## Page Contents

- SIDRA for Highway Capacity Manual
- HCM roundabout capacity model in SIDRA INTERSECTION
- Extensions for HCM roundabout capacity model
- Back of Queue Model in SIDRA INTERSECTION
- HCM & SIDRA INTERSECTION Methodology
- References

**SIDRA INTERSECTION** software complements **Highway Capacity Manual (HCM Edition 6)** as an **advanced intersection analysis** tool. Using SIDRA INTERSECTION, you can evaluate intersection and network treatments involving signalized intersections (fixed-time / pretimed and actuated), signalised and unsignalised pedestrian crossings, roundabouts (unsignalised), roundabouts with metering signals, fully-signalized roundabouts, two-way stop sign and give-way / yield sign control, all-way stop sign control, single point interchanges (signalised), freeway diamond interchanges (signalised, roundabout, sign control), diverging diamond interchanges and other alternative intersections and interchanges all in one package. Intersections with up to 8 legs, each as a two-way road, one-way approach or one-way exit.

The **Highway Capacity Manual software setup ****of SIDRA INTERSECTION** offers options for US Customary and Metric units.

The Highway Capacity Manual is prepared by the US Transportation Research Board (TRB) Committee on Highway Capacity and Quality of Service.

SIDRA INTERSECTION is compatible with the US **Highway Capacity Manual (HCM)**. However, unlike some software packages available in the USA, the **HCM software setup of SIDRA INTERSECTION **does not claim to be a simple replication of the HCM procedures. This means that:

- Generally (for all types of intersection and network), SIDRA INTERSECTION uses more advanced models and methods, including lane-by-lane analysis rather than analysis by lane groups, modeling of approach and exit short lanes, detailed modeling of geometric delays, and the use of drive cycles (cruise, acceleration, deceleration and idling) for detailed modelling of delay and travel time components as well as operating cost, fuel consumption and emission estimation.
- The
**HCM software setup of SIDRA INTERSECTION**is based on the calibration of most model parameters using the HCM defaults as applicable. - SIDRA INTERSECTION includes various configuration options to allow the user to choose between the HCM and standard SIDRA model options, for example the HCM roundabout capacity model options, the HCM Delay and Queue model options, intersection and network level of service options, and so on.
- The
**HCM Edition 6**and**HCM 2010 roundabout capacity models**are fully integrated in SIDRA INTERSECTION with significant extensions. Together with the**SIDRA Standard**roundabout capacity model with calibration for US conditions, these are offered as options for roundabout capacity modeling. SIDRA INTERSECTION offers**major extensions**to the HCM roundabout capacity and performance estimation method. For detailed discussion including a summary of the extensions to the HCM roundabout capacity model offered by SIDRA INTERSECTION, refer to**HCM Roundabout Capacity Model in SIDRA INTERSECTION**. - For
**signalised intersections**, in addition to the general features mentioned above, advanced signal timing methods are available, and the use of two green periods for modeling slip lanes, RTOR and permitted-protected left-turns provides more accurate capacity estimates. For detailed discussion, refer to**SIGNALS**. - Some HCM models or methods are used in all versions of SIDRA INTERSECTION, including the HCM Level of Service method and criteria, the delay and queue progression factors, roundabout capacity model, various aspects of saturation flow estimation for signals and critical gap and follow-up headway estimation for Two-Way Stop Control, and so on. For All-Way Stop Control, SIDRA INTERSECTION allows the user to specify the Departure Headway values estimated by the HCM method as input and offers model extensions for performance estimation.
- The HCM and SIDRA INTERSECTION
**capacity and performance models**are compatible in their basic structures and principles, for example the HCM delay models for**signals**were derived (originally in HCM 1985 edition) from the basis of the model used in SIDRA INTERSECTION model (Akçelik 1981).

According to the US Transportation Research Board document "**Roundabout Practices**", SIDRA INTERSECTION is the most widely-used software tool in the USA for roundabout capacity and performance analysis, followed by Synchro and HCS, based on a survey of state transportation departments (based on 28 of the 37 state agencies responding to the survey question).

The report also states "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; about one-quarter use some form of the United Kingdom equations. (Because states were allowed to select more than one operational performance model, there is some overlap.)"

A detailed paper on the HCM version of SIDRA was published by Akçelik (1990). Some aspects of this paper are outdated due to the changes introduced in later editions of HCM and the adoption of those changes in SIDRA INTERSECTION.

Detailed explanations of all models related to the Highway Capacity Manual are included throughout the SIDRA INTERSECTION User Guide.

The roundabout capacity model for single-lane and multi-lane roundabouts based on research on US roundabouts as described in Chapter 22 of **Highway Capacity Manual Edition 6** is available in SIDRA INTERSECTION.

SIDRA INTERSECTION offers a unique implementation of the HCM Edition 6 roundabout capacity model with **major extensions** to the capacity and performance estimation method. A summary of the extensions offered by SIDRA INTERSECTION are given below.

The roundabout capacity model is the only analysis method in HCM Edition 6 (as in HCM 2010) which uses lane-by-lane analysis, and as such it is best implemented by SIDRA INTERSECTION which has been using a lane-by-lane methodology for all types of intersection since 1980s, now with the ability to allocate vehicles belonging to individual Movement Classes (Light Vehicles, Heavy Vehicles, Buses, Bicycles, Large Trucks, and so on) to specific lanes.

The Highway Capacity Manual (HCM) software setups of SIDRA INTERSECTION use the **HCM Edition 6** roundabout capacity model as the default model. The **HCM 2010** roundabout capacity model continues to be available as an option. The **SIDRA Standard** roundabout capacity model is the default option for all other software setups (Standard Left, Standard Right, New Zealand, New South Wales). These three models are available as options for user to select in all software setups of SIDRA INTERSECTION.

SIDRA INTERSECTION is the * most widely-used software tool* in the USA according to a recent NCHRP survey of state transport agencies as reported in the following publication which is available for download:

TRB (2016).** Roundabout Practices**. A Synthesis of Highway Practice, NCHRP SYNTHESIS 488. Transportation Research Board, Washington, DC, USA.

The report also states that, to analyze roundabout performance, about three-quarters of the reporting states use some form of the Highway Capacity Manual model and SIDRA Standard Model; about one-quarter use some form of the UK equations.

Highway Capacity Manual Edition 6 (as in HCM 2010) acknowledges the need to use alternative models in view of various shortcomings it lists about the HCM roundabout capacity model. SIDRA INTERSECTION is one of the alternative tools referred to in the HCM.

The limitations listed in the HCM are addressed by SIDRA INTERSECTION through extensions to the HCM Edition 6 and HCM 2010 roundabout capacity model options or as part of the SIDRA Standard roundabout capacity model option. The extensions offered by the HCM HCM Edition 6 and HCM 2010 roundabout capacity models in SIDRA INTERSECTION are listed below.

**Network Applications**: SIDRA INTERSECTION provides a sophisticated network model for analysis of any combination of signalised and unsignalised intersections. Closely-spaced multiple ("paired") roundabout designs, roundabout corridors, roundabouts with upstream signalised intersections, pedestrian crossings near roundabouts, and so on can be analysed to model interactions between intersections including lane blockage by downstream queues (queue spillback), capacity constraint at oversaturated upstream intersections, extra bunching from upstream signals, midblock lane changes, and so on. Using this method, Sites forming the network can be modeled using the HCM roundabout capacity model options.**More Than Two Entry and Circulating Lanes**: Roundabouts with more than 2 lanes and up to 8 legs can be analysed with any configuration of number of approach and circulating lanes, lane types and lane disciplines using the*HCM Edition 6*,*HCM 2010*and*SIDRA Standard*roundabout capacity model options. These include single and multiple*shared and exclusive slip lanes*(yielding bypass lanes) controlled by yield or stop signs and*continuous bypass lanes*.

The*SIDRA Standard*roundabout capacity model is sensitive to many parameters related to roundabout geometry, namely roundabout diameter, entry radius, entry angle, entry lane width, circulating lane width, number of entry lanes and circulating lanes, short lanes for approach flaring, exit short lanes and other geometric parameters. However, the he*HCM Edition 6*and*HCM 2010*roundabout capacity models are only sensitive to the number of entry and circulating lanes.**Roundabout Metering Signals**: Roundabout metering signals can be used to create gaps in the circulating stream in order to solve the problem of excessive queuing and delays at approaches affected by highly directional (unbalanced) flows. The SIDRA INTERSECTION method for analysis of roundabout metering signals is available using the capacities from the HCM roundabout capacity model options.**Capacity Constraint**: The HCM roundabout capacity model treats the roundabout as a series of independent T-intersections. Instead, the SIDRA Standard roundabout capacity model treats the roundabout as a single interactive system of intersection approaches and circulating flows. An important aspects of this model is the capacity constraint which is necessary to apply when one or more roundabout entry lanes are oversaturated (v/c ratio above 1). This method limits the amount of traffic that can enter the roundabout circulating road from each oversaturated lane to its capacity value. This affects the circulating and exiting flow rates of downstream approaches, thus requiring iterative calculations. This essential element of roundabout modelling is not included in the HCM roundabout capacity model. The SIDRA capacity constraint method is applied to the HCM roundabout capacity model in SIDRA INTERSECTION.**Critical Gap and Follow-Up Headway Reduction with Increasing Demand Flows**: The HCM roundabout capacity model uses fixed critical gap and follow-up headway values whereas the SIDRA Standard roundabout capacity model reduces the critical gap and follow-up headway parameter values with increasing circulating flow rates. This has an important effect on design life analysis since increased demand flow rates in future years mean increased circulating flow rates. Reductions in the critical gap and follow-up headway parameter values with increasing circulating flow rates emulate more aggressive driver behaviour in future years, thus compensating for capacity losses and increased delays to some extent.**Upstream Signal Effect on Capacity**: Effect of upstream signals on roundabout capacity is modeled using the*Extra Bunching*parameter. In network analysis, SIDRA INTERSECTION will determine Extra Bunching automatically. Theoretically, the Extra Bunching parameter does not affect gap acceptance capacity in the case of random arrival distributions as in the case of the HCM roundabout capacity model. However, SIDRA INTERSECTION will apply the program-determined and user-specified extra bunching values for the effect of upstream signals on roundabout capacity to the HCM roundabout capacity model. For this purpose, an Extra Bunching Adjustment Factor is determined from capacities obtained with and without extra bunching using the bunched exponential model used in the SIDRA Standard model.**Unbalanced Flow Conditions**: An important aspect of treating a roundabout as a single interactive system in modelling roundabout capacity is the ability to model unbalanced flow conditions. Under these conditions, the circulating flow originates mostly from one (dominant) approach and is highly queued on the approach before entering the roundabout with uniform queue discharge headways, thus reducing downstream entry capacities at high demand flow conditions. Refer to the Articles page for numerous publications discussing the effect of unbalanced conditions at roundabouts.

The SIDRA Standard roundabout capacity model uses an iterative method to allow for the effect of unbalanced flow conditions considering the effects of origin-destination pattern of entry flows, proportion queued and any unequal lane use at entry lanes of upstream approaches. The SIDRA method for unbalanced flow conditions can be applied as an extension of the HCM roundabout capacity model as well. The use of the calibration parameter*Entry /Circulating Flow Ratio*will further improve the modeling of unbalanced flow conditions. This is useful in dealing with specific conditions when the entry flow rate is high and the circulating flow rate is low rather than relying on a regression method for general average conditions.**Priority Reversal and Priority Emphasis**: Priority reversal (priority sharing) between entering and circulating vehicles under high flow rates is related to low critical gap values at high circulating flow rates. Using the*SIDRA Standard*model, which uses a bunched exponential distribution of arrival headways, this condition is identified and indicated in output. Under most conditions (except low circulating flow rates), gap-acceptance parameters estimated by the*SIDRA Standard*model imply priority sharing.

The Origin-Destination (O-D) Factor in the SIDRA Standard model makes adjustment for the limited-priority gap-acceptance process although the process can be one of priority emphasis (opposite of priority reversal) in the case of unbalanced flow patterns.

The priority reversal condition cannot be identified by the HCM Edition 6 or HCM 2010 model which assumes a random distribution of circulating stream headways (Siegloch M1 model). This model assumes zero intrabunch value. However, values of the follow-up headway and critical gap values used by the HCM 2010 model indicate possibility of priority reversal in reality.**Flared Entries and Short Lanes**: In the SIDRA Standard roundabout capacity model, approach flaring effects are predicted through the use of entry lane width parameter (extra width at the give-way line which is not sufficient for a separate queue to form) and short lane modeling (extra width which allows for an additional queue to form).

The SIDRA INTERSECTION approach short lane model applies to the HCM roundabout capacity model as an important extension. Modeling of short lane capacity is an important part of roundabout capacity modeling since such short lanes (flares) may be very effective in providing additional capacity at roundabouts. SIDRA INTERSECTION uses a unique capacity model for short lanes making use of gap acceptance cycles (blocked and unblocked intervals) to determine stop-line departure rates of a short lane and the full-length lane receiving its overflow queue which is normally an adjacent lane. For the HCM roundabout capacity model, its gap-acceptance characteristics are used for this purpose.

Since the SIDRA Standard and HCM roundabout capacity models are lane-based, and with the use of short lane models, approach flaring parameters are not needed unlike the UK approach-based linear regression model.

The effectiveness of short lanes depends on flow conditions. Short lanes allocated to turning streams exclusively, or flares on single-lane approaches, do not necessarily reduce the v/c ratio of the approach when the flow rates of traffic using short lanes (flares) are low. Modeling of short lanes (flares) using geometric parameters only can therefore underestimate the degree of saturation (v/c ratio) of the approach.**Model Calibration**: Model calibration is important for the applicability of the HCM roundabout capacity model to different local conditions, and for accommodating changes in driver characteristics over time. The HCM recommends calibration of the model by specifying values of model parameters using known follow-up headway and critical gap values. While this is still possible, SIDRA INTERSECTION provides the Model Calibration Factor (HCM 6) and Model Calibration Factor (HCM 2010) parameters for easy calibration of the HCM 6 and HCM 2010 roundabout capacity models.**Entry Lane Flow Calculations**: SIDRA INTERSECTION determines entry (approach) lane flows according to the equal degree of saturation principle subject to user-specified and program-determined lane under-utilisation factors including exit short lane effects. Furthermore, de facto exclusive lane cases are identified and taken into account appropriately during iterative lane flow calculations. Vehicles belonging to different Movement Classes can be assigned to specific lanes, e.g. Bus slip / bypass lanes. Special Movement Classes can also be used for improved estimation of unequal approach lane use considering downstream intersection destinations of movements. The method applies to the HCM roundabout capacity model including roundabouts with more than 2 lanes.

The lane flow calculation method used in SIDRA INTERSECTION allocates lower volumes for lanes with lower capacities (subdominant lanes). The lane volume factors in the HCM roundabout capacity model allocates higher volumes to dominant lanes. Determining lane flows without considering lane capacities may end up allocating unduly high volumes into low-capacity lanes, resulting in high degree of saturation (v/c ratio) and high delay, and therefore unreasonable lane flow distributions (i.e. implying that drivers choose lanes with higher delay). This may also manifest itself through inconsistent "critical lane" definition where the critical lane is defined as the lane with highest degree of saturation.**Circulating Lane Flows**: SIDRA INTERSECTION calculates circulating lane flow rates in front of each entry. The circulating lane flow rates are given in output reports. The SIDRA Standard roundabout capacity model takes into account the effect of flow distribution in circulating lanes in addition to the total circulating flow in determining entry lane capacities. The HCM roundabout capacity model considers only the total circulating flow and not the flow rates in individual circulating lanes.**Heavy Vehicle Effects**: SIDRA INTERSECTION allows for the effects of heavy vehicles and other Movement Classes on capacity using the Gap Acceptance Factor and Opposing Vehicle Factor input parameters that vary according to the Movement Class. For the HCM roundabout capacity model, the capacity estimates are adjusted directly. The SIDRA Standard capacity model adjusts the follow-up headway and critical gap values for the effects of heavy vehicles and other movement classes rather than adjusting the capacity estimate directly. Ability for the analyst to specify the Gap Acceptance Factor and Opposing Vehicle Factor parameters as input by OD (turning) movement and Movement Class is useful for model calibration in specific situations, e.g. when there are large commercial vehicles in particular turning movements.

It is important that the effects of movement classes are determined for each entry lane rather than the whole approach since different movement class percentages for individual turning movements result in different movement class percentages per lane according to lane flow allocations.**High Pedestrian Levels**: The SIDRA Standard and HCM roundabout capacity models use the same method described in the HCM for the effect of pedestrians on roundabout entry capacity. Exit lane capacities as a function of pedestrian flows are also determined for all roundabout legs using a gap acceptance method. More detailed analysis of unsignalized and signalized pedestrian crossings on roundabout approaches can be carried out using network modelling in SIDRA INTERSECTION.**Performance Calculations**: Back of queue and stop rate estimates, as well as fuel consumption, emission (including CO2) and operating cost estimates determined using the HCM roundabout capacity model results are important model extensions offered by SIDRA INTERSECTION.

Roundabout negotiation speeds and negotiation distances are estimated and geometric delays are calculated as a function of approach, exit and negotiation speeds and distances, thus allowing for speed variations of vehicles negotiating roundabouts. Geometric delays can be added to the delay results from the HCM roundabout capacity model (optional).

Back of queue models, including percentile queue and probability of blockage modeling, are used for roundabouts and sign-controlled intersections (not available in the HCM). The models are consistent with back of queue models for signalised intersections (available in the HCM) and are useful for modeling of short lanes and for estimating the probability of blockage of upstream lanes. This is an essential element of the SIDRA INTERSECTION Network model.

For unsignalised intersections, the HCM method gives a*cycle-average queue*rather than a*back of queue*. SIDRA INTERSECTION will always give the back of queue as the queue length in all output reports for analysis consistency. The back of queue is the parameter relevant to estimating the blockage of upstream intersection lanes (for queue spillback modeling) as well as short lane overflow probabilities. Cycle-average queues are given in the queue length tables in the Detailed Output report for information only.

Estimates of stop rates and proportion queued are provided for all types of intersection including roundabouts.

Time-proven models of fuel consumption, emissions and operating cost, implemented using the HCM 6 or HCM 2010 roundabout capacity model results as input, are important model extensions offered by SIDRA INTERSECTION.**Level of Service**: SIDRA INTERSECTION offers options for choice of alternative Site Level of Service methods and the Site Level of Service Target parameter is available for specifying the*acceptable (target) Level of Service levels*for different intersection types, e.g. for Design Life analysis.

SIDRA INTERSECTION also allows for alternative Level of Service Methods for roundabouts, namely*Same as Sign Control*,*Same as Signalised Intersection*and a unique*SIDRA Roundabout LOS*. These options are available for choice of alternative LOS criteria for roundabouts since the use of LOS thresholds which are the same as those used for sign-controlled intersections may not be appropriate for roundabouts as this would create a bias against roundabouts when compared with signalized intersection treatments.**Templates**: SIDRA INTERSECTION offers numerous TEMPLATES for roundabouts including all roundabout examples given in MUTCD 2009 and TRB Roundabout Informational Guide (NCHRP Report 672).

Different guides and software packages refer to two types of queue length, namely the **back of queue** and the **cycle-average queue***.* The SIDRA INTERSECTION output reports and displays include the back of queue estimates for all types of intersection. Cycle-average queues are given in the queue length tables in the Detailed Output report for information only.

SIDRA INTERSECTION uses a unique method for estimating back of queue values for unsignalised intersections by modelling gap acceptance cycles Akçelik (1994). A gap acceptance cycle consists of a block period and an unblock period, i.e. vehicles waiting due to lack of an acceptable gap, then departing when an acceptable gap occurs, similar to a signal cycle that consists of a red period and a green period.

The cycle-average queue is the average value of the number of vehicles in the queue during each cycle. This would be based on a queue count recorded frequently, e.g. every 5 seconds. The cycle-average queue length incorporates all queue states including zero queues observed towards the end of the cycle.

The back of queue is the maximum extent of the queue that occurs once each cycle, usually during green time or unblock time. Zero queue states are not relevant to the back of queue.

The back of queue is a more useful performance measure since it is relevant to the design of appropriate queuing space, e.g. for short lane design to avoid queue spillback into adjacent lanes, for phasing design to avoid blockage of upstream intersection lanes in networks situations, and for signal coordination offset design to prevent interruption of platoons by downstream queue. The back of queue is also used for the prediction of such statistics as the saturated portion of the green period and for modelling short lane capacities.

The signalised intersection chapter of the Highway Capacity Manual includes a back of queue model which was originally developed by Akçelik (1995, 1996). The HCM back of queue model for signalised intersections will be used instead of the standard SIDRA INTERSECTION model when the **HCM Queue Formula** option is selected. The two models are based on the same modelling methodology and give close results.

On the other hand, ** HCM does not give back of queue models for roundabouts and sign-controlled intersections**. For model consistency, the standard SIDRA INTERSECTION back of queue model will be used for roundabouts and sign-controlled intersections rather than the cycle-average queue given in the HCM. The results of this back of queue model are unique to the HCM software setups of SIDRA INTERSECTION since the model uses parameters based on the defaults used in the HCM versions of the software. For example, for roundabouts, gap-acceptance parameters of the HCM roundabout capacity model are used in the back of queue model.

Back of queue models, including **percentile queue** modeling, used for roundabouts and sign-controlled intersections are consistent with back of queue models for signalised intersections and are useful for modeling of short lanes and for estimating the ** probability of blockage** of upstream lanes. This is an essential element of modelling queue spillback (upstream lane blockage) in the SIDRA INTERSECTION **Network model**.

It should also be noted that the **HCM Delay Formula** option in SIDRA INTERSECTION will cause delays for signalised intersections, roundabouts and two-way stop-sign control and all-way stop to be calculated using the HCM equations regardless of the software setup chosen. Otherwise, the standard SIDRA INTERSECTION delay equations will be used in all cases. Independent of the HCM Delay Formula option, the *geometric delay* component of delay can be included or excluded by the user. However, to be consistent with the Highway Capacity Manual equations, geometric delay is automatically excluded when the HCM Delay Formula option is selected. The HCM Delay Formula option is used by default in the HCM software setups of SIDRA INTERSECTION.

HCM Edition 6, Chapter 22 makes the following statement about the strengths of the HCM model: "*The procedures in this chapter were based on extensive research supported by a significant quantity of field data. They have evolved over several years and represent a body of expert consensus. They produce unique deterministic results for a given set of inputs, and the capacity of each approach is an explicit part of the results. Alternative tools based on deterministic intersection models also produce a unique set of results, including capacities, for a given set of inputs, while those based on simulation may produce different results based on different random number sequences. Unique results from an analysis tool are important for some purposes such as development impact review.*"

SIDRA INTERSECTION shares this basic premise with HCM as a "deterministic intersection model" using the HCM terminology. However, it should be noted that such mathematical models make allowance for stochastic nature of traffic behavior as evident from the randomness effects in delay and queue length equations, percentile queue values, probabilities of lane blockage, effect of random arrival headways and random size and occurrence of bunches in traffic on gap-acceptance capacities, and so on.

AKÇELIK, R. (1981). **Traffic Signals: Capacity and Timing Analysis**. Research Report ARR No. 123. ARRB Transport Research Ltd, Vermont South, Australia. (7th reprint 1998)

AKÇELIK, R. (1990). ** SIDRA for the Highway Capacity Manual SIDRA for the Highway Capacity Manual.** *Compendium of Technical Papers, 60th Annual Meeting of the Institute of Transportation Engineers*, pp. 210–219.

**Gap acceptance modelling by traffic signal analogy.** Traffic Engineering and Control, 35(9), pp 498-506.

AKÇELIK, R. (1995). **Extension of the Highway Capacity Manual Progression Factor Method for Platooned Arrivals**. Research Report ARR No. 276. ARRB Transport Research Ltd, Vermont South, Australia.

AKÇELIK, R. (1996). **Progression Factor for Queue Length and Other Queue-Related Statistics**. Transportation Research Record, 1555, pp 99-104.

TRB (2010). **Roundabouts: An Informational Guide**. NCHRP Report 672. Transportation Research Board, Washington, DC, USA.

TRB (2010). **Highway Capacity Manual (HCM 2010)**. Transportation Research Board, National Research Council, Washington, DC, USA.** **

TRB (2016a). **Highway Capacity Manual 6th Edition - A Guide for Multimodal Analysis**. Transportation Research Board, National Research Council, Washington, DC, USA.

TRB (2016b).** Roundabout Practices**. A Synthesis of Highway Practice, NCHRP SYNTHESIS 488. Transportation Research Board, Washington, DC, USA.