Hi i am trying to make a Wig Wag sequence for a traffic light using the Atmega32

I have already programmed in Atmel AVR software the 0.5 second delay routine based on a processor speed of 8MHz.

now i need to create a routine that will produce flashing red lights (Wig Wag sequence see image below or go to this site http://img8.imageshack.us/img8/5445/wigwagsequence.jpg ) on the major and minor roads. The flash rate is to be a repetitive 0.5 second on 0.5 second off.

Can someone please help me to program this. Thanks