Hi:
 
Vic may be right - bees splattered on windshields can be a problem.
 
However, we have documented that levels of lead in or on forager bees not
only is higher in colonies near busy roads, but that the levels change
from day - to -day.  For example, we saw peaks around the 4th of July and
Labor Day, big holidays in the U.S.
 
Also, tires give off cadmium.
 
 
Now, before anyone panics - there is a big difference between being able
to measure pollutants in or on bees, pollen, honey, etc.  AND  any
imagined or real risk to bees or people.
 
We and others have found that heavy metals and other pollutants can get
in to honey -  but the levels are likely to be much lower than the levels
in the bees themselves or pollen.  In addition, the levels in honey, while
measurable generally are not high enough to be of concern to human health.
When compared to flowering plants and vegetables, the honey usually looks
a lot better than the plants (in these areas).
 
Now, the levels in bees can get fairly high, as can the levels in pollen.
So, I don't advise eating pollen (or bees) from colonies kept in heavily
polluted areas.  However, I would be more concerned about bees near
smelters, chemical plants, and some other heavy industries, than bees near
highways.
 
Bottom line, if  you live by the highway, you are already exposed to the
fumes, metals, etc. from the cars every time that you take a breath.  If
you grow a garden, you ingest some more.  If your kids play in the yard
and stick their fingers in their mouth, even more exposure.  So, the bit
more (if any) from honey will only be a small part.  I'd be a bit more
cautious about pollen, if the traffic is really heavy.  Lead and cadmium
tend to get in to the hive in a particulate form that is deposited onto
plants and bees from with the dust in the air.  As such, we have seen
levels of these chemicals in or on pollen at concentrations several fold
higher than in honey.  I envision pollen as a "sticky lollipop" that gets
coated with the junk, whereas the necataries are likely to be less
exposed to chemicals drifting out of the air.
 
All of this ignores plant uptake from soil into nectar or pollen, but our
experience is that the fallout from the air is more important.
 
Finally, you will also find things like napthaline in your hives. But
again, remember the distinction between measurable and effects, presence
does not equal effect.
 
At some levels, pollutants do harm bees and people.
 
However, very few of us live in "pristine" environments.  So, we have to
use common sense.
 
Beyond that, the only way to really know is to analyze the honey, pollen,
and bees.  If your bees appear to be healthy and thrive, I wouldn't worry.
If you can't keep them alive, or if you see lots of patchy brood (and you
have medicated and treated for mites), I'd suggest moving the bees to
another location.  If they recover, I'd find a new place to live.
 
Cheers
 
Jerry Bromenshenk