Some riders have to wear their load as they use road bikes to tour and commute. Road bikes are useless for anything except as a way of going fast and getting fit. They do not normally come with lugs to attach a carrier. Others do not have any convenient way of attaching a bag to the bike rack. The standard way to attach a bag is to buy a couple of cheap and cheerful bungees. These give me the willies. When I stretch them I have a vision of a skewered eyeball when the hook slips. I have just come across a set of straps with a safer design: ROK Commuter Straps (http://www.rokstraps.com). You loop the strap onto the bike rack, click the buckles together and tighten the strap around the bag.
As soon as we can find a dealer they will be bought.