Untreated (end-of-pipe) highway runoff can exhibit direct aquatic toxicity in some situations. The variations in runoff toxicities based on highway metrics and watershed characteristics is vague, at best. The exact causes of these toxic discharges are unknown, but have been speculated to result mostly from "high" concentrations of dissolved (non-complexed) metals and combustion polynuclear aromatic hydrocarbons (multiple benzene rings).
Increases in impervious cover on watershed scales, as measured though total impervious area, effective impervious area, forest cover, or other metrics, have been positively correlated with degraded stream conditions in small, lowland watersheds. The widely accepted theory is that impervious cover changes stream hydrology, which degrades stream habitat, which in turn leads to decreased stream biodiversity. The specific role of state highways in a watershed context on this topic, if any, is unknown.