Microscopic motorway simulation

SISTM

SISTM (SImulation of Strategies for Traffic on Motorways) is a microscopic motorway simulation model owned by the Highways Agency that TRL has developed for studying and evaluating methods for reducing congestion on motorways. It can model a single carriageway of motorway together with entry and exit slip roads. The traffic management measures that can be introduced include ramp metering, fixed and variable speed limits and dedicated lanes. It has been used to model the variable mandatory speed limit system on the M25. SISTM has also been used in studies of merge and diverge layouts, the effect of fitting vehicles with Autonomous Intelligent Cruise Control and the provision of climbing lanes.

The model is microscopic in that it aims to replicate the behaviour of individual drivers. It contains car following and lane changing algorithms which are carried out on a fixed time increment basis. The rules on which SISTM is based are derived from observations of traffic in heavy flow conditions. Extensive testing has been carried out to ensure that the algorithms reproduce traffic behaviour, both at the individual vehicle level and collectively, over a period of peak demand. The programs contain modules for estimating vehicle exhaust emissions, fuel consumption and traffic noise.

Recent improvements have been made to the lane changing algorithm so that lane changing rates are closer to those on real motorways and the software has been further enhanced so that it can reproduce shockwaves in motorway traffic. These occur when the motorway is operating close to capacity and where a vehicle suddenly braking can cause a ripple effect which travels upstream. It is important for micro-simulation models to replicate shockwave characteristics as the traffic speeds close to capacity can be highly variable.

SISTM consists of a suite of programs which run on IBM compatible PCs. The main simulation is fully interactive and has an on-line graphical representation of the vehicles on the motorway. The software includes 3D visualisation and data entry components developed by QinetiQ. Input requirements to SISTM include data defining the geometric characteristics of the motorway, vehicle and driver parameters and traffic flows in the form of origin/destination matrices for the simulation period. Data output is comprehensive and includes summary statistics on flows, speeds, queue lengths and journey times for the network and detailed traffic operational data on any specified link of the motorway. Output can be processed by spreadsheet packages or by the MTV (Motorway Traffic Viewer) software.

The software can be distributed to other organisations subject to agreement of the Highways Agency’s terms and conditions. Training in the use of the package can be provided by TRL to suit individual users’ needs.