Building on the comment about lead (Pb), other metals such as Cadmium and Zinc are common alongside roads, and especially PAH's (polycyclic aromatic hydrocarbons) that are made from the burning of fossil fuels (so car exhaust!) and deposited within 25m of roadways and bind in the soil. A good thing to consider is how the folks living near there would be exposed to contaminants as a result of the huge increase in traffic from the overpass. Are they growing gardens near there? Are children playing near there in the soil?
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There's a community who lives just off or, nearly under the overpass- as in, they look directly up at it at probably an 85 degree angle, so children play there. There are also some gardens, and a small intersection with a green way with a field there where children often play.
You could get a local lab to test for metals using atomic absorption or atomic emission. You would have to look up the best technique for the metals you are interested in. Anyway, you would do an extraction of a soil sample with nitric acid and get the lab to do the analysis. If you talk to someone at your local university chemistry department you might be able to get them to have a student do it in the fall semester as a project.
Poly-cyclic aromatics you could probably do a qualitative test for using uv spectroscopy. Extract the organics with something like hexane, evaporate it to a known volume and record the spectrum. This is something that your local college will definitely be able to do.