  The efficiency of traffic flows in urban areas is known to crucially depend
on signal operation. Here, elements of signal control are discussed, based on
the minimization of overall travel times or vehicle queues. Interestingly, we
find different operation regimes, some of which involve a "slower-is-faster
effect", where a delayed switching reduces the average travel times. These
operation regimes characterize different ways of organizing traffic flows in
urban road networks. Besides the optimize-one-phase approach, we discuss the
procedure and advantages of optimizing multiple phases as well. To improve the
service of vehicle platoons and support the self-organization of "green waves",
it is proposed to consider the price of stopping newly arriving vehicles.
