These guys found that road-side dust contains 1000 DKK worth of platinum per 1 ton of dust.
Cody explains that this concentration of platinum equals the amount found in good-quality platinum ore.
Does the stretch of road matter?
In a word, yes. The user Jafromobile commented that a car is likely to eject platinum particulates into the exhaust stream when the catalytic converter is the hottest, when the exhaust system is under pressure, and both of these conditions are basically achieved at the same time. In other words, at full throttle.
Where do cars use full throttle? In an urban area it could be at traffic lights. Other sections of road might in a similar manner reveal consistently-high platinum deposits.