Editing Roundabout (section)

=== Capacity and delays ===
[[File:ChivertonCrossRoundaboutA30Cornwall.jpg|thumb|Traffic approaching Chiverton Cross roundabout in Cornwall, UK]]

The capacity of a roundabout varies based on entry angle, lane width, and the number of entry and circulating lanes. As with other types of junctions, operational performance depends heavily on the flow volumes from various approaches. A single-lane roundabout can handle approximately 20,000–26,000 vehicles per day, while a two-lane design supports 40,000 to 50,000.<ref name="modern">{{cite web|title=Modern Roundabouts, an Informational Guide|url=https://www.fhwa.dot.gov/publications/research/safety/00068/|website=Federal Highway Administration Research and Technology|access-date=15 September 2015}}</ref>

Under many traffic conditions, a roundabout operates with less delay than signalised or [[all-way stop]] approaches. Roundabouts do not stop all entering vehicles, reducing both individual and queuing delays. Throughput further improves because drivers proceed when traffic is clear without waiting for a signal to change.

Roundabouts can increase delays in locations where traffic would otherwise often not be required to stop. For example, at the junction of a high-volume and a low-volume road, traffic on the busier road would stop only when cross traffic was present, otherwise not having to slow for the roundabout. When the volumes on the roadways are relatively equal, a roundabout can reduce delays, because half of the time a full stop would be required. Dedicated left turn signals (in countries where traffic drives on the right) further reduce throughput.

Roundabouts can reduce delays for pedestrians compared to traffic signals, because pedestrians are able to cross during any safe gap rather than waiting for a signal. During peak flows when large gaps are infrequent, the slower speed of traffic entering and exiting can still allow crossing, despite the smaller gaps.

Studies of roundabouts that replaced stop signs and/or traffic signals found that vehicle delays were reduced 13–89 percent and the proportion of vehicles that stopped was reduced 14–56 percent. Delays on major approaches increased as vehicles slowed to enter the roundabouts.<ref name="iihs"/>

Roundabouts have been found to reduce [[carbon monoxide]] emissions by 15–45 percent, [[nitrous oxide]] emissions by 21–44 percent, [[carbon dioxide]] emissions by 23–37 percent and [[hydrocarbon]] emissions by 0–42 percent. Fuel consumption was reduced by an estimated 23–34 percent.<ref name=iihs/>

==== Capacity modelling ====
Many countries have researched roundabout capacity. The software can help calculate capacity, delay and queues. Packages include [[ARCADY]], Rodel, Highway Capacity Software and [[Sidra Intersection]]. ARCADY and Rodel are based on the Transport Research Laboratory mathematical model. The TRL approach is derived from empirical models based on geometric parameters and observed driver behaviour with regard to lane choice. Sidra Intersection software includes roundabout capacity models developed in Australia and the US.

Research on Australian roundabouts was conducted in the 1980s at the Australian Road Research Board (ARRB).<ref>{{cite book|url=http://www.arrb.com.au/Information-services/Publications.aspx|title=Effectiveness Audit of ARRB Intersection Capacity Research|location=Research Report ARR 242. ARRB Transport Research Ltd, Vermont South, Australia|author=Taylor, M.P., Barton, E.V., Bliss, J. and O'Brien, A.P. (1993)|url-status=dead|archive-url=https://web.archive.org/web/20160227020155/https://arrb.com.au/Information-services/Publications.aspx|archive-date=27 February 2016}}</ref> Its analytical capacity and performance models differ from the TRL model significantly, following a lane-based gap-acceptance theory including geometric parameters.

Research on U.S. roundabouts sponsored by the [[Transportation Research Board]] (TRB) and [[Federal Highway Administration]] (FHWA) culminated in a capacity model that was included in the ''[[Highway Capacity Manual]]'' (HCM) Edition 6<ref>{{cite book|url=http://www.trb.org/publications/hcm6e.aspx|title=Highway Capacity Manual, Sixth Edition: A Guide for Multimodal Mobility Analysis|author=TRB (2016)|location= Transportation Research Board, National Research Council, Washington, DC, US}}</ref> and the ''TRB-FHWA Roundabout Informational Guide'' ([[NCHRP]] Report 672).<ref>{{cite book|url=http://www.trb.org/Main/Blurbs/164470.aspx|title=Roundabouts: An Informational Guide |author=TRB|date=2010 |location=NCHRP Report 672. Transportation Research Board, National Research Council, Washington, D.C., US, in cooperation with US Department of Transportation, Federal Highway Administration|doi=10.17226/22914 |isbn=978-0-309-15511-3 }}</ref> The HCM Edition 6 model is based on lane-based gap-acceptance theory. A recent NCHRP survey of US state transport agencies found that [[Sidra Intersection]] is the most widely used software tool in the US for roundabout analysis.<ref>{{cite book|url=https://www.nap.edu/catalog/23477/roundabout-practices|title=''Roundabout Practice, A Synthesis of Highway Practice''|publisher=  Transportation Research Board|year=2016|location=National Cooperative Highway Research Program, NCHRP SYNTHESIS 488. Washington DC, US.|doi=10.17226/23477|isbn=978-0-309-27208-7 |last1=Pochowski |first1=Alek |last2=Paul |first2=Andy |last3=Rodegerdts |first3=Lee A. }}</ref>