is discussed in more detail and the Positive Kinetic Energy (PKE) model is explored. In Part 4, problems associated with fuel consumption measurement are discussed.  The elemental and the PKE models are then discussed in detail and criticism of the elemental model is provided. In Part 5, the authors of the four previous papers present a joint statement of the resolution of differences in the approaches adopted by them for developing simple fuel consumption models. It is shown that, subject to various simplifications and an unexplained term, the elemental and PKE models are very similar. In the last paper (Part 6), results of further studies are reported which answer some of the questions raised in previous parts of the report.

This report presents techniques for the analysis of capacity and timing requirements of traffic at signalised intersections. Its purpose is to both provide methods that can be used by the traffic engineer for detailed geometric and operational design of signalised intersections and to also explain their background to help traffic engineering students develop a better understanding of the subject. The methods given in this report follow the basic framework established in earlier publications which have influenced the Australian and U.K. signal design practices. The present report introduces several changes to the traditional techniques, a basic change being from phase-related methods to movement-related methods. After the publication of this report, the SIDRA computer program was developed as a design and research aid to implement the techniques described in the report.

New expressions are presented for delay, stop rate and queue length at traffic signals operating in a fixed-time mode for flow conditions which last for a finite period of time. The emphasis is on the use of the overflow queue concept which facilitates a better understanding of the transition from undersaturated conditions to oversaturated conditions. Each one of the new expressions for delay, stop rate and queue length is presented in a simple form consisting of a uniform component and an overflow component. The latter is an explicit function of the average overflow queue. Transition functions for the average overflow queues at isolated as well as co-ordinated signals are presented.

The queue length, delay and stop rate at isolated undersaturated signals are discussed.  A formula is given for predicting stop rates.  The formula allows for multiple stops in oversaturated cycles as well as a reduction due to partial stops, i.e. vehicles which slow down without coming to a complete stop.  Formulae for pedestrian stops, delay and queues are given.