Heavy-Metal Contamination in Agroecosystems From Road Transportation: A Global Threat to Food Safety and Security

Road transport has emerged as a major global contributor to heavy-metal contamination in agroecosystems, threatening sustainable soil functioning, food safety, and public health. This PRISMA guided review synthesized to evaluate vehicular and infrastructure sources of metals, transport processes governing transfer to soils and crops, and implications for food safety and human health. Pb, Cd, Zn, Cu, Cr, Ni, and Hg consistently accumulate in roadside soils, with concentrations influenced by traffic intensity, road age, soil properties, and distance from carriageways. Brake and tire wear, together with road surface abrasion, are identified as principal contemporary contributors, while runoff and atmospheric deposition act synergistically to redistribute metals into adjacent fields. Crop uptake is highly species dependent and controlled largely by metal bioavailability and foliar deposition rather than total soil levels, leafy vegetables frequently show higher burdens than cereals. Widely used indices (I_geo, EF, BAF, TF) aid interpretation of enrichment and transfer, but cross study comparability is limited by inconsistent background values, sampling designs, and neglect of bioavailable fractions. Health-risk frameworks (THQ/HI/CR) reveal that dietary exposure may be significant in high-traffic corridors even when soil concentrations meet some screening thresholds. The review highlights key contradictions, their mechanistic drivers, and the need for standardized methods. Effective risk reduction requires combined source control, buffer zones, soil amendments, and crop focused monitoring to limit heavy-metal transfer from roadside environments to the food chain.