Source? As a counter anecdote: Madrid, where most people live near traffic, has the highest life expectancy in the EU at 85.4 years.

Poor people live next to highways?

Or when we build the highway, we don’t raze the homes of the elite/wealthy/well-connected to do it, we do it in “bad neighbourhoods”

Or indirect effects: factories move/build next to the new highway (ease of moving materials+density of employees+ease of commuting because of the highway), contaminating the neighbourhood.