Our soil in one part of our yard has tested positive for lead, being somewhat close to a road. This is where we have our comfrey plants. Typically how deep is lead contamination from traffic? Would the leaves contain kead?
Generally speaking, if a plant can uptake excess heavy metals, it will sequester what isn't used in metabolism in its own biomass. That's why, when biological remediation projects clean up land with in-situ plant resources, a managed project will actively harvest that biomass and take it out of the biosphere in some way, either by processing into a more stable, long-lived form, or by locking it away from the biological activity that would release its' carbon and heavy metals to the environment.
If comfrey is acting as a dynamic accumulator in a polluted area, it may, indeed, be sequestering heavy metals, in which case, those heavy metals would be transferred to whatever soil that biomass decomposed in.
I don't know that you could find out for sure without having the comfrey tested.
Not all is lost, though, should your comfrey be sequestering lead out of the environment. Some fruit tree species need higher heavy-metal content for proper vegetative growth. Also, the comfrey could be managed, cut and moved to a place where you're growing wood for heating or lumber.
This is actually not a bad thing. I would suggest you look at drainage off of your road and consider if it is possible to get some wood chips in a trench, even a narrow, deep one, between your road and the soil you wish to protect. I would then make up some actively aeratedcompost extract and some Oyster mushroom fungal slurry and inoculate it. The trench will nurture soil life whose job it is to break down hydrocarbons and move toxic amounts of nutrients around to where they're lacking.
This would require a little management, but the fruiting bodies could be harvested and sequestered as well as any bordering dynamic accumulators. If the area were perpetually wet, I would actually encourage a reed bed system as well.
So to directly answer your question, BeverlyR, it is certainly probable in some situations, but it is not something unique to comfrey, but something that dynamic accumulators are really good at, that can be harnessed to cleanse your soil of lead, and can be used to prevent any future such issues, and to actively fix them should the unavoidable occur through flooding.
One thing occurs to me, though. Are you sure the road is the source of the lead? How long have you been on that property, and how far back can you trace what activities occurred there? Lastly, I don't know that this is possible in your area, but if the cause was geological in nature, that would have ramifications, likewise if the property had, at some time in the past, been involved in the sale or storage of petrochemicals.
I hope you find it's not as bad as all that, though. I wouldn't underestimate the power of biological remediation through sequestration, and having an active system in place to deal with unknowable future ills is right up there in my personal permacultural planning handbook.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein