  Due to that the existing traffic facilities can hardly be extended,
developing traffic signal control methods is the most important way to improve
the traffic efficiency of modern roundabouts. This paper proposes a novel
traffic signal controller with two fuzzy layers for signalizing the roundabout.
The outer layer of the controller computes urgency degrees of all the phase
subsets and then activates the most urgent subset. This mechanism helps to
instantly respond to the current traffic condition of the roundabout so as to
improve real-timeness. The inner layer of the controller computes extension
time of the current phase. If the extension value is larger than a threshold
value, the current phase is maintained; otherwise the next phase in the running
phase subset (selected by the outer layer) is activated. The inner layer adopts
well-designed phase sequences, which helps to smooth the traffic flows and to
avoid traffic jam. In general, the proposed traffic signal controller is
capable of improving real-timeness as well as reducing traffic congestion.
Moreover, an offline particle swarm optimization (PSO) algorithm is developed
to optimize the membership functions adopted in the proposed controller. By
using optimal membership functions, the performance of the controller can be
further improved. Simulation results demonstrate that the proposed controller
outperforms previous traffic signal controllers in terms of improving the
traffic efficiency of modern roundabouts.
