Traffic signal controller using 555

I've been assigned the project to make a traffic signal controller using advanced electronic circuits. Of the few circuit diagrams I've referred to over the net, I can see the use of a 555 timer. Although, I'm not sure which mode (monostable, bistable, astable) to go for. There have been a few circuits with monostable mode and some with astable mode. Can anyone please help me out about this?

regards,
Smita