Well, on a L2 switch each switchport is a collison domain i.e collision supected on one port will not effect any other port.So In directly when you are create separate vlans and assiging the ports to different vlans so it is decreasing the number of collison domains in a vlan. for example.. if you have all 24 ports in vlan so you have say u have 24 collison domain in a single boradcast domain and when you assign the ports to diff vlans say 12 ports you are decreasing the number of collision domain in a vlan
HTH,
-amit singh
