SATURN (Simulation and Assignment of Traffic to Urban Road Networks) is a suite of flexible network analysis programs developed at the Institute for Transport Studies, University of Leeds and distributed by Atkins Limited since 1982. It has six basic functions:
i) as a combined traffic simulation and assignment model for the analysis of road-investment schemes ranging from traffic management schemes over relatively localised networks (typically of the order of 100 to 2,000 nodes) through to major infrastructure improvements where models with over 10,000 junctions are not infrequent;
(ii) as a “conventional” traffic assignment model for the analysis of much larger networks (e.g., up to 7,500 links in the smallest standard PC version, 200,000 in the largest)
(iii) as a simulation model of individual junctions;
(iv) as a network editor, data base and analysis system;
(v) as a matrix manipulation package for the production of, e.g., trip matrices.
(vi) as a trip matrix demand model covering the basic elements of trip distribution, modal split etc.
As a combined simulation and assignment model - its original function - SATURN is most suitable for the analysis of relatively minor network changes such as the introduction of one-way streets, changes to junction controls, bus-only streets, etc. (which can be loosely categorised as “traffic management measures”) and whose evaluation requires a detailed analysis of traffic behaviour at junctions.
However, from its starting point as a combined simulation and assignment model, SATURN has been extended in both directions so that it can function both as a conventional traffic assignment model and as a pure junction simulation model. As a “conventional” traffic assignment model (with or without simulation) SATURN can deal with large conurbation, regional or even national models.
These may then interface with smaller localised models using the greater precision of the junction simulation models.
With both simulation and conventional network representations SATURN provides a wide range of assignment options such as generalised cost, all-or-nothing, Wardrop equilibrium, Burrell multiple-route assignment (SUE) and, more recently, demand-responsive (elastic) assignment to deal with induced traffic. All these are founded on theoretically consistent modelling frameworks and convergent algorithms reflecting the academic background of SATURN’s development.
Releases of “major” updates to SATURN occur on a roughly annual basis with the latest release, 11.5, becoming generally available in July 2020.
Post release 10.9, SATURN includes options to carry out Origin-based Assignment (OBA) as developed by Dr. Hillel BarGera while a PhD student at the University of Illinois at Chicago in the late 1990’s. His work revolutionised traffic assignment in that his methods solve for Wardrop Equilibrium solutions to an accuracy limited only by the numerical accuracy of the computer and within comparable CPU times to existing algorithms such as Frank-Wolfe. See Section 21. More recently, Dr Yanling Xiang of Atkins extended the theoretical framework for use in Multiple User Class assignments.
Also post 10.9 SATURN includes further features facilities to undertake “warm starts” to speed up one run of SATURN using data extracted – in several different ways – from previous runs. See Section 22.
At the other extreme SATURN allows for the interactive simulation and editing of individual junctions, so that the user is able to vary parameters such as flows, saturation flows, signal settings, etc and to analyse one node in isolation.
SATURN, through module P1X, also possesses powerful graphical display capabilities for network, junction and matrix-based data. Junction details including link and turn data may be shown at both individual junction and network wide levels. The wide range of options available also allows for on-screen cordoning, select link reassignments, GIS-style background displays, animated queues, data editing and tree building.
The interactive and graphical facilities of P1X may also be used to “edit” existing networks on screen and/or to build networks from scratch on the screen. The latter function may use an existing bitmap file (e.g. a normal road map which has been scanned) as the background and to “trace” the SATURN network on top of it.
In a related fashion SATURN may be applied to the analysis of network-based data which need not be in any way related to traffic assignment problems. For example, data relating to accident rates per link, or the last dates of road resurfacings stored, etc may be input and analysed.
Through its general matrix manipulation program MX SATURN offers a full range of interactive matrix operations as required by standard transport planning applications, e.g. matrix building, editing, factoring, furnessing, transposing etc. It also provides easy transfer between SATURN and other transport and spreadsheet packages.
While originally conceived as a purely traffic assignment package with a fixed user-defined trip matrix SATURN now also provides a number of standard modelling steps for matrix estimation, e.g. modal split and distribution. These models have been integrated with the assignment in a self-consistent mode to provide a mathematically convergent form of “Variable Demand Modelling”.
A further long-standing example of matrix estimation within SATURN is the ME2 Model (Matrix Estimation from Maximum Entropy) developed at Leeds and University College London by Dr. L.G. Willumsen, enabling O-D trip matrices to be estimated directly from traffic counts, thus reducing significantly the survey costs required to run an assignment model such as SATURN as well as increasing its accuracy. In effect ME2 generates the ‘most likely’ trip matrix which is consistent with all available traffic information, including where possible any prior estimates of the trip matrix.
Other important features of SATURN include:
♦ fully interactive analysis of results
♦ optimum green splits for traffic signals
♦ traffic signal co-ordination modelled
♦ lane structure of intersections and choice of lane modelled
♦ the growth and decay of queues modelled quasi - dynamically
♦ facilities to “skim”, e.g., inter-zonal time, distance, etc. matrices
♦ bus routes, bus-only roads and bus-only lanes included explicitly
♦ both left-hand and right-hand drive accepted
♦ special features of Australian and New Zealand rules of the road have been incorporated
♦ selected link analysis
♦ multiple User Class Assignment differentiating between, e.g., cars, taxis, HGV’s, etc.
♦ full analysis of O-D routes generated by the assignment
♦ network and trip matrix cordoning for sub-areas
♦ a SATURN to TUBA interface module (SATTUBA) to assist in the economic evaluation of schemes within the UK
♦ modelling tolls or road charges on specified links
♦ modelling area charges, ie charges for entering or using roads in an area
♦ facilities to both “dump” SATURN data into ASCII files for input into, e.g., spreadsheets or other suites of transport programs and equally to re-input data files from these external procedures
♦ support for the Open Matrix (OMX) binary Format for exchanging matrix information between different (transport-related) software applications using SATOMX