SOS - SELF OPTIMISING SIGNAL CONTROL: DEVELOPMENT AND FIELD TRIALS OF THE SOS ALGORITHM FOR SELF OPTIMIZING SIGNAL CONTROL AT ISOLATED INTERSECTIONS

A new traffic signal control algorithm for isolated intersections, called SOS - Self Optimising Signal Control, has been specified, implemented, simulated and finally tested in a field trial at one intersection. Many signalised intersections are not part of co-ordinated networks. More than 60% of the 3,500 signalised intersections in Sweden are operating isolated and many of them are located on trunk roads with high traffic volumes. The Swedish Lhovra control is a modular tool box of traffic engineering functions. Lhovra was mainly developed to increase safety on high speed roads, and to reduce the risk for rear-end accidents. The Finnish HUTSIM simulator was used during the development work and trials indicated a socio economic pay back period of 6 months for a SOS installation. Field trials indicated that the number of vehicles in the option zone when the signals changed from green to yellow were reduced by 38%, compared with Lhovra control. The number of vehicles exposed to a risk of rear-end collision risks were reduced by 58%. The delay was about the same as with Lhovra control. However, SOS did not perform as well as expected during rush hours, mainly due to problems with the traffic model but also due to the fact that the capacity optimising algorithm was not used. The total traffic safety improvement can be estimated to be worth about SEK 200,000 per year. It is important to note that: the test intersection has very high traffic volumes and is not representative for normal intersections. To conclude, the Scandinavian tradition of signal control has been combined with an optimising algorithm. The number of vehicles exposed to the risk of a rear-end collision on high speed approaches can be reduced dramatically if traffic safety is favoured in the parameter settings. The traffic model needs further improvements during peak hours. A combination of the properties of SOS and Lhovra could possibly give good results.