NCHRP 572 Roundabout Capacity Model
For US Users of SIDRA INTERSECTION
The roundabout capacity model for single-lane and multi-lane roundabouts based on research on US roundabouts as described in Chapter 4 of NCHRP Report 572 has been implemented in SIDRA INTERSECTION Version 4 (see references below).
SIDRA INTERSECTION offers a unique implementation of the NCHRP 572 research results with major extensions to the capacity and performance estimation method.
SIDRA INTERSECTION for NCHRP 572
The extensions offered by the NCHRP 572 Roundabout Model implementation in SIDRA INTERSECTION are listed below.
- Roundabouts with more than 2 lanes can be analysed.
- Roundabout Metering Signals: This analysis can be carried out using the NCHRP 572 capacity model option.
- Closely-spaced roundabouts: Closely-spaced multiple intersections, or pedestrian crossings near intersections, can be analysed using 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.
- Pedestrian Effects: The effect of pedestrians on roundabout entry lane capacity is determined in accordance with the NCHRP Report 572 Appendix B. 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.
- Lane Flow Calculations: The method is extended to more than two lanes in a lane group. SIDRA INTERSECTION determines lane flows according to the equal degree of saturation principle subject to user-specified and program-determined lane underutilisation cases. This method allocates lower volumes for lanes with lower capacities. Furthermore, in SIDRA INTERSECTION:
- de facto exclusive lane cases are taken into account appropriately during iterative lane flow calculations, and
- heavy vehicles are not assumed to distribute themselves equally across all lanes as discussed above.
- Short Lane Capacity: SIDRA INTERSECTION short lane capacity model is used with equivalent gap-acceptance parameters from the NCHRP model. This is a space-based capacity model which uses gap-acceptance characteristics to determine excess flows overflowing from short lanes into adjacent lanes.
- 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.
- The Origin-Destination factor and adjustment factor for Entry /Circulating Flow Ratio for unbalanced flow conditions are applicable for improved modelling that can deal with specific conditions rather than relying on a regression method for average conditions. HCM recognises the situation where drivers can be more aggressive when the entry flow rate is very high.
- 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 NCHRP 572 exponential regression models for capacity estimation are important model extensions offered by SIDRA INTERSECTION.
For unsignalised intersections, HCM 2000 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.
- Level of Service: The use of LOS criteria which are the same as those used for sign-controlled intersections may not be appropriate for roundabouts since this would create a bias against roundabouts when compared with alternative intersection treatments. SIDRA INTERSECTION offers options for choice of alternative LOS criteria for roundabouts. 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.
- As an analytical tool, SIDRA INTERSECTION produces unique deterministic results for a given set of inputs, and the capacity of each approach is an explicit part of the results, while those based on microsimulation generally do not.
Potential Differences in Capacity Estimates
The factors which may cause differences in capacity estimates between the original NCHRP 572 method and its implementation in SIDAR INTERSECTION are listed below.
- SIDRA INTERSECTION solution is based on the equal lane utilisation (equal lane degree of saturation) principle unless exclusive lanes are specified by the user or defacto exclusive lane cases are identified by the program. This means that lane flows will differ for cases of multi-lane entry and circulating flows since dominant and sub-dominant lane capacities are different.
- Default setting for the Origin-Destination factor is "Not Applicable". If the input data is changed to "Applicable", this will affect the capacity estimates significantly, resulting in lower capacity estimates depending on balance of O-D demand patterns. The default setting for Entry /Circulating Flow Ratio adjustment is "Medium", and this may affect the capacity estimates, resulting in slightly higher values than the original NCHRP 572 estimates. This effect can be removed by setting the parameter to "None".
- If the HV Method for Gap Acceptance is selected as "Include HV Effect if Above 5 per cent", then the HV effect on capacities will be less, therefore capacity estimates will be larger, generally.
- Delay formula used will be the standard SIDRA INTERSECTION model, including geometric delays determined according to the approach and exit cruise speeds and roundabout negotiation speeds.
- LOS thresholds are the same as signalised intersections as in the standard SIDRA method.
References
RODEGERDTS, L. (2005). State-of-the-Art in U.S. Roundabout Practice. Paper presented at the Institute of Transportation Engineers 2005 Annual Meeting, Melbourne, Australia.
RODEGERDTS, L. (2007). Updated Roundabout analysis procedures for the next Highway Capacity Manual. Presentation at the National Roundabout Conference, Transportation Research Board, Kansas City, MO, USA. http://teachamerica.com/RAB08/index.htmlTRB (2007). Roundabouts in the United States. NCHRP Report 572. Transportation Research Board, National Research Council, Washington, DC, USA.