HCM Roundabout

The roundabout capacity model for single-lane and multi-lane roundabouts based on research on US roundabouts as described in Chapter 21 of HCM 2010 is available in SIDRA INTERSECTION Version 5.1. SIDRA INTERSECTION offers a unique implementation of the HCM 2010 model with major extensions to the capacity and performance estimation method (this replaces the NCHRP 572 option in previous versions). A summary of the extensions offered by SIDRA INTERSECTION is given in the following document, and further discussed below.

 Integration of the new HCM 2010 roundabout capacity model into SIDRA INTERSECTION 5.1.pdf 

The roundabout capacity model is the only analysis method 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.

The Highway Capacity Manual (HCM) versions of SIDRA INTERSECTION use the HCM 2010 roundabout capacity model as the default model. The SIDRA Standard model is the default option for all other versions (standard left, standard right, New Zealand, RTA NSW). Both models are available as options in all versions. 

HCM 2010 acknowledges the need to use alternative models in view of various shortcomings it lists about the HCM 2010 roundabout capacity model. SIDRA INTERSECTION is one of the alternative tools referred to in HCM 2010. The limitations listed in HCM 2010 are addressed by SIDRA INTERSECTION through extensions to the HCM 2010 model option or as part of the SIDRA Standard model option.

The extensions offered by the HCM 2010 roundabout capacity model in SIDRA INTERSECTION are listed below.

• 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 2010 model or SIDRA Standard capacity model. These include single and multiple shared and exclusive slip lanes controlled by yield or stop signs and continuous bypass lanes.
  The SIDRA Standard capacity model for roundabouts 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 and other geometric parameters. However, the HCM 2010 model is only sensitive to the number of entry and circulating lanes.
• Lane Flow Calculations: SIDRA INTERSECTION determines lane flows according to the equal degree of saturation principle subject to user-specified and program-determined lane under-utilisation factors including downstream short lane effects. Furthermore, de facto exclusive lane cases are identified and taken into account appropriately during iterative lane flow calculations. The method applies to the HCM 2010 model including roundabouts with more than 2 lanes. 
• Capacity Constraint: This essential element of roundabout modeling (limiting circulating flow rates by upstream capacity flow rates where there are oversaturated lanes) is readily available in SIDRA INTERSECTION. 
• Templates: SIDRA INTERSECTION offers numerous TEMPLATES for roundabouts including all roundabout examples given in MUTCD 2009 and TRB Roundabout Informational Guide (NCHRP Report 672).
• Roundabout Metering Signals: This analysis can be carried out using the HCM 2010 capacity model option. 
• Level of Service: While allowing for LOS thresholds which are "Same as Sign Control", SIDRA INTERSECTION offers options for choice of "Same as Signalised Intersections" or a unique "SIDRA Roundabout LOS".  Furthermore, the LOS Target parameter available in SIDRA INTERSECTION can be used to specify the acceptable LOS level for particular intersection types, e.g. for design life analysis.
• Pedestrian Effects: The effect of pedestrians on roundabout entry lane capacity is determined in accordance with HCM 2010.  Exit lane capacities as a function of pedestrian flows are also determined for all roundabout legs.  The method is available generally for all models. 
• Heavy Vehicle (HV) Effects: The HV adjustment factor needed for capacity adjustment is calculated for each lane rather than the whole approach.  SIDRA INTERSECTION carries out iterative calculations to find the flows and HV percentages per lane, and adjusts lane capacities accordingly.  The heavy vehicle equivalent (default value of 2) can be specified as input per movement.  This helps with model calibration in specific situations where there are large commercial vehicles in particular turning movements. 
• Short Lane Capacity: SIDRA INTERSECTION short lane capacity model is used with equivalent gap-acceptance parameters from the HCM 2010 model.  This is a space-based capacity model which uses gap-acceptance characteristics to determine excess flows overflowing from short lanes into adjacent lanes.  Since the SIDRA Standard and HCM 2010 models are lane-based, and with the use of short lane models, approach flaring parameters are not needed unlike the UK TRL 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: The parameters of the exponential regression models can be calibrated as lane-based model parameters.  Additionally, gap-acceptance parameters can be specified to calibrate individual movement capacity estimates, for example in the case of right-turn bypass (slip) lane movements.
• 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 which may result in the condition "follow-up headway + intrabunch headway > critical gap". 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 2010 model which assumes a random distribution of circulating stream headways (Siegloch M1 model). This model assumes zero intrabunch value. However, close values of the follow-up headway and critical gap values used by the HCM 2010 model indicate possibility of priority reversal in reality. Through the O-D Factor option, the priority reversal and priority emphasis can be allowed for to some extent with the HCM 2010 model in SIDRA INTERSECTION.

• Unbalanced Flow Conditions: Unbalanced conditions at roundabouts cause loss of capacity at high demand flow conditions. The Origin-Destination factor and adjustment factor for Entry /Circulating Flow Ratio for unbalanced flow conditions, which are important aspects of the SIDRA Standard roundabout model, can also be used for the HCM 2010 model in the software (optional). These are useful in dealing with specific conditions rather than relying on a regression method for general average conditions. For example, it is recognized that drivers can be more aggressive when the entry flow rate is very high. Iterative calculations are needed to apply the Origin-Destination factor since this factor depends on the demand flow pattern as well as the amount of queuing on approach lanes. 
• Performance Estimates: Geometric delays calculated as a function of speed are added to delay estimates as speed variations of vehicles negotiating roundabouts are an important factor in delay estimation. 
• Back of queue and stop rate estimates, as well as fuel consumption, emission and operating cost estimates consistent with the HCM 2010 exponential regression models for capacity estimation are important model extensions offered by SIDRA INTERSECTION. 
  For unsignalised intersections, HCM 2010 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.  Cycle-average queues will also be given in the queue length tables in Detailed Output report for information only.
• Closely-spaced roundabouts: Parameters to help with analysis of closely-spaced multiple roundabouts, or pedestrian crossings near roundabouts, are provided in SIDRA INTERSECTION:  The Capacity Adjustment parameter can be used to specify the amount of capacity reduction for upstream intersection lanes using the Probability of Blockage estimated for downstream intersection lanes where queue storage spaces are limited.

HCM 2010, Chapter 21 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 2010 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, effect of random arrival headways and random size and occurrence of bunches in traffic on gap-acceptance capacities, and so on.

TRB (2007).  Roundabouts in the United States. NCHRP Report 572. Transportation Research Board, National Research Council, Washington, DC, USA.

TRB (2010). Roundabouts: An Informational Guide. NCHRP Report 672. Transportation Research Board, National Research Council, Washington, DC, USA, in cooperation with US Department of Transportation, Federal Highway Administration.