Software Used When Designing Roundabouts:

  • Junctions (formerly ARCADY): TRL's software program for the assessment of roundabout capacity, delay and safety at standard, mini and grade-separated roundabouts.
  • VISSIM: Microscopic simulation software for multi-modal traffic flow modeling and modeling of corridors of varying control types.
  • SIDRA Intersection: Software program with a lane-based model for the analysis of roundabout capacity, delay, and emissions. Supports the use of Sidra and HCM capacity models. Can also be used to analyze signalized and sign controlled intersections and has the capability to model multiple intersections as a network.
  • AutoTrack: Vehicle simulation software, modeling all types of vehicles, including oversize/overweight, transit, rail, air, and independent secondary-steered vehicles. (Lateral and vertical clearances in a 3D environment can be tested.)
  • AutoTURN: Vehicle simulation software, modeling a vast array of vehicles, including oversize/overweight, transit, and linked secondary-steered vehicles. (Lateral and vertical clearances in a 3D environment can be tested.)
  • GuidSIGN: Sign design software for creating custom, MUTCD-based signs in MicroStation or AutoCAD.



Simulation is an effective way to evaluate interaction between intersection control types such as corridors of roundabouts, or roundabouts and traffic signals. Other key benefits include assessment of weaving and merging behaviors between intersections.

A micro-simulation model produces a qualitative depiction of the analysis of a corridor to compare operational impacts of alternative traffic controls, but it has limitations. It is immensely important to understand the capabilities and time requirements of micro-simulation. Calibration and validation is difficult and time-consuming for high demand, highly complex situations, such as large multilane roundabouts. Prediction of future driver behavior is challenging, due to changing driver habits as they become accustom to the roundabout. Thus, we use this tool as an indicator of potential problems relative to us online casinos the corridor rather than for absolute metrics of queuing and delay.

Nevertheless, simulation can be an invaluable tool for public display, discussion, and education. Micro-simulation picks-up where other analysis software leave-off. No other analysis tool allows designers and planners to visualize how a roundabout will interact when placed in a corridor, with multiple modes (transit, rail, cyclists, pedestrians, cars & trucks), varying lane configurations, control types, and vehicle speeds. These models can then be linked with CAD design software for accurate 3D simulations.


Click here to view some of our VISSIM Simulations. 


Operational Analysis

The purpose of Junctions 8 is to:

  • Improve design quality
  • Drastically reduce design time
  • Reduce land and service costs
  • Allow rapid exploration of many options especially with respect to safety
  • Derive the optimum layout within the conflicting constraints of cost, delay and safety


Junctions 8 is based on the empirical capacity equations developed through extensive experiments at the Transport Research Laboratory in the United Kingdom. Rather than simply checking designs after they have been drawn, Junctions 8 generates geometry prior to scheme drawing. This avoids the time consuming practice of repeated drawing and checking. Junctions 8 allow the designer to optimize a balanced design in respect to delay, safety and cost.

The empirical models found in Junctions 8 is derived from extensive research, and with simple calibration are proven accurately predict roundabout capacity at all ranges of traffic flows.  Junctions 8 allows for calibration to the recent NCHRP 3-65 research on U.S. roundabout capacities. 

Other roundabout analysis programs use gap-acceptance models that are not empirically proven. Gap-acceptance theory is familiar to transportation professionals from sources such as the Highway Capacity Manual. Capacity is predicted based on the size in time of the gaps between successive vehicles motorists choose when entering a major traffic stream, such as the circulating flow of a roundabout. The gap-acceptance time can be calibrated to reflect differences in motorist behavior between countries.  Unfortunately, gap parameters vary according to traffic flow and are difficult to measure.

Gap-acceptance models can adequately predict roundabout capacity within certain ranges of traffic flows. However, outside these ranges gap-acceptance models can over-predict capacity at low traffic flows and under-predict capacity at high traffic flows. At low flows, motorists often react to enter a major traffic stream more slowly. At high flows, “gap-forcing” and “priority reversal” take place, which are not explained well by conventional gap-acceptance theory. These phenomena are described in a paper by the TRL’s R.M. Kimber entitled Gap-Acceptance and Empiricism in Capacity Prediction (Transportation Science, Vol. 23, No. 2, 1989).

Find out more about the Junctions Training Manual.