I do live on a bus route to work, so mostly I wimp out if it's raining. If dry, I usually have some "just in case" waterproofs. Have you seen "Rainlegs"?
Rainlegs Kind of like a farrier's split apron, it protects the top part of your legs which catch the worst of the wet. Light to carry, and might be good to reinforce waterproof trousers that aren't quite up to the job.
Mary