Metering Signals

SIDRA INTERSECTION includes a unique model for timing and perfromance analysis of roundabout metering signals. 

SIDRA INTERSECTION for Roundabouts

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The use of metering signals is an effective measure to alleviate the problem of excessive delay and queuing that may be observed at unsignalised roundabouts, especially with heavy unbalanced flows. The Australian roundabout and traffic signal guides acknowledge the problem of unbalanced flows and discuss the use of metering signals (AUSTROADS 1993, Sections 3.9.4 and 12.1; and AUSTROADS 2003, Section 15.7).  Case studies have been discussed in numerous papers by Akçelik (see the reference list and web page links below). 

The signalised roundabout solution has been used extensively in the UK as well (Huddart 1983, Lines and Crabtree 1988, Hallworth 1992, Shawaly, et al 1991) though the solution may differ from the Australian metering signals.  Ken Huddart (1983) explained the problem clearly: "... the proper operation of a roundabout depends on there being a reasonable balance between the entry flows ... an uninterrupted but not very intense stream of circulating traffic can effectively prevent much traffic from entering at a particular approach." and "The capacity of roundabouts is particularly limited if traffic flows are unbalanced. This is particularly the case if one entry has very heavy flow and the entry immediately before it on the roundabout has light flow so that the heavy flow proceeds virtually uninterrupted. This produces continuous circulating traffic which therefore prevents traffic from entering on subsequent approaches."

A recent US paper discusses the use of metering signals for the Clearwater roundabout in Florida (Sides 2003).

The unbalanced flow problem may not manifest itself when the overall demand level is low but may appear with traffic growth even at medium demand levels.  Demand flow patterns and demand levels may change significantly after the introduction of a roundabout, sometimes in a relatively short period of time.

Modeling of effects of heavy unbalanced flows on roundabout capacity and level of service is important in optimizing the roundabout geometry (including lane use) to alleviate the associated problems.  This can be achieved to a good extent for a new roundabout subject to the reliability of traffic demand information, or for an existing roundabout to a smaller extent, given the design constraints (see O'Brien, et al 1997).  Part-time metering signals on selected roundabout approaches, operational only when needed under peak demand conditions, can be an effective measure preventing the need for a fully signalized intersection treatment.

Extract from the Australian Traffic Signal Guide (AUSTROADS 2003, Section 15.7):

Roundabout metering signals may be used where excessive queuing and delays are observed on one or more legs of a roundabout due to heavy circulating flow rates, especially in the case of heavily directional origin-destination movements.  In this case, a dominant approach stream constitutes the major proportion of traffic in the circulating stream that causes a significant reduction in the capacity of the approach that has to give way to that circulating stream (Akçelik, Chung and Besley 1998).  These signals are usually employed on a part-time basis since they may be required only when heavy demand conditions occur during peak periods. 

Two-aspect yellow and red signals are used for roundabout metering.  The sequence of aspect display is Off to Yellow to Red to Off.  When metering is not required neither aspect is displayed.  

The signalised approach is referred to as the "metered approach", and the approach with the queue detector as the "controlling approach".  When the queue on the controlling approach extends back to the queue detector, the signals on the metered approach operate so as to create a gap in the circulating flow.  This helps the controlling approach traffic to enter the roundabout.  When the red display is terminated on the metered approach, the roundabout reverts to normal operation. 

The introduction and duration of the red signal on the metered approach is determined by the controlling approach traffic.  The duration of the blank signal is determined according to a minimum blank time requirement, or extended by the metered approach traffic if detectors are used on that approach.

A minimum of two signal faces, one primary and one tertiary, shall be installed.  A regulatory sign STOP HERE ON RED SIGNAL shall be fixed to any signal post erected adjacent to the stop line, as drivers do not expect to stop at the advance stop line location.  Stop lines shall be located not less than 3 m in advance of the approach holding line but preferably, should be positioned approximately 20 m from the holding line.  Queue detector setback distance on the controlling approach is usually in the range 50 m to 120 m. 

Various site-specific methods may also be used to meter traffic, e.g. using an existing upstream midblock signalised crossing on the metered approach.

In some cases, it may be necessary to supplement the traffic signals with explanatory fixed or variable message signposting.  Where sight restrictions exist, advance warning signals should be considered.

HEAVY ENTRY FLOWS

At a roundabout with an unbalanced flow pattern, a traffic stream with a heavy flow rate enters the roundabout against a circulating stream with a low flow rate.  Three such roundabout cases from Melbourne, Australia are described below.

Small single-lane roundabout at the intersection of Stanhope Grove with Broadway in Camberwell: 1524veh/h per lane entering against a circulating flow rate of 60 veh/h has been reported. Sum of entering and circulating flows is 1584 veh/h.

Small-Medium single-lane roundabout at the intersection of Grange Rd, St Georges Rd and Alexandra Avenue in Toorak: 1693 veh/h per lane entering against a circulating flow rate of 67 veh/h has been reported.  Sum of entering and circulating flows is 1760 veh/h.  The measured follow-up headway and critical gap values for this entry lane are 1.992 s and 2.423 s, respectively.  The maximum capacity at zero circulating flow (corresponding to the follow-up headway) is 3600 / 1.992 = 1808 veh/h.  A photo of this roundabout is shown below.

Large roundabout at the intersection of Mickleham Rd and Broadmeadows Rd in Westmeadows (this is a case described in the AUSTROADS Roundabout Guide):  1397 veh/h per lane against a circulating flow rate of 83 veh/h in am peak (sum of entering and circulating flows = 1480 veh/h) and 1501 veh/h per lane against a circulating flow rate of 112 veh/h in pm peak (sum of entering and circulating flows = 1613veh/h)s. 

Some of these articles are available for download on this web site:

Articles    More Articles on Roundabouts

AKÇELIK, R. (2008). An investigation of the performance of roundabouts with metering signals. Paper presented at the National Roundabout Conference, Transportation Research Board, Kansas City, MO, USA, 18-21 May 2008.

AKÇELIK, R. (2006).  Operating Cost, Fuel Consumption and Pollutant Emission Savings at a Roundabout with METERING SIGNALS.  Paper presented at the ARRB 22nd Conference, Canberra, 29 Oct - 2 Nov 2006.  Updated: 22 October 2007.

E. NATALIZIO (2005).  Roundabouts with Metering Signals.  Paper presented at the Institute of Transportation Engineers 2005 Annual Meeting, Melbourne, Australia, August 2005.

AKÇELIK, R. (2005a).  Roundabout Model Calibration Issues and a Case Study.  Paper presented at the TRB National Roundabout Conference, Vail, Colorado, USA, May 2005.

AKÇELIK, R. (2005b).  Capacity and Performance Analysis of Roundabout Metering Signals.   Paper presented at the TRB National Roundabout Conference, Vail, Colorado, USA, May 2005.  

AKÇELIK, R. (2004).  Roundabouts with Unbalanced Flow Patterns.   Paper presented at the ITE 2004 Annual Meeting, Lake Buena Vista, Florida, USA, Aug 2004.  (2.8MB)

AKÇELIK, R. (2003).  A Roundabout Case Study Comparing Capacity Estimates from Alternative Analytical Models.  Paper presented at the 2nd Urban Street Symposium, Anaheim, California, USA, 28-30 July 2003.  (470KB).

AKÇELIK, R., CHUNG, E. and BESLEY, M. (1998).  Roundabouts: Capacity and Performance Analysis. Research Report ARR No. 321.  ARRB Transport Research Ltd, Vermont South, Australia (2nd Edition 1999).

AUSTROADS (1993).  Guide to Traffic Engineering Practice Part 6 - Roundabouts.  AP-G11.6, Sydney.

AUSTROADS (2003).  Guide to Traffic Engineering Practice Part 7 - Traffic Signals.  AP-G11.7, Sydney.

HALLWORTH, M.S. (1992).  Signalling roundabouts - 1. Circular arguments.  Traffic Eng. and Control, 33 (6), pp 354-363.

HUDDART, K.W. (1983). Signalling of Hyde Park Corner, Elephant and Castle and other roundabouts. PTRC 11th Summer Annual Meeting, Proceedings of Seminar K, pp 193-208.  (2100KB)

LINES and CRABTREE (1988).  The use of TRANSYT at signalised roundabouts.  Traffic Eng. and Control, 29 (6), pp 332-337.

O'BRIEN, A., AKÇELIK, R., WILLIAMSON, D. and PANTAS, T. (1997).  Three-laning a two-lane roundabout - the outcomes. Compendium of Technical Papers (CD).  67th Annual Meeting of the Institution of Transportation Engineers. (500KB)

SHAWALY, E.A.A, LI, C.W.W. and ASHWORTH, R. (1991).  Effects of entry signals on the capacity of roundabout entries - a case study of Moore Street roundabout in Sheffield.  Traffic Eng. and Control, 32 (6), pp 297-301.

SIDES. K. (2000).  Assessing the Clearwater Beach Entryway Roundabout. 
ITE 70th Annual Meeting Compendium.  (790KB).