As a former commercial truck driver and touring cyclist, as well as putting well over 1,200,000 miles on my private vehicles, being very visible is your best asset when riding on busy roads. Wearing bright clothing or a vest, especially fluorescent light green or orange, and using lights such as those mentioned by "Carbonfiberboy" go a long way toward allowing vehicle drivers to see you even in heavy rain or foggy conditions. Far too often, when driving in poor visibility conditions such as heavy rain and spray from the other vehicles on the road, have I noticed cyclists only just before, or as I was passing them, because they weren't wearing bright clothing or using good lights. Once, while touring through the Fraser Canyon, I stopped for lunch at a cafe and had 2 truck drivers come up to me and thank me for wearing my high visibility vest because it was the only thing they saw of me as they approached in such conditions. Using a mirror to monitor traffic is wise as well, as "Gus Riley" suggests, moving well off to the side, or stopping, to allow 2 large vehicles such as semis, to pass.