>  This explains why bees are attracted to freshly-laid tarmac, but never to old road edges. 

I think you are way over-thinking this one. Bees collect stuff that's sticky because its useful. They also collect random stuff like sawdust, as if it were food. Personally, I would be reluctant to eat propolis (or sawdust).

> Bees are not too fussy about what they collect and road tar, drying paint and caulking compounds are collected too. It can accumulate environmental pollutants, such as lead from paints on hives, in metallic hive spacers, in the air from petrol, and industry, as well as drugs used to cure bee illness, and hive waste. South American propolis has been reported as having excessive lead contamination. 

Source:
http://www.skeptics.org.nz/download/flybeeproducts.pdf

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